ApplyingChemical

ideas

Module 8: Applying Chemical Ideas

Outcomes

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A student:

› develops and evaluates questions and hypotheses for scientific investigation CH11/12-1

› designs and evaluates investigations in order to obtain primary and secondary data and

information CH11/12-2

› conducts investigations to collect valid and reliable primary and secondary data and

information CH11/12-3

› selects and processes appropriate qualitative and quantitative data and information using a

range of appropriate media CH11/12-4

› communicates scientific understanding using suitable language and terminology for a

specific audience or purpose CH11/12-7

› describes and evaluates chemical systems used to design and analyse chemical processes

CH12-15

Content Focus

The identification and analysis of chemicals is of immense importance in scientific research,

medicine, environmental management, quality control, mining and many other fields.

Students investigate a range of methods used to identify and measure quantities of chemicals.

They investigate and process data involving the identification and quantification of ions

present in aqueous solutions. This is particularly important because of the impact of adverse

water quality on the environment. Students deduce or confirm the structure and identity of

organic compounds by interpreting data from qualitative tests of chemical reactivity and

determining structural information using proton and carbon-13 nuclear magnetic resonance

(NMR) spectroscopy.

Working Scientifically

In this module, students focus on developing and evaluating questions and hypotheses when:

designing, evaluating and conducting investigations; analysing trends, patterns and

relationships in data; and communicating scientific understanding about applying chemical

ideas. Students should be provided with opportunities to engage with all the Working

Scientifically skills throughout the course.

Analysis of Inorganic Substances

Inquiry question: How are the ions present in the environment identified and measured?

● analyse the need for monitoring the environment

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Background:

The state of our environment is an important issue for society. Pollution of air, land and water

in urban, rural and wilderness areas is a phenomenon that affects the health and survival of all

organisms, including humans. An understanding of the chemical processes involved in

interactions in the full range of global environments, including atmosphere and hydrosphere,

is indispensable to an understanding of how environments behave and change. It is also vital

in understanding how technologies, which in part are the result of chemical research, have

affected environments.

Some modern technologies can facilitate the gathering of information about the occurrence of

chemicals — both those occurring in natural environments and those that are released as a

result of human technological activity. Such technologies include systems that have been

developed to quantify and compare amounts of substances.

Many environmental problems have been generated by humans and their activities. One

problem is excess salinity, especially of the Murray Darling Basin. Primary production in this

basin makes a major contribution to the economies of both NSW and Victoria. The chemical

problems caused by excess salinity will need to have chemical solutions and will require the

cooperation of all parties including farmers, the public, state and federal governments and

industries that rely on water, either directly or indirectly. Management practices that ensure

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the sustainability of the environment and long-term viability for all parties using the

environment will need to be developed and implemented.

● conduct qualitative investigations – using flame tests, precipitation and complexation

reactions as appropriate – to test for the presence in aqueous solution of the following

ions:

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– cations: barium (Ba2+), calcium (Ca2+), magnesium (Mg2+), lead(II) (Pb2+), silver ion

(Ag+), copper(II) (Cu2+), iron(II) (Fe2+), iron(III) (Fe3+)

– anions: chloride (Cl–), bromide (Br–), iodide (I–), hydroxide (OH–), acetate

(CH3COO–), carbonate (CO32–), sulfate (SO4

2–), phosphate (PO43–)

Investigation 14.1

Investigation 14.2

Complexation Reactions

Some ions form complexes.

These ions are called complex ions.

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A complex ion forms when one or more small molecules or ions attach themselves to

a central atom.

The central atom, is often, but not always, a transition metal ion.

The surrounding molecule or ions, called ligands, must contain at least one lone pair

of electrons.

The resultant complex has different properties to the central cation, attached

molecules and ions.

The ligand molecule or ion acts as an electron pair donor.

For example:

When copper (ll) salts dissolve in water they form the complex, hexaaquacopper (ll),

([(H2O)6]2+.

Identifying Anions in Solution

Table 14.4

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Investigation 14.3 p424

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● conduct investigations and/or process data involving:

– gravimetric analysis

– precipitation titrations

Gravimetric Analysis

Revision from year 11

Gravimetric analysis is a quantitative analytical technique which determines the percent

composition of component of a mixture.

