Industrial Chem

8/16/2019 Industrial Chem

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Copyright © 2006 keep it simple scienceHSC Chemistry Option Topic “Industrial Chemistry”

Trinity Catholic College, Auburn SL#606238

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HSC Chemistry Option Topic

INDUSTRIAL CHEMISTRY

What is this topic about? To keep it as simple as possible, (K.I.S.S.) this topic involves the study of:1. THE CHEMICAL INDUSTRY

2. CHEMICAL EQUILIBRIUM

3. PRODUCTION OF SULFURIC ACID

4. PRODUCTION OF SODIUM HYDROXIDE

5. SOAP & DETERGENTS

6. THE SOLVAY PROCESS FOR Na2CO3...all in the context of the applications of Chemistry in human society.

1. THE CHEMICAL INDUSTRY

The “Invisible” IndustryMost people are familiar with some aspects of the

production and manufacture of the many goods we needand use every day, but do not understand the vast chemical

industry which underlies it all.

You might never have seen inside a paper mill, but you can

at least imagine that it is a big factory where wood chips goin one end, and paper comes out the other. You are familiar

with paper itself, so you can get your head around the ideathat it is made in a factory somewhere.

What most people do not realize is that paper manufacture

uses not only wood chips, but huge quantities of chemicalssuch as sodium hydroxide (NaOH) and chlorine (Cl2 ).

Where do these come from?

They are supplied to the paper mill by the chemical industry.

You might never have visited a sugar plantation or wheat-

growing farm, but you eat bread and sprinkle sugar on yourbreakfast cereal, so producing these foods seems quite

understandable.

What is “hidden” is that farming (in general terms) uses

vast quantities of fertilizers, pesticides and other chemicals which are the products of an “invisible” chemical industry.

We do not generally buy and use chemicals such as sulfuric

acid, sodium hydroxide, ammonia, or 1,3-butadiene, so wedon’t appreciate that these chemicals are consumed in vastquantities to manufacture the everyday things we use and

need.

This topic will give you a glimpse of that “invisible”,underlying industry which is vital to all the others...

...the Chemical Industry.

Photo ©Robert Lincolne 2006Used with permission

Sugar Cane Harvesting

Paper MillBurnie, TasmaniaPhoto by Diana


2/32Copyright © 2006 keep it simple scienceHSC Chemistry Option Topic “Industrial Chemistry”


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A Brief History of Changed SupplyNatural rubber comes from the sap (“latex”) of a tree which is native to tropical South America, but now widely grown in plantations in tropical Asia, especially Malaysia,Indonesia and Thailand.

Rubber became a vital resource for making tyres as motor vehicles became more and more common and important inthe early 20th century.

In World War II, the major rubber-growing areas of South

East Asia were conquered by Japan, so a major proportionof world supply was suddenly cut off. This stimulatedmajor research projects in Germany and USA to develop asynthetic substitute for rubber.

Today, most rubber products are made from varioussynthetic rubbers (e.g. “neoprene”) which are polymersmade from petrochemicals. Even without the impetus of WW II, this switch to synthetic rubber was inevitablebecause:

• natural rubber production could never have kept up withthe accelerating demand as world population andeconomies grew rapidly after 1950. Rubber plantations

require a lot of land which is needed for food productionin developing tropical countries.

• natural rubber varies in quality, and cannot match the versatility and hard-wearing properties of the varioussynthetic polymers.

• natural rubber can only be grown in the tropics, andsupplies (and prices) vary with the weather and theuncertainties of political events and long-distance shipping.Synthetic rubber supplies are based on petrochemicals andcan be made anywhere with much more reliability,efficiency and predictable price structures.

The Modern Day IssuesIn the modern world we have the “Global Economy”,growing populations, increasing wealth and living standards, and rubber continues to be a vital resource.

Its major use is still the manufacture of vehicle tyres anddemand continues to grow. As well as tyres, its uses includemedical gloves and dressings, wetsuits, fittings for taps and valves (e.g. tap washers), tool handles, foam cushions andtoy making.

While natural rubber continues to be grown in the tropics,70% of world production is made synthetically frompetrochemicals, so the modern issues of rubber supply are:

• It is unlikely that natural rubber production can besignificantly increased because the vast areas of tropicalland needed for rubber tree plantations are also vital forfood production in developing countries.

• Petrochemicals for synthetic rubber are a non-renewableresource and supplies are likely to fall below demand within20-30 years. (It may be happening already)

• The chemical technology to replace many petrochemicals with renewable chemicals such as ethanol is quite feasible,but would require vast areas of land being devoted togrowing sugar cane (or similar crop).

These issues can be revised in the topic “Production of Materials”.

Evaluation of Progress

In June 2006, a consortium of universities and companiesheaded by the Mazda Motor Corporation, announced thedevelopment of a new biopolymer suitable for replacing various petrochemical polymers for motor vehicles. Thiscannot replace rubber for tyres, but is progress towards that.

Made from corn starches and sugars, with the involvementof a type of bacteria, the biopolymer is “GreenhouseFriendly”. The corn can be grown in temperate climates

Replacement of Natural Chemical ResourcesCase Study: Rubber

Rubber Plantationand Latex Harvestingin BrazilPhotos courtesy of

Tiago Pantaleao

The major use of rubber(over 60%)

is for vehicle tyres

where there is lesspressure to use land only for food production.

This is progress, but not

yet a solution.

As discussed in aprevious topic, the key tofully replacing petrochemicals may lie indeveloping thetechnology toeconomically convertcellulose (from plant wastes) into glucose.

Research continues...


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Equilibrium and Le Chatelier’s PrincipleIn previous topics you were introduced to the idea of chemical equilibrium and how it can be affected by changesin temperature, concentration and gas pressure.

Many chemical reactions do not just run forward and go tocompletion. Instead, both forward and reverse reactionsmay proceed. Eventually a “dynamic equilibrium” isreached where the forward and reverse reactions arerunning at the same rate.

For example, in the industrially important reaction to makeammonia from its elements...

...in a sealed container an equilibrium is reached with both product (NH3 ) and reactants (N2 & H2 ) present.

At equilibrium, the concentration of all 3 gases does notchange; it seems the reaction has stopped. It has NOT really stopped, but rather the forward and reverse reactions areoccurring at the same rate, so the concentrations remain fixed.

If certain reaction conditions are altered, the equilibriumposition can shift either forwards or in reverse. You arereminded of Le Chatelier’s Principle:

Effect of Temperature on Equilibrium The ammonia reaction above is exothermic, so heat energy can be thought of as a reaction product.

If the temperature was increased, the equilibrium will shiftto the left (more reactants, less product) because thisabsorbs heat and counteracts the disturbance.

If the temperature was decreased, the equilibrium will shiftto the right (more product, less reactants) because thisproduces heat and counteracts the disturbance.

Effect of Temperature (continued)

An endothermic reaction will respond in exactly theopposite way.

Reactants + heat Products

Increasing the temperature will shift the equilibrium to theright (more products). Decreasing the temperature will

shift the equilibrium to the left (more reactants).

Effect of Concentration

If the concentration of a reactant is increased, theequilibrium will shift right, attempting to use up the

reactant and counteract the change.

If the concentration of a product is increased, theequilibrium will shift left, attempting to use up the product

and counteract the change.

Effect of Volume and Pressure Changes

In reactions involving gases, changing the pressure and/orthe volume of the reaction chamber may have the effect of

changing the concentration, and shift the equilibrium one way or the other.

The effect of increasing pressure this way is to shift the

equilibrium in the direction resulting in less gases,according to Le Chatelier’s Principle.

For the ammonia reaction at left, increasing the pressure will shift the equilibrium to the right, since there are less

total gas molecules on the product side.

NOTE:If pressure was increased by pumping another, unrelatedgas (e.g. inert argon gas) into the chamber, this wouldincrease the pressure, but would not alter the concentrationof the reactant and product gases. This method of changing the pressure would NOT affect the equilibrium.

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2. CHEMICAL EQUILIBRIUM

N2(g)+ 3H2(g) 2NH3(g) + heat(exothermic)

Le Chatelier’s Principle:

If a system in equilibrium is disturbed,

the system will adjust itself in the direction which

counteracts the disturbance

Time

C

a

o

o

C

m

i

c

s

NH3

N2

H2

InitialEquilibrium at(say) 300oC

NewEquilibrium at400oC has lessproduct, more

reactants

Temp increased to400oC at this time

Temp decreased to200oC at this time

E q u i l i b r i u m h

a s m o r e p r o d u c t ,

l e s s r e a c t a n t s

( b u t t a k e s l o n g e r t o r e a c h b e c a u s

e

r a t e o f r e a c t i o n i s s l o w e r )

Mixture of Gas Reactants & Product at Equilibrium

Moving pistonchanges thevolume of thecontainer

Decreased = Increased volume Pressure

The same number of molecules are now crammedinto a smaller volume, at higher pressure.

This means they all have ahigher concentration, in molL

-1




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Prac Work: Modelling an Equilibrium Reaction You may have carried out an activity designed to model adynamic equilibrium. There are many ways to do this, butone of the simplest is as follows:

Use 2 measuring cylinders to represent “reactants” and“products”. The quantity of water in each represents the

concentration of each. Water is transferred from one to theother (using glass tubes or pipettes) to represent theforward and reverse reactions.

The glass tubes must be allowed to fill to the level in eachcylinder before transfer... no sucking water up!

After each back-and-forth cycle, the volume of water ineach cylinder can be recorded, and later graphed.

You can try using different diameter glass tubes(representing different forward and reverse reaction rates)and you can start with one cylinder full, one empty, orbegin with both half-full... lots of variations possible.

“Reactant” Cylinder “Product” Cylinder

When the tube fills to the level inthe cylinder, the top is closed witha finger, and the water transferred

to the “product” side. Thisrepresents the forward reaction.

Then the tube is allowed to fill tothe level in the “product” cylinder,and some water transferred back.

This represents the reversereaction.

Typical Results

It may take a while, but eventually the amount of water ineach cylinder reaches a (more-or-less) constant amount.

Despite water still being transferred back-and-forth, theamount in each cylinder stays the same... a dynamic

equilibrium. The larger the diameter of the glass tube(s)

used, the faster the equilibrium will be reached.

V o l u m e o f w a t e r i n c y l i n d e r

( s i m u l a t e s C o n c e n t r a t i o n )

R e a c a n

t

l i

e

P

u c t

C y l i n

d er

Results Graph, if :• begin with reactant cylinder

full, product cylinder empty.• use same diameter tube for

transfer in both directions.(Simulates forward & reversereactions having same rate)

V o l u m e o f w a t e r i n c y l i n d e r

( s i m u l a t e s C o n c e n t r a t i o n )

R e a c a n

t

l i

e

P

u c t C

y l i n d e

r

Results Graph, if :• begin with reactant cylinder

full, product cylinder empty.• use larger diameter tube for

transfer towards products.(Simulates forward reactionrunning faster than reverse)

Prac Work: Observing an Equilibrium Shift Another experiment you may have done, is to observe(qualitatively) an equilibrium shift occurring. This ofteninvolves observing a colour change in an equilibriummixture, when reaction conditions are changed. Two common examples are:

Temperature Effect on an Equilibrium Your teacher may have prepared a sample of the (toxic)gas nitrogen dioxide (NO2 ) by reaction of concentratednitric acid on copper. This gas rapidly reacts to reach theequilibrium:

2NO2(g) N2O4(g) + heat (exothermic)brown colour colourless

As temperature increases, the equilibrium shifts left, which forms more (brown coloured) NO2 gas.

No. of “transfer cycles” (simulates Time)

ice bath water bath hot water bath4oC 40oC 80oC

Colour Darker as Temperature Increases

NO2

&N

2

O4

NO2

&N

2

O4

NO2

&N

2

O4

Concentration Effect on an Equilibrium

Another commonly studied reaction ischromate ions dichromate ions

2CrO42-

(aq) + 2H+

(aq) Cr2O72-

(aq) +H2O(l) yellow orange

If this equilibrium shifts, the colour will become moreyellow, or more orange.

You might start with a yellow solution of chromate ions in

water. If a few drops of 2 molL-1

sulfuric acid (H2SO4 ) are

added the solution becomes more orange; the equilibriumhas shifted right because the concentration of H+

ionsincreased.

If a a few drops of NaOH

solution are added, the colourchanges to yellow again.

The NaOH reacts with H+

ions and lowers theirconcentration. The

equilibrium shifts left,

according to Le Chatelier’sPrinciple.




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Mathematical Analysis of EquilibriumIn a closed chemical system, at a given temperature, achemical reaction may reach a dynamic equilibrium whereboth forward and reverse reactions are occurring at thesame rate, and the concentrations of all species presentbecome constant.

Regardless of the amounts of chemicals you begin with,

the equilibrium will reach a point where the ratio of concentrations of reactants and products becomes aconstant value.

Consider a generalized chemical equation:

aW + bX cY + dZ

W & X are the reactant chemicals. Y & Z are the product chemicals.a,b,c & d are the molar coefficients(“balancing numbers”) for each of the chemicals.

Then, at equilibrium:

Interpreting the Value of K Values for the constant K vary enormously from one

reaction to another, and also change with temperature. What do high and low values mean?

If K is very large... e.g. K > 103

(more than 1,000)

This means that the concentrations of the PRODUCTS are

much higher than those of the reactants at equilibrium. Theproducts must be favoured in the equilibrium, which is said

to “lie to the right”.

If K is very small... e.g. K < 10-2

(less than 0.01)

This means that the concentrations of the REACTANTS

are much higher than those of the products at equilibrium. Very little product has been made, and the equilibrium issaid to “lie to the left”, favouring the reactants.

If K is middle-sized... e.g. between 0.01 and 1,000

This means that the concentrations of the PRODUCTSand REACTANTS are very roughly equal at equilibrium.

The reaction reaches an equilibrium “in the middle”.

In example 1, the value for K was a “middle-sized” value,

and if you look at the concentrations of reactants andproducts you will see that they were roughly equal.

Now study example 2 for quite a different situation...

EquilibriumConstant, K = [Y]

cx [Z]

d

[W]a

x [X]b

You are reminded that, in chemical shorthand,[square brackets] means “concentration in molL

-1”

So, K= (Molar conc. of PRODUCTS)(raised to powers)

(Molar conc. of REACTANTS)(raised to powers)

Each chemical reaction has its own value for K, which is constant at any given temperature.

The value of K changes at different temperatures.

Units of Measurement: All concentrations must be in moles per litre (molL

-1 )

The units for the constant K vary from reaction toreaction. The syllabus does NOT require you to useany units for “K”... values only.

Example Problem 1Calculating the Value of K for an Equilibrium

The reaction below has reached equilibrium at 200oC.

The molar concentrations of each species is shown.

0.100 0.20 0.22molL

-1molL

-1molL

-1

Calculate the value of the equilibrium constant, K.

Solution:K = [NH3 ]

2= (0.22)

2

[N2 ] x [H2 ]3

0.10 x (0.20)3

= 61 (2 sig. figs) At 200

oC, the value of K = 61

Example Problem 2 Another Equilibrium

The reaction below has reached equilibrium at 0oC in a

5.00 litre container. The number of moles of each

species present in the container is given.2NO2(g) N2O4(g)

0.0173 0.213mol mol

Calculate the value of the equilibrium constant, K andinterpret this value.

Solution:K = [N2O4 ]

[NO2 ]2

You must NOT jump in and substitute the values given!

The values are mole amounts, not concentrations. Notethat it is a 5 litre container, so the concentrations are:

[NO2 ] = 0.0173 / 5 = 0.00346 molL-1

and [N2O4 ] = 0.213 / 5 = 0.0426 molL-1

Now substitute: K = 0.0426 / (0.00346)2

= 3.56 x 103

At 0oC, the value of K = 3.56 x 10

3, which indicates

that the equilibrium lies well to the right, favouring

the production of N2O4.

N2(g)+ 3H2(g) 2NH3(g)

Try the WORKSHEET at the end of section




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Reciprocal Values of K A reaction can reach equilibrium from either direction, and

the equation can be written in either direction.

For example, imagine it was possible to begin with a puresample of NO2(g) at 0

oC. Example 2 (previous page) tells

us what would happen:

Now, imagine starting with a pure sample of N2O4(g) at the

same temperature of 0o

C.

The exact same equilibrium will be reached, but

approached from the opposite direction.

Notice that the expressions for the equilibrium constants

are reciprocals of each other:

K R = 1 = 1 = 2.81 x 10-4

K 3.56x103

Values of K Change with TemperatureFor any chemical reaction at equilibrium at a particular

temperature, the value of K is a constant regardless of thequantities involved.

However, the value will change at different temperatures.

With an understanding of Le Chatelier’s Principle, andknowledge of the expression K = [Products]

[Reactants]

you can predict the way that K changes with temperature.

Exothermic Reactions

In an exothermic reaction, heat is a product:

Reactants Products + heat

As temperature increases, the equilibrium will shift left, so

at the new equilibrium the [reactants] is higher and[products] is lower. Therefore, the value of K will decrease.