Worked example 14.3 and Try These yourself page 431

Investigation 14.5

Precipitation Titrations

Volhard’s Method

Volhard’s method uses back titration of an acidic solution to determine the quantity of

particular anions in solutions.

Investigation 14.4

● conduct investigations and/or process data to determine the concentration of coloured

species and/or metal ions in aqueous solution, including but not limited to, the use of:

– colourimetry

– ultraviolet-visible spectrophotometry

– atomic absorption spectroscopy

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Colourimetry

Colourimetry is the method for determining the concentration of a chemical in a solution

using its colour and concentration in a solution.

Colourimetry focuses on the use of light in the visible spectrum.

A colourimeter measures the amount of a specific wavelength of light absorbed by the

chemical being analysed.

To determine concentration a calibration curve is construct using sample of know

concentration and their absorbance.

Investigation 14.6

Check Your Understanding 14.6 Q5 & 6

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Ultraviolet-visible spectrophotometry (UV-vis spectrophotometry)

Ultraviolet-visible spectrophotometry is used to determine the concentration of inorganic and

organic molecules as well as metal ions.

Most organic molecules are colourless so they do not absorb in the visible region of the

spectrum.

Metal irons often produce coloured compounds, so they tend to absorb in the visible region of

the spectrum.

In the spectrophotometer, the light source provides light of wavelengths between 200 –

800nm.

A simplified diagram of a UV-vis spectrophotometer.

To calculate the absorbance A at a particular wavelength, the density of light at that

wavelength before it passes through the sample Io by the intensity at that wavelength after it

passes through the sample I then takes log to the base 10 of the ratio.

A = log10(Io/I)

The absorbance is related to concentration of the solution.

The Beer-Lambert Law states that the quantitative relationship between absorbance and

concentration is:

A = εlc

Where

A is absorbance

ε is the molar absorbtivity: which has the units Lmol-1cm-1

l is the path length of light through the sample – usually 1cm

c is the concentration of the solution

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Since ε and l are constants then A α c

Quantitative Analysis

UV-Visible spectrophotometry can be used to:

a. confirm the identity of the substance by comparing the spectrum of a sample to the

spectrum of a pure sample.

b. Calculate concentrations of a substance using a calibration curve.

Investigation 15.3 p477

Worked Example 15.4 and Try This Yourself p479

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Atomic Absorption

Atomic Absorption Spectrometry (AAS) is a common technique used to identify the

concentration of metal ions. In testing water quality the concentration of the following

cations are usually determined: sodium, magnesium, calcium and potassium.

AAS allows the detection of very small concentrations from samples of air, water or

food. This activity depends on your ability to manipulate data and dilution factors.

The absorbance values obtained using solutions of known concentration enable you to

draw a calibration graph. Use the specific absorbance data provided to read off the

corresponding concentration for the sample. The following information relates to the

monitoring of arsenic and its analysis will allow you to evaluate the use of AAS.

A case study in the monitoring of arsenic

Arsenic-rich ground water is a serious threat to 20 million people in Bangladesh. Solar

oxidation and removal of arsenic (SORAS) is a simple method that uses irradiation of water

with sunlight in PET plastic, or other UV transparent bottles, to reduce arsenic levels in

drinking water. 

Groundwater in Bangladesh contains Fe2+ ions and Fe3+ ions. Fe3+ forms an insoluble

hydroxide precipitate. Arsenic with an oxidation state of three, As(III), is only weakly

adsorbed but arsenic with an oxidation state of five, As(V), is strongly adsorbed to the

surface of iron(III) hydroxide particles as they precipitate out of solution. 

The SORAS method involves adding about 6 drops of lemon juice to a litre of water in a 1.5

L PET bottle. The bottle is shaken vigorously for 30 seconds, then placed horizontally in

sunlight for 4 to 5 hours. The UV energy, oxygen and water in the bottle produce oxidising

conditions:

At the end of the day, the bottle is stood vertically. The As5+ is adsorbed onto the surface of

the brown Fe(OH)3 as it precipitates overnight. The next morning, the liquid is decanted off

or filtered through fine cloth leaving the last 100 mL, containing iron(III) hydroxide and

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arsenic(V), to be discarded. The citric acid from the lemon juice enhances the photochemical

oxidation of the arsenic(III) and leads to much faster formation and settling out of precipitate.