Example:

Temperature (oC) Approx.Value of K

0 5.0x108

200 60400 0.25

600 0.005

Endothermic Reactions

In an endothermic reaction, heat is a reactant:

Reactants + heat Products

As temperature increases, the equilibrium will shift right, soat the new equilibrium the concentration of products is

higher and concentration of reactants is lower.

Therefore, the value of K will increase.

C o n c e n t r a t i o n

Time

2

N 2O

Starting with pure NO2

(& no N2

O4

)the reaction reaches equilibrium:

2NO2

N2

O4

and K = [N2

O4

] = 3.56x103

[NO2

]2 at 0oC

C o n c e n t r a t i o n

Time

N 2

2 4

Starting with pure N2

O4

(& no NO2

)the reaction reaches the sameequilibrium from the other direction:

N2

O4 2NO2

and KR

= [NO2

]2

[N2

O4

]

If an equation is written in reverse

then the expression and value for K

is the reciprocal.

K reverse = 1K

Exothermic Reactions

K decreases as temperature increases

Endothermic ReactionsK increases as temperature increases

N2(g)+ 3H2(g) 2NH3(g) ∆H= -92 kJmol-1

Effect of Catalysts on Equilibrium

You are reminded that catalysts:• lower the activation energy

and • speed up chemical reactions

What about equilibrium?Catalysts speed reactions up so that they reach equilibrium

sooner, but have no effect on the position of theequilibrium, and do not change the value of K . C o

n c e n t r a t i o n o f p r o d u c t

Time

Equilibrium

r e a c t i

o n w i t h

o u t c

a t a l y

s t

With catalyst,Equilibrium reached faster


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Worksheet 1&2Fill in the blanks. Check your answers at the back.

Rubber Supply Case StudyNatural rubber is made from the a)............................... of atree native to b)..................................................., but now grown also in tropical c).................................. Since World War II, rubber has also been made synthetically from

d)................................................ Synthetic rubber makes upabout e)................% of world production. Over 60% of total production is used for f)....................................................

The issues/problems with rubber are that demandcontinues to grow, but:• natural rubber production cannot easily be increasedbecause g)..............................................................................................................................................................................................• synthetic rubber is dependant on petroleum, a non-h)........................................ resource.

There has been some progress towards solving theseproblems, including development of new i).............................

made by bacteria from sugars and starch derived fromj)......................... Ultimately we need to be able to usek)................................................ from plant wastes to makesynthetic rubber (and other polymers).

1. (5 marks)

Write the expression for the equilibrium constant for:

a) PCl5 PCl3 + Cl2b) 2H2O 2H2 + O2c) 2NO + Cl2 2NOCl

d) Al(OH)3(s) Al3+

(aq) + 3OH-(aq)

e) 2H2S 2H2 + S2

2. (5 marks) The reaction H2(g) + I2(g) + heat 2HI(g)reached equilibrium at 400

oC.

Predict the effect on the equilibrium concentration of HI(g)of: i) increasing the temperature

ii) compressing the mixture to a higher pressureiii) increasing pressure by pumping in extra H2(g)iv) adding a catalyst v) increasing pressure by pumping in argon gas

...explaining your answer in each case.

3. (4 marks)

In an equilibrium mixture as described in Q2, theconcentrations (at 400

oC) were [H2 ]= 0.0195molL

-1,

[I2 ] = 0.0211molL-1

and [HI] = 0.153molL-1

.

a) Calculate the value of K, the equilibrium constant.b) The temperature of this same gas mixture was changed,

and the new value for K = 22.3. Was the mixture heated or cooled? Explain.

Chemical EquilibriumMany chemical reactions do not go to completion, butreach a l)................................... ............................................... in which both forward and reverse reactions are running atm).............................................................. According ton)..........................................’s Principle, if an equilibriumsystem is disturbed, the equilibrium will shift in thedirection which o)........................................................................

The main factors which can disturb an equilibrium arep).............................. and ......................................... If gases areinvolved, a change in q)............................. or ............................of the container will change the concentration andtherefore will shift the equilibrium.

For any reaction, the equilibrium can be describedmathematically by “K”, the “r)......................................... ........”If this value is very high, it means the equilibrium favoursthe s)........................................, while if it is very low theequilibrium favours the t).......................................... The value of K for the “reverse equation” is theu)............................................... of the original. The value of K is constant only at a particular v)..........................................

The values of K for an exothermic react ion w)......................................... .. as temperature increases. The values for an x).......................................... .... reaction willy)........................................... as temp. increases.

4. (5 marks) A sample of pure hydrogen iodide gas (HI) was placed into

a container and heated to 400o

C. It decomposed in thereverse reaction to that described in Q2 & Q3, and themixture reached an equilibrium.

a) Write an equation for the reaction.b) Write an expression for the equilibrium constant, andpredict its value for equilibrium at 400

oC. (Use answer Q3)

c) At equilibrium, the concentration of hydrogen iodide gas[HI]=0.0552molL

-1. Find the concentrations of H2 and I2

gases. (Hint: they will be equal to each other)

5. (5 marks)Carbon monoxide reacts with steam as follows:

CO(g) + H2O(g) CO2(g) + H2(g)

A mixture containing 10.0 moles each of CO gas and steam was introduced at high pressure and temperature into a 4.00litre container. After equilibrium was reached it was foundthat there were 6.50 moles of CO2 in the container.

a) Calculate the equilibrium concentrations of each reactantand product.b) Calculate the value of the equilibrium constant for thistemperature.c) What would have been different if the reaction had beencarried out in the presence of a catalyst?

7



Practice Problems for Section 2 These are not intended to be "HSC style" questions, but to challenge your basic knowledge and

understanding of the topic, and remind you of what you NEED to know at the K.I.S.S. principle level.


8/32

The Uses of Sulfuric AcidSulfuric acid (H2SO4 ) is one of those “hidden” chemicalsdescribed in the introduction on page 1. People do notusually buy it, or use it, so its importance to industry is not

generally understood.

In fact, it is one of the most vital industrial chemicals andis produced in greater quantities than any other

manufactured chemical in the world.

Its major uses are:Fertilizer production (over 70% of production) The main world fertilizer is “superphosphate”, which is

made from “rock phosphate” by treating it with H2SO4. The other important fertilizer is “sulfate of ammonia”

((NH4 )2SO4 ) which is made by reacting ammonia withsulfuric acid.

Titanium Dioxide (TiO2) production

Titanium dioxide is a white pigment used in huge quantitiesin paint, paper, plastics, synthetic fibres and even cosmeticsand good-old zinc cream. Sulfuric acid is used to make

TiO2 from its ore.

Cleaning of Steel

Before galvanizing (coating with zinc for corrosioncontrol) or electroplating any steel products it is essentialthat any surface rust, grease or dirt is removed. This isgenerally done by treatment with sulfuric acid which is very

corrosive and “eats” surface impurities away very efficiently.

In addition to these uses, sulfuric acid is used as a reagent,

catalyst or solvent in a wide variety of other industries,including petroleum refining, mining, and the production

of detergents, explosives, nylon and other synthetic fibres,pharmaceuticals, dyes, etc, etc...

Extracting Sulfur from Mineral Deposits The basic raw material for the manufacture of sulfuric acid

is elemental sulfur. A lot of sulfur is collected as a by-product of metal smelting and the petroleum industry, but

there are some places where mineral deposits of elementalsulfur occur naturally, due to ancient volcanic activity.

These deposits are mined by the “Frasch Process”, whichtakes advantage of the properties of sulfur to extract it

from the Earth without the usual trouble and expense of traditional mining methods.

A triple pipe is drilled down into the sulfur deposit. Hot

water (160oC) is pumped down under pressure in one pipe,

and compressed air down another. The sulfur melts and theair pressure forces the mixture of sulfur and water up

through the third pipe.

The properties of sulfur which allow this process to work efficiently are:

• Low melting point: Sulfur melts at about 120o

C, sopressurized hot water at about 160

oC is well able to liquify it.

• Low density: Sulfur has a low density so that compressed

air is easily able to force it upwards.

• Solubility: Sulfur is insoluble in water, so the mixture of molten sulfur and water that rises to the surface is easily

separated without any need to evaporate the water. Withslight cooling, the sulfur solidifies and rapidly settles out. The still-warm water can be re-cycled.

8



3. PRODUCTION OF SULFURIC ACID

Deposit ofSulfur

Surface

Triple Pipedrilled down

into sulfur deposit

Sulfur meltsin this zone

Compressed air

injected

Superheated waterinjected

Molten Sulf urand hot water

mixture




9

Environmental Issues of Extracting Sulfur While sulfur itself is not an environmental threat, the various sulfur compounds, and other impurities, releasedby the mining or smelting processes, are of great concern.

• Sulfur dioxide gas is an acidic oxide which has thepotential for serious environmental damage, including the“acid rain” phenomenon studied in a previous topic.

• Hydrogen sulfide (H2S) is a gas more toxic than cyanideand its release could have serious consequences in theimmediate vicinity of a mining operation.

There may be soluble minerals which are brought to thesurface by the Frasch Process. For example, volcanic sulfurdeposits often contain small amounts of soluble arseniccompounds. If any waste water is discharged into theenvironment this could poison ecosystems or human waterand food supplies.

Reaction Rate & Equilibrium ConsiderationsStep 1: No problem! The reaction proceeds rapidly (insidea furnace) and goes to completion.

Step 2: This is the tricky one! The reaction reaches anequilibrium, so to get maximum yield and rate it is necessary to adjust the reaction conditions. By Le Chatelier’s Principle,getting maximum yield should involve:

• Reacting at low temperature should shift equilib. to right.

However, this slows down the rate, so use a catalyst.• High pressure should favour the products.• Increasing the reactant concentration and decreasing the

product concentration should drive the equilibrium to theright.

In practice, (as is often the case) some compromises arenecessary... study the diagram, next page.

Step 3: It’s not practical to simply combine SO3 gas with water because the resulting “fog” of tiny droplets isdifficult to handle. In practice this is achieved in a different way... this is explained on the next page.

Production of Sulfuric Acid from Sulfur To produce H2SO4 from sulfur there are just 3 mainchemical steps:

Step 1: Burn sulfur to make sulfur dioxide gas

S(s) + O2(g) SO2(g)+ heat

(This step is not needed if SO2 is supplied directly from ametal smelter)

Step 2: Oxidize SO2 to SO3

2SO2(g) + O2(g) 2SO3(g)+ heat

Step 3: React SO3 with waterSO3(g) + H2O(l) H2SO4(l)

+ heatHowever, it’s not quite that simple!

Sulfur Dioxide from Metal SmeltingIn an earlier topic you learned that some metal ores containmetal sulfide compounds. When these ores are smelted to

obtain the metal, the gas sulfur dioxide is also produced.

Example: A common ore of lead is “galena” containing

lead(II) sulfide, PbS. This is roasted in a furnace with a blast

of air :

PbS(s) + O2(g) Pb(l) + SO2(g)

In earlier years the SO2 was simply released into the air, andcaused a lot of environmental damage. These days it is

collected and either sent directly to an ajacent sulfuric acidplant, or is chemically reduced to elemental sulfur.

Sulfur from Petroleum & Natural GasPetroleum and natural gas often contain small butsignificant amounts of hydrogen sulfide (H2S). This mustbe removed so that it will not end up being burnt with the

fuel to release polluting SO2.

2H2S(g) + 3O2(g) 2H2O(g) + 2SO2(g)

Often the first step in refining petroleum or natural gas isto separate the H2S and convert it to solid sulfur.

Environmental Damage

Photo by Diana




10

Molten Sulfurinjected

The reaction is exothermic, so the

SO2

(and the unused air)

comes out at about 1000oC.

It must be cooled before the next

step. The excess heat is used to

make steam f or other processes,

or to make electricity.

Excess

of dry air

Sulfur

burning

furnace

Heat

Exchanger

S(s) + O2(g) SO2(g)

Steam out

Cooling water in

Multi-stageCatalytic

SO2Converter

steam out

water in

heatexchanger

SO2

70% conversion to SO3

at fast reaction rate

97% conversion to SO3

at slower reaction rate

conversion of virtually all

remaining SO2

Any unreacted SO2

returns to catalytic converter

Catalyst bed.Pellets of

vanadium(V)

oxide (V2O5)

Temperature = 550oC

Temperature = 400oC(pushes equilibrium

to the right)heatexchanger

SO3

+

=

H

2SO

4

H2S2O7“oleum”

Temperature = 400oC

Sulfuric acid in

Absorption

Tower

Oleum

diluted

H2SO4Final

Product

Water in

2SO2(g)+ O2(g) 2SO3(g)

The “Contact Process”Industrial production of H2SO4 is called the “ContactProcess” because, during that critical second step, the

reactants must be in contact with a catalyst.

Further Explanations

Temperatures chosen for the “Contact

Process” are a compromise between yield(higher at lower temperature) and rate of

reaction which is faster at higher temperatures.

Passing the gases through the catalytic converter

a second time (at lower temperature) shifts theequilibrium toward greater yield and achieves

about 97% conversion to SO3.

The Catalyst vanadium(V) oxide (V 2O5 ) speedsthe reaction up by lowering activation energy.

The catalyst is in the form of small pellets sothere is a huge surface area

for the gases to react on.

Although the equilibrium yield would be favoured by

High Pressure, the conditions described aboveachieve the desired yield and rate already, so

there is no need for the great expense of high-pressure equipment. The entire process is

operated at just above atmospheric pressure,sufficient to push chemicals through.

The Final Step:

Although SO3 will simply react with water...SO3(g) + H2O(l) H2SO4(l)

this results in a “fog” of sulfuric acid whichis difficult to handle.

Instead, the SO3 is absorbed by H2SO4forming “oleum”:

SO3(g) + H2SO4(l) H2S2O7(l)“oleum”

This is then diluted with water...

H2S2O7(l) + H2O(l) 2H2SO4(l)

... which achieves the same overall

result, in a way that is more convenient




11

Reactions as an Oxidising Agent You are reminded of the concept of Oxidation-Reduction:

Concentrated sulfuric acid is an oxidising agent: it willcause oxidation to other chemicals, while it undergoesreduction.

Examples: although copper metal (and others) will notreact with any dilute acid to form hydrogen gas, it will react

with hot, concentrated sulfuric acid as follows:

Copper oxidises Cu(s) Cu2+

(aq) + 2e-

(loses electrons)

Sulfuric acid reduces (gains electrons)

2H2SO4(aq) + 2e-

SO2(g) + 2H2O(l) + SO42-

(aq)

So, the overall reaction is

2H2SO4 + Cu SO2(g) + 2H2O + CuSO4(aq)

Concentrated sulfuric acid will also oxidise bromide (Br- )

or iodide (I- ) ions to the respective element

Iodine oxidation: 2I-(aq) I2(aq) + 2e

-

Sulfuric acid reduces as above, so if it is added to (say)sodium iodide solution the overall equation is

2NaI + 2H2SO4 SO2(g) + 2H2O + I2 + Na2SO4

Reactions of Sulfuric AcidIn a previous topic you studied Acid-Base reactions, and know that H2SO4 is a common laboratory acid.

However, it is much more than just an acid. Its wide usage in industry is not just because of its acid properties.

Loss of Electrons = “OXIDATION”

Gain of Electrons = “REDUCTION”

Reactions where this occurs are called

“OXIDATION-REDUCTION”

“REDOX”

Prac Work: Observing H2SO4 REDOX You may have carried out one or more of the reactions

above in the laboratory.

A “Risk Assessment” will have indicated the need to takeall the usual acid precautions, but additionally you may

have used a fume cupboard because SO2(g) is acidic, toxicand very nasty smelling!

The diagram shows the main observations you might makeif copper metal is reacted with concentrated H2SO4 in a

small beaker.Bubbles of gas (SO

2

)

Copper eaten away

Solution turns blue(CuSO

4(aq)

)

Moist blue litmus paper turns pink(SO

2

is acidic)

Reactions as a Dehydrating AgentSulfuric acid has a great affinity for water; it will absorb

water from other substances and so is used widely inindustry to remove moisture from gases and non-aqueous

liquids when these are needed in a totally dry state.

It will even “suck” water out of chemical compounds.

For example, the dehydration of ethanol:

+ H2O

ethanol ethylene + water

(This reaction may become important in the future if plant-

derived ethanol is used to replace ethylene for the plasticsindustry)

You should also remember the “condensation” reaction

which is used to make esters:

alkanoic acid + alkanol ester + waterIn this reaction the role of sulfuric acid is described asbeing a catalyst, but what’s really happening is a chemical version of a mugging... H2SO4 is very aggresive atdragging water molecules out of anywhere it can findthem!

In the laboratory you may have seen, or carried out, the

experiment where conc. H2SO4 is poured onto a sampleof ordinary sugar. The reaction is violent, spectacular and very smelly... a chemical classic!

C12H22O11 12 C(s) + 11H2O(g)sucrose (sugar) carbon + steam

The result is the growth of a charcoal stalagmite.H2SO4 can do the same to cotton, wood and paper,because they all contain cellulose, a sugar polymer.

H

H

H

C C

H

H

C C

H OH hot, conc.H

2

SO4

conc. H2

SO4




12

Ionisation of H2SO4 To remind you of some basics: H2SO4 is an acid because it

readily donates a proton to other chemical species. Whenmixed with water, it ionizes in 2 steps:

H2SO4(aq) + H2O(l) HSO4-(aq) + H3O

+(aq)

hydrogen- hydroniumsulfate ion ion

Then:HSO4

-(aq) + H2O(l) SO4

2-(aq) + H3O

+(aq)

Sulfuric acid is classed as a strong acid because the firstionization occurs to 100% of the H2SO4 molecules. Thesecond ionization reaches an equilibrium, so the

hydrogensulfate ion (HSO4- ) is a weak acid.

The dissolving and ionization of H2SO4 in water isstrongly exothermic:

H2SO4 + H2O HSO4-

+ H3O+

∆H= -75kJmol-1

The result is that a great deal of heat is liberated whensulfuric acid is mixed with water, and this leads to some

important safety considerations.

Procedure for Diluting Concentrated H2SO4If you pour water into concentrated sulfuric acid, thesudden heat release in a small volume of liquid can easily

cause the liquid to boil. This can cause “spitting” of smalldroplets of acid, or even cause the container to crack.

The correct procedure is to add the acid (slowly, withstirring) to the water. This way there is a relatively larger volume of liquid to absorb the heat and keep the

temperature manageable. If “spitting” does occur, it is amore dilute acid solution and less dangerous

Safety Precautions When UsingConcentrated H2SO4

Concentrated sulfuric acid is extremely corrosive to skinand clothing and its usage requires extreme care.

• always wear safety glasses to protect the eyes.

• wear safety gloves and laboratory coat/apron.• work near a supply of running water to dilute and wash

any slashes or spills.• have a supply of sodium hydrogencarbonate (NaHCO3 )

to neutralize floor spills.

Safe Storage and TransportationDiluted sulfuric acid contains hydronium ions (H3O

+ )

which react corrosively with most metals. It must be storedand transported in glass or plastic containers. These then

need careful packaging to avoid breakage.

Pure sulfuric acid with no water in it, is molecular H2SO4and therefore will not react with metals. This means that itcan be transported in steel tanks, which are strong enough

to provide safe transportation and storage.

It is vital that the containers have air-tight seals, not just to

prevent leaks, but to prevent the acid absorbing water vapour from the air. Remember that H2SO4 is a great

dehydrating agent and will readily absorb moisture. Asmoisture is absorbed H3O

+ions will form, and these will

begin to corrode the metal container.

ACID

WATER

Wrong

Dangerous!

WATER

ACID

Correct

Stir

Never add water

to the acid

P h o t o b y “ c o n n m a n 2 1 ”

Photo courtesy of Dave Gostisha


13/32

Worksheet 3Fill in the blanks. Check your answers at the back.

The biggest single use of sulfuric acid in industry is to makea).........................................., such as b).......................................

and ............................................................ Other importantuses include the production of c)............................................. .

(which is used as a white pigment in paper, paint, etc) andin the d).......................................... of steel prior to

galvanizing or electroplating.

Sulfuric acid is often made from elemental e).........................in 3 main chemical steps. Firstly, the sulfur is reacted withoxygen to make f)......................................... gas. This is then

converted to g)............................................ Finally, this isreacted with h)....................................... to form H2SO4.

Sulfur is sometimes found in its elemental state in mineral

deposits which can be mined by the “i)..................................Process”. This involves drilling a triple pipe into the

deposit, then pumping down hot j).............................. andcompressed k)............................... The sulfur melts, and isforced to the surface. It is easily separated from the water

mixture because it is l)................................................................

Other major sources of sulfur include the smelting of somem)................................................ .., and from the refining of n)......................................... and ................................................

which usually contain small amounts of sulfur compoundsas impurities.

1. (3 marks)Describe 3 properties of elemental sulfur which allow itsextraction from mineral deposits by the “Frasch process”,and briefly relate each property to the extraction process.

2. (4 marks)

a) Apart from mining elemental sulfur, name 2 other majorsources of sulfur for industrial use.b) Outline any environmental issues associated with theextraction of sulfur from these sources.

3. (5 marks) An important industrial reaction is

2SO2(g) + O2(g) 2SO3(g) + heat

a) Predict the temperature and pressure conditions whichmight maximize the yield of SO3 and explain your answer.b) Explain why, in practice, the temperature requirementmay have to be compromised.

The industrial production of H2SO4 is done by the“o)................................ Process”. Sulfur dioxide is producedby burning sulfur in a furnace. This is catalytically

converted to p)................................... by the catalystq).......................................... .......... The temperature chosen is

a compromise between r)............................... and ...................of reaction. The equilibrium yield of product is favoured

by s).............................. (higher/lower) temperatures, but therate is faster at t)............................... (higher/lower)temperatures.

The SO2 is passed through the catalytic converter a second

time at u)................................. (higher/lower) temperature, which gives a total conversion of about 97%.

Instead of reacting the SO3 directly with water, it is firstabsorbed into sulfuric acid to form “v).................................”

(H2S2O7 ). This is then reacted with water to form sulfuricacid.

As well as being a strong acid, H2SO4 is also an w)......................................... . agent and (because it has a greataffinity for water) a x)............................................. agent.

The ionization of H2SO4 in water is strongly y)...............-thermic. Therefore, when diluting the concentrated acid,

always add the z).............................. to the ...............................to avoid dangerous spitting.

4. (4 marks)In the “Contact Process” of industrial production of

sulfuric acid, explain why:a) the gas stream containing SO2 and O2 is passed throughthe catalyst twice, the second time at a lower temperature.

b) SO3 gas is absorbed into sulfuric acid and later reacted

with water, rather than being directly reacted with water.

5. (4 marks)

Write balanced chemical equations for:a) the oxidation of bromide ions.

b) the reduction of sulfuric acid.c) the dehydration of ethanol, and explain the role of concentrated H2SO4 in the reaction.

6. (5 marks)

Identify 4 safety precautions you should take when using concentrated H2SO4, and outline the method for diluting it.

13



COMPLETED WORKSHEETS

BECOME SECTION SUMMARIES

Practice Questions 3

These are not intended to be "HSC style" questions, but to challenge your basic knowledge andunderstanding of the topic, and remind you of what you NEED to know at the K.I.S.S. principle level.

When you have confidently mastered this level,it is strongly recommended you work on questions from past exam papers.


14/32

Sodium Hydroxide, NaOHSulfuric acid is the most important industrial acid, and themost important base is sodium hydroxide, NaOH. It isused in huge quantities in the paper industry and in the

manufacture of soap, detergents and various syntheticpolymer fibres, to name just a few uses.

The industrial production of NaOH involves electrolysis:

the process of using electricity to cause a chemical change. All electro-chemistry processes involve Oxidation-Reduction (“REDOX”) reactions, which were revised

earlier in this topic.

In a previous topic you learnt about Galvanic Cells, whichare the basis of electrical cells and batteries. At the right is

a quick revision, and an outline of Electrolytic Cells. Asyou will see, the production of NaOH involves anelectrolytic cell.

Production of NaOH in Outline The overall reaction used to make NaOH is:

2NaCl(aq) + 2H2O(l) 2NaOH(aq) + Cl2(g) + H2(g)salt water sodium chlorine hydrogen

hydroxide gas gas

(The Cl2 and H2 gases are produced as by-products, but are

very useful and valuable in their own right.)

As usual, the actual process is not as simple as the equation

suggests... there’s no way you can just mix salt and waterand expect to get the reaction above to occur.

To get a better idea, consider the reaction in ionic form:2Na

++ 2Cl

-+ 2H2O 2Na

++ 2OH

-+ Cl2 + H2

Now you can see that the sodium ions (Na+ ) are spectators

and are not changed. The real changes are:

• chloride ions are oxidized (lose electrons) to chlorine2Cl

-(aq) Cl2(g) + 2e

-

and• water is reduced (gains electrons)

to hydroxide ions & hydrogen gas

2H2O(l) + 2 e-

2OH-(aq) + H2(g)

The real “trick” is to carry out these reactions in such a way that the products are separated from each other so they

cannot react to produce other, unwanted products.

Historically, there have been 3 main industrial processes toachieve this: • the “Mercury Process”

• the “Diaphram Process”• the “Membrane Process”

Each is detailed a little later... first revise REDOX

14



4. PRODUCTION OF SODIUM HYDROXIDE

Review of “Galvanic Cells”In a previous topic you studied “Galvanic Cells”... here’s aquick revision:

Galvanic cells are the basis of electrical cells and batteries which produce electricity from REDOX reactions.

The reaction is spontaneous, and exothermic. (Energy

release is mostly electricity rather than heat.)

The sum of the Standard Half-Cell Potentials (Eo ) is +ve.

Electrolytic Cells

These are, in energy terms, the opposite of galvanic cells.

Electrolytic cells:• do not run spontaneously, and cannot produce electricity.• are endothermic, and must be supplied with energy (in

the form of electricity) to force the reaction to occur.

The Standard Cell Potential (Eo) is negative, and

indicates the minimum voltage which needs to beapplied for the reactions to proceed.

V

ANODE CATHODE

is where is whereOXIDATION REDUCTION

occurs occurs

electrons flow fromAnode to Cathode inthe external circuit

Electrodesmay bereactants, ormay be inertelectricalcontactse.g. graphite

Reactantsin

Electrolytesolutions

Electron flow

At the ANODE,electrons are

“sucked out”,forcing OXIDATION

to occur

At the CATHODE,electrons are

“pushed in”,forcing REDUCTION

to occur

Voltmeter measurescell potential

Electrical

source+ -

Ions canmigrate inelectrolytesolution

Electrolytic Cells are often constructed in asingle container to allow easy ion migration

Salt Bridge allowsions to migrate




15

Electrolysis of Pure, Molten ElectrolytesIf electricity is passed into (say) pure, molten sodiumchloride, the result is the decomposition of the compound

into its elements.

At the ANODE, chloride ions are OXIDIZED:E

o

2Cl-(l) Cl2(g) + 2e

--1.36 V

At the CATHODE, sodium ions are REDUCED:

Na+

(l) +e-

Na(l) -2.71 V

Cell Potential = - 4.07 V

This means that the cell requires at least 4.07 volts (under

standard conditions) for the reactions to occur.

In this cell there are only 2 chemical species present,so it is quite easy to predict what will happen.

-ve+ve

molten

NaCl

containsNa+ & Cl-

Cl2(g)

Molten sodiummetal

forms at cathode

CATHODE ANODE

(both inert electrodes)

The salt needs tobe molten so it is

an electrolyte,and ions can

migrate

Electricalvoltage

Refer to a Table of “Standard Potentials”

for “Half-Equations” and Eo Voltages

Electrolysis of an Aqueous SolutionIn an aqueous Electrolytic Cell, such as applying electricity to a salt solution, the reactions may be very differentbecause the water molecules can also react, and often do soin preference to other possible reactions.

For example, at the CATHODE...E

o

Na+

(aq) + e-

Na(s) -2.71 V and...

H2O(l) + e-

1/2 H2(g) + OH-(aq) -0.83 V

...are both theoretically possible.

However, the water reduction is far more likely because of its lower voltage requirement. These 2 half-reactions are“competing” with each other, and the water reaction winsthe competition by having a significantly lowerenergy/voltage requirement.

(If any sodium metal did form, it would instantly react withthe surrounding water anyway!)

Meanwhile, at the anode there are also 2 possibleoxidations that can occur:

Eo

2Cl-(aq) Cl2(g) + 2e

--1.36 V

andH2O(l) 1/2 O2(g) + 2H

+(aq)+ 2e

--1.23 V

In this case there is no clear winner because the voltagesare quite similar. Which reaction predominates will dependon the concentrations of the reactants.

In a very dilute salt solution (i.e. lots of water) the waterreaction would predominate, and the gas produced would

be oxygen. In a more concentrated salt solution there would be a mixture of oxygen and chlorine gasesproduced. In the industrial process, the salt solution used isa highly concentrated “brine”, so the chloride oxidationpredominates, and the gas produced is pure chlorine.

Prac Work: Electrolysis of Salt Solutions You may have carried out experiments in which youelectrolysed aqueous salt solutions, perhaps using a voltameter, as shown in the photo at right.

An acid-base indicator can be added to detect theproduction of either H

+ions or OH

-ions in the solution

around each electrode. The starting solution is neutral.

Any gas produced can be collected and subjected to simpletests, such as a “flame test”, or tested with wet litmus paper.

If you use a very dilute salt solution you may observe:

• the reaction is very slow (because the solution is nota good electrolyte).

• at the negative electrode the indicator shows a base(OH

-ions) being produced, and the gas collected will

“pop” when ignited... hydrogen.• at the positive electrode the indicator shows production

of acid (H+

ions) and the gas re-ignites a glowing ember... oxygen. Overall, water is being decomposed. - +

For a more concentrated saltsolution:• the reaction runs faster.• at the negative electrode,

same result... OH-

and H2

2H2O+ 2e

-

2OH

-

+H2

• at the positive electrode, thegas collected may be foundto bleach the colour frommoist litmus, and the odourof chlorine may be noticed.

2Cl-

Cl2(g) + 2e-

These are the same reactionsthat occur in the industrialproduction of NaOH.

H

2

O

2 or Cl2depending on concentration




16

Industrial Production Techniques for Producing NaOH There are 3 different processes to know about.

Environmental Problems with the Mercury Process

The Mercury Process is very efficient and economical, but has been almost entirely phased out because of its seriouspotential to cause environmental problems. Despite all efforts, there is always some escape of mercury from a factory.Mercury vapours escape, and leakages occur. Any mercury which escapes into the environment enters the food chains

and accumulates in living things. It causes serious health problems in animals and humans. The infamous case of themany deaths in Minamata, Japan, in the 1950’s was due to mercury pollution from a Sodium Hydroxide plant.

Mercury poisoning is actually called “Minamata disease” from this tragic episode.

Cl2

+ve

Graphite Anode

Chlorine gasout

SaltBrine

in

Waterin

NaOHsolution

out

Hydrogengasout

Mercury Pump

The Mercury Process

Flowing Mercury Cathode

-ve

Normally in an aqueous solution the sodium ions will not be reduced to sodium metal.

However, with mercury present (and suf ficient voltage) this does occur. Sodium metalcombines with mercury to form a liquid alloy called an “amalgam”. The sodium is carriedaway by the flowing mercury before it gets a chance to react with water in the cell.

Flow of mercury-sodium amalgam

Reactions Occurring

In the Electrolytic Cell Anode: 2Cl- Cl2(g)+ 2e

-

Cathode: Na+

+ e-

Na

Then, the mercury-sodiumamalgam is sprayed into areactor tank of water. Thesodium reacts: 2Na + 2H2O

2NaOH(aq) + H2(g)

The mercury is re-cycled backthrough the electrolytic cell.

NaOH is collected in water solution.Solid NaOH can be collected byevaporation.

The Cl2 and H2 gases are collected asvaluable by-products for sale tovarious industries.

ELECTROLYTIC CELL

SODIUM

REACTOR

-ve

+ve

ELECTROLYTIC CELL

NaCl

brine

Chlorine gasout

Hydrogengas out

NaOH solution accumulatesby dripping from cathode

NaOHsolution

out

SteelMesh

Cathode

Steam is sprayed onthe cathode mesh.

Some condenses andwashes the NaOH of f,so that it accumulatesin the bottom of the

reactor vessel.

AsbestosDiaphram

The Diaphram Process

How it Works At the Anode: 2Cl

-Cl2(g)+ 2e

-

Meanwhile, the Na+

ions are attracted tothe negative cathode and, with some water,can seep through the asbestos diaphram.

At the Cathode:2H2O+ 2e

-H2(g)+ 2OH

-

In an aqueous environment, and low voltage, the reduction of sodium ions doesnot occur.

The Na+

ions are spectators, and are washed off the cathode mesh along withthe OH

-ions which form. Thus NaOH

solution collects in the tank.

Problems:• Some Cl

-ions seep through the diaphram,

so the NaOH is contaminated with salt andmust be purified• Asbestos is carcinogenic and a serious

health hazard. Its use is now banned, so diaphram plants havebeen phased out.

G r a p h i t e A n o d e




17

A Final Summary

You need to understand that the 3 different processes for making NaOH all achieve the same overall reaction:

2NaCl(aq) + 2H2O(l) 2NaOH(aq) + Cl2(g) + H2(g)salt water sodium chlorine hydrogen

hydroxide gas gas

The details of the different processes are simply 3 different ways to separate the oxidation product (Cl2 gas) from thereduction products (H2 gas and OH

- ). If these are not kept apart, they will react to produce undesirable by-products.

You must also understand the impact that developing technologies, and growing awareness of environmental and health

issues, have had on the changes from one process to another. Study each process carefully to see:

• How the chemical reactions always add up to thesame overall result. (example given at right)

• How each process separates the products.

• The environmental and health issues associated with changing from one process to another.

• How modern technology has contributed tochanging from one industrial process to another.

In most countries, both the Mercury Process and the

(asbestos) Diaphram Process have been replaced by the“Membrane Process”. This relies on a modern, hight-tech.polymer membrane (Teflon) which allows positive ions

(Na+ ) to flow through, but will not allow negative ions

(Cl-, OH

- ) to pass.

Such membranes are said to be “semi-permeable”.

The Membrane Process

Water in

Semi-permeable Membrane

Na+ ions

dif f usethrough themembrane.

Cl- & OH-

cannot getthrough

NaOHsolution

out

NaClbrine in

Cl2

gasout

H2

gasout

depletedNaCl

brine out(con-

centrated&

re-cycled) I n e r t C a t h o d e

+ve -ve

I n e r t A n o d e

Anode Reaction:2Cl

-Cl2(g) + 2e

-

Cathode Reaction:2H2O+ 2e

-H2(g)+ 2OH

-

Sodium ions diffuse through the membrane toward the

negative cathode. The water flowing around the cathodecarries away Na+

and OH-ions, which can be collected (by

evaporation) as pure NaOH.

Example:

Finding the Overall Reaction for the Mercury Process

The Mercury Process involves 3 distinct equations:2Cl

-Cl2(g)+ 2e

-

2Na

++ 2e- 2Na (multiplied x2 so

electrons cancel)2Na + 2H2O 2NaOH(aq) + H2(g)

2Na+

+ 2Cl-+ 2H2O 2NaOH + Cl2 + H2

The overall equation is found by simply adding all specieson the left side, and all species on the right, but cancelling

any species which occur on both sides.


18/32

Worksheet 4Fill in the blanks. Check your answers at the back.

The industrial production of NaOH starts with the 2 raw materials a)................................... and .......................................

It uses an b)...................................................... cell to causec)................................................ ............ reactions to occur. The

3 final products are d)............................................. solution,plus the gases e)................................. and ...................................

which are valuable by-products. It is vital to keep theproducts apart so that they cannot f).......................................

........................................................................................................

Electolytic cells are (in energy terms) the opposite of

g)............................................ cells. An electrolytic cell ish)....................-thermic and will never run without the input

of i).............................. in the form of j).................................. The Standard Cell Potential is k).................................. and the

value indicates the minimum l)... .... .... .... .... .... .... .... .... ..required under standard conditions.

If pure, molten salt is electrolysed, the products arem)........................ gas at the n)..................................(electrode)

and o).............................. metal at the p)...................................However, in aqueous solutions of salt, the water may react

in preference to the salt. In a dilute solution this results inq).............................. and ............................ at the cathode, andr).................................. and ............................. at the anode.

1. (5 marks)

Compare and contrast “Galvanic” and “Electrolytic” cells.

2. (6 marks)Outline, giving relevant equations, the different resultsobtained by electrolysing molten sodium chloride,

compared to electrolysing a dilute solution of sodiumchloride.

3. (4 marks)

Write an equation (or half-equation) fora) the oxidation of chloride ionsb) the reduction of sodium ions

c) the reaction of sodium metal with waterd) the balanced, overall sum of these 3 reactions

In the “Mercury Process” a salt brine is electrolysed withliquid s)...................................... as the t)....................................

This allows sodium ions to be u).................................... tosodium metal, which immediately combines with themercury to form an “(v).......................................”. This is

carried away to be reacted with w).............................. to formhydrogen and x)................................................... solution.

Meanwhile, at the anode, y)................................... gas isproduced. Although quite efficient, this process has been

phased out, mainly because of z)............................................concerns about the impacts of escaped aa).............................

In the “Diaphram Process” anode and cathode areseparated by a diaphram made of ab).......................................

This keeps the products apart while allowing ac)..................................... ions to seep through to the

ad)....................................... where they combine with theae).................................... ions being formed by reduction of

water. Steam is used to wash the af).................................... .....product off the steel mesh. While this process is moreacceptable environmentally, the use of ag)...............................

is a serious health concern, and the process is less efficientbecause the NaOH is contaminated with ah)........................

The modern “ai).......... ........................ Process” uses a semi-

aj).................................... membrane to separate anode andcathode. This membrane allows ak)............................. ions toflow through, but does not allow al)................................ or

....................................... ions through. This is both efficientand (relatively) environmentally friendly.

4. (4 marks)

Historically, there have been 3 industrial processes used tomanufacture NaOH. The key difference between them was

the way in which the products were kept apart as they formed.

a) Why is it desirable to keep the products apart?b) Outline how each (named) process attempts to keepproducts apart as they form.

5. (4 marks)

a) Briefly discuss any environmental and/or technicalproblems which have contributed to the phasing out of the

“Mercury” and “Diaphram” Processes.b) give an outline of how a modern technology contributedto the “Membrane Process” replacing other processes.

18



COMPLETED WORKSHEETS

BECOME SECTION SUMMARIES

Practice Questions 4 These are not intended to be "HSC style" questions, but to challenge your basic knowledge and

understanding of the topic, and remind you of what you NEED to know at the K.I.S.S. principle level.

When you have confidently mastered this level,it is strongly recommended you work on questions from past exam papers.

Mark values shown are suggestions only, and indicate the depth of answer required.


19/32

Soap, and the Chemistry of Fats While we all may have difficulty appreciating the industrial production of ammonia, sulfuric acid or sodium hydroxide, soapis something we can understand. Soap is one of the most common and best-known household chemicals. Its production isone of the oldest “industrial” chemical processes, dating back several thousand years and practiced by ancient civilizations.

Soap is made from fat (oil) and a strong base such as NaOH or KOH, and the process is called “Saponification”.

Before you can understand soap, you need to learn about fats and oils... the chemistry of the “triglycerides”.

Chemistry of Triglycerides“Triglyceride” is the general name given to a compound made from 3 molecules of “fatty acid” joined to a “glycerol” molecule.

“Fatty Acids” are long-chain, hydrocarbon molecules with a carboxylic acid group (-COOH) on one end.

Example:

This is “stearic acid”,

CH3

(CH2

)16

COOH

There are many different fatty acids;

• some have more (or fewer) carbon atoms in the chain;• some have one or more double (-C=C-) bonds

(and less hydrogen atoms) within the hydrocarbon chain. To keep it simple (K.I.S.S. Principle!) all the fatty acids can

be represented schematically by the diagram at right

Glycerol is a “triple alcohol” molecule containing 3 OH

“-ol” groups. Its systematic name is “propanetriol”.

Structural Formula Schematic Diagram

Triglycerides are molecules in which 3 fatty acids arebonded to a glycerol molecule.

Schematically, it can be depicted as follows:

Triglycerides are Esters The chemical link between each fatty acid and the glycerolmolecule is formed by esterification. You are reminded thatesters form by a “condensation reaction” between analcohol and a carboxylic acid.

What’s the difference between fats and oils?“Fats” are simply triglycerides which are solid at roomtemperature (like beef fat), while oils are liquid at roomtemperature (like olive oil). The difference is simply themelting point. This in turn is due to the different fatty acidsin the triglyceride... animal fats tend to contain “saturated”fatty acids (all single C-C bonds like stearic acid above) while the vegetable oils often contain “unsaturated” fatty acids containing many double C=C bonds and lesshydrogen.

19



5. SOAP & DETERGENTS

H

H

C

H

H

H

H

C C C

H

H

C

H

H

C

H

H

H

H

C C C

H

H

C

H

H

H

C C

O

O

C

H

H

H

H

C

H

H

H

H

C C C

H

H

C

H

H

C

H

H

H

H

H

H

C

C

O

O

O

This long, non-polar, hydrocarbon section The -COOH group isis “hydrophobic” (=”water-hating”) polar and “hydrophilic”

(=”water-loving”) Note these properties...

they become important later.

This represents the hydrophobichydrocarbon chain

This represents thehydrophilic -COOHend of each molecule

3 fatty acids

G

y

o

H-OR

2

H

2

R

2

H

2

O-H

O

O

H

H

1

C

Alkanoic acid(R1 represents therest of the molecule)

Alkanol

(R2 represents therest of the molecule)

These atoms formwater, and the 2molecules join

O

O

R

1

C

Water

+ Ester




20

Saponification The making of soap involves reacting a triglyceride with a

strong base such as NaOH. It can be depictedschematically as follows:

+ 3 NaOH(aq)

Triglyceride (fat) glycerol+ +

3NaOH 3 salts of fatty acid(soap)

If the fatty acid was stearic acid, CH3(CH2)16COOH,

then the soap molecule would be sodium stearate,CH3(CH2)16COO

-Na

+.

Laboratory Reaction Compared toIndustrial Saponification

How does your little soap-making experiment compare with the industrial process? The overal chemical reaction is

the same, but there the similarities end.

Obviously, on an industrial scale, the quantities of soap

being made are measured in tonnes rather than grams. Inthe laboratory you probably used just one type of fat or oil,

but in industry a blended mixture of several fats and oilsare usually used. For example, “Palmolive” brand soap is

made from a blend of palm oil and olive oil.

The main difference, however, is that your experiment wasa “batch” process with one reaction step taking place atnormal pressure and a temperature between 50-100

oC.

In contrast, the most common industrial process uses a

continuous through-put process, with the reaction taking place in 2 distinct stages, one of which occurs at high

pressure and temperatures, in the presence of a catalyst.

A Common Industrial Saponification Process

Under high pressure and temperature, in the presence of

water (steam) and a ZnO catalyst, the triglyceride moleculesare “hydrolysed” (=split with water) to form fatty acids and

glycerol. This step is the exact reverse of esterification.

The fatty acids flow upwards, while glycerol flows down

with the water and is collected as a valuable by-product.

After drying, the fatty acids are reacted with preciseamounts of base to form soap... the sodium or potassium

salt of a fatty acid. This step is an acid-base neutralization.

N a

+ -

N a -

Na

+ -

The hydroxide ions(OH-) attach to

re-formthe glycerol

molecule

Each f atty acidmolecule becomes asodium salt.

This is soap.

Prac Work: Saponification

You may have carried out a simple experiment in the

laboratory to make some soap from vegetable oil andNaOH.(Strong base... use risk analysis & safety precautions!)

Simply mixing the ingredients in hot water, then stirring orshaking for a few minutes is enough to make the soap.

Adding some salt and allowing the mixture to cool isusually effective in causing a layer of soap to separate intoa solid layer on top. This is best washed to remove NaOH

residues before handling.

This is how soap has been made for thousands of years.

Your crude cake of soap can be tested by shaking a piecein warm water to see it “lather” and form a soapy foam.

ZnO

catalyst

Fat is“hydrolysed”(split with

water)

Hydrolysis

Tower

Vacuum Drier

Fats/oils

injected

Fatty acids

HighPressuresteaminjected

Water &Glycerol

Fatty acidsreact withbase toform soap

base added(NaOH

orKOH)

Soap

Glycerol separatedby distillation

Neutralization

Chamber




21

The Range of Fats & OilsUsed to Make Soap

Any fat or oil can be used to make a soap. Traditionally,animal fats from meat wastes and scraps were used. The fatcollected by boiling beef scraps is known as “tallow”, whilepork scraps gives “lard”. Tallow and lard have been themain sources of soap for centuries. Whale oil was usedextensively until about the mid-20th century.

In modern large-scale industry, vegetable oils arepredominantly in use because they can be more reliably supplied in large quantities at consistent quality and price.

The main oils used include palm, olive, coconut andsoybean oil.

Soap as an Emulsifier The reason that soap works for washing and cleaning isthat it is able to emulsify fats in water... it is an emulsifier.

You need to realize that water by itself will often not washaway “dirt” because usually there is some fatty material in

the “dirt”. This is not water soluble, and tends to cling toskin and fabrics and resist being simply washed away.

As you know, when water and oil are mixed, they rapidly separate with the oil forming a layer on top. This is becauseoils (triglycerides) are large, non-polar molecules which areimmiscible with water.

However, if water, oil and soap are mixed and agitated, theoil becomes broken up into tiny droplets which remainmixed into the water, and tend to separate more slowly ornot at all. Why?

Remember that soap is the sodium salt of a fatty acid.

If each soap molecule is represented bythen what happens when soap emulsifies a fat is shown inthis diagram.

Notice how the non-polar end of each soap moleculedissolves in the fat droplet, with the ionic, hydrophilic endsticking out into the water.

This is how emulsification works.

Na

+ -

This end is ionic,hydrophilic andwater-soluble.

This end is non-polar and hydrophobic.It will readily dissolve in oil.

Each droplet of f at becomessurrounded by soap molecules.These suspend each droplet inthe water and prevent droplets joining together

Water

Water

Water

Fat Fat

Fat

Prac Work: Soap as an Emulsifier

A simple experiment is outlined by the following photos:

If a little vegetable oil isshaken with water, thenallowed to stand, the oildroplets rapidly join

together and 2 layersseparate.

However, if a small

amount of soap isadded and the mixture

shaken again, the oil isemulsified.

Even after severalminutes the emulsion

remains quite stable,and the oil does not

separate.

Prac Work: Properties & Uses of an Emulsion

Acrylic Paint (e.g. house paint) is a good example to study and relate properties to uses.

Acrylic paints are a mixture of colour pigments andpolymer compounds emulsified in water.

Property How Property Relates to Usage

Water-based Convenient for washing brushesand rollers, etc.

Emulsion is very Remains uniformly liquid &stable and does not coloured for easy application

easily separate and attractive appearance.

As water evaporates The layer of dried paint ispolymers & pigments durable and washable and lastsform a solid film on for many years.

the wall.

Other common, everyday

emulsions include

• milk

• mayonnaise

• sorbolene cream and

other cosmetic lotions.

Oil

Water

Foam

Oil-wateremulsion

Common Emulsions




22

DetergentsDetergents are “artificial soaps”. They were first invented

during the 1940’s and have largely replaced soap in many cleaning roles because they are more powerful as

emulsifiers, can be made to have various properties suitablefor specialist tasks, and work well in a wider variety of

applications.

Chemistry & Molecular Structure There are many different detergents, but one of the mostcommon types has a structure as shown. Note the basic

similiarity (in general terms) to a soap molecule, but alsothe differences in detail

The molecule can be represented schematically as:

This molecule acts very efficiently to emulsify fats into water, so that greasy “dirt” is cleaned off or washed away.

Effect of “Hard Water”Some water supplies contain significant concentrations of

Ca2+

and/or Mg 2+

ions. This water tends to cause mineraldeposits to build up in pipes and kettles and so has come to

be called “hard water”.

Apart from clogging up water pipes, hard water has one

other major problem... soap will not work in hard water.

If you attempt to use soap in hard water, it will not lather,and a greasy “scum” forms. It does not cause emulsification

and will not clean anything.

The problem is that Ca2+

orMg

2+ions will combine with thefatty acid ions to form an

insoluble compound. Themolecules precipitate and cling

together forming the “scum”.

An important characteristic of detergents is that their ion (alkyl-

benzene sulfonate ion) remains in

solution, so detergents will work quite normally in hard water.

This is one of the reasons that detergents have mostly

taken over from soap as cleaning products.

You can test this effect yourself by trying to produce a soap

lather in seawater, or bore water. Then try with somedishwashing detergent or hair shampoo... big difference!

SO3-

Na

+

H

H

C

H

H

H

H

C C C

H

H

C

H

H

C

H

H

H

H

C

H H

S

O

O

O

-Na

+

H

H

H

C

H

H

H

H

C C C

H

H

C

C

C

C

C

C

The long hydrocarbon chain is non-polarand hydrophobic.It is derived from an alkane from petroleum.

A Typical Sodium Alkylbenzene Sulfonate Detergent Molecule

The ring structure comesfrom benzene, derivedfrom petroleum.

This end group isderived fromH

2

SO4

and is ionic& water soluble.

Definition of an Emulsion

What exactly is meant by an emulsion anyway?

Two liquids which will not dissolve in each other andalways separate when mixed together are said to be“immiscible”... like water and oil.

An emulsion is when 2 immiscible liquids are

combined so that they form a stable, uniform

mixture which does not separate.

It is NOT a solution. The molecules are not intimately associated at all. In an emulsion, one of the liquids is

broken up into tiny, microscopic droplets which are

spread uniformly throughout the other liquid.

To achieve an emulsion, another chemical (“emulsifier”)

is required to keep the droplets dispersed, and stopthem joining together.

For each pair of immiscible liquids there are 2 possibletypes of emulsion. For example, with cosmetic creams:

• a sorbolene (or “vanishing cream”) is an emulsion where the oil phase is dispersed throughout the water.

• a “cold cream” is an emulsion where the water phaseis dispersed throughout the oil.

Ca

2+ -

-

In hard water, soap molecules becomeinsoluble and will not emulsif y

Ca

2+

Ca

2+

Ca

2+

Ca

2+

Ca

2+

Ca

2+

S O 3-

S O 3

-

In “hard water”, detergentmolecules remain in

solution and still work asemulsifiers



Trinity Catholic Coll

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