Here are data that can be used to produce an AAS calibration graph for the arsenic levels in

this study.

Here are some AAS arsenic absorbance measurements for an investigation into the SORAS method:

 

Draw a calibration curve of absorbance vs total arsenic concentration. Use the curve

to gather data to determine the arsenic concentration before and after SORAS

treatment.

People generally require about two litres of water a day and the recommended daily

intake of arsenic by an adult is set at 150 micrograms (150 µg). Process the

information extracted from the data by assessing the importance of the data and

information gathered in relation to the acceptable levels of arsenic.

Present your findings. By referring to the precision of AAS and to the quantities of

arsenic in drinking water before and after treatment, evaluate the effectiveness of:

o the SORAS method in reducing arsenic levels in drinking water to acceptable

levels

o the use of AAS in monitoring and controlling pollution in this situation.

Each element has its own characteristic absorption spectrum that is related to its

electron energy levels.

Atomic absorption spectroscopy (AAS) detects minute concentrations of an element

in a sample of solution.

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The flame containing the vapourised sample absorbs light at the particular

wavelengths characteristic of the element in the flame and re-emits it in all directions.

A detector records the intensity of light emerging from the flame. The intensity of

light detected drops sharply at the wavelengths of light absorbed by the elements in

the flame, thus producing an absorption spectrum. The relative intensity and pattern of

changes of intensity within each of the bands in the absorption spectrum indicate the

concentration of the element in the test sample.

The study of the concentration of pollutants in our environment has been greatly enhanced

and is more accurate and reliable since the development of AAS by the CSIRO scientist, Alan

Walsh, in the 1950s. As Alan Walsh stated "the AAS method is a quick, easy, accurate and

highly sensitive means of determining the concentrations of over 65 elements". It is used in

a range of areas, such as medicine, agriculture, mineral exploration, metallurgy, food

analysis, biochemistry and environmental monitoring. It has been described as the most

significant advance in chemical analysis of the 20th Century.

Trace elements are elements needed in very small amounts by living things. AAS enabled the

measurement of the concentrations of many metals in the bodies of plants and animals and

in their surrounding environments. This has proved to be enlightening in many practical

situations. Two such situations include the following:

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o In coastal south-western Australia, animal health could not be maintained on

seemingly good pastureland. AAS showed cobalt deficiencies in the soil and the

pasture.

o Arid parts of Victoria could not support legume crops until molybdenum deficiencies

were detected by AAS and rectified. Alan Walsh and AAS Australian Academy of

Science.

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Analysis of Organic Substances

Inquiry question: How is information about the reactivity and structure of organic

compounds obtained?

Students:

● conduct qualitative investigations to test for the presence in organic molecules of the

following functional groups:

– carbon–carbon double bonds

– hydroxyl groups

– carboxylic acids (ACSCH130)

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● investigate the processes used to analyse the structure of simple organic compounds

addressed in the course, including but not limited to:

– proton and carbon-13 NMR

– mass spectrometry

– infrared spectroscopy (ACSCH130)

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Proton and carbon-13 NMR

Principles of NMR Spectroscopy:

The NMR spectrometer consists of a large magnet, a sample holder, a source of radio waves

and a detector.

The sample is placed into the magnetic field.

The sample is then irradiated with a range of different frequencies of radio waves.

A number of these frequencies will be absorbed as a particular “nuclear flip”, (Change in

orientation of nuclear spin upon excitation or relaxation).

The detector records the energy waves emitted when the nuclear spin returns to the lower

energy state.

This produced the NMR spectra.

NMR spectroscopy is used to determine the structure of an organic compound and is able to

distinguish between isomers.

The spectrum produced shows the calibration peak (at zero) and a series of lines along the

horizontal axis scale labelled ‘chemical shift’.

The following tables are used when interpreting NMR spectra.

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Examples:

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Check Your Understanding 15.4 p469 Q9

Mass Spectroscopy

Mass spectroscopy can assist in determining the structure of a substance, such as molar mass

or elements present. It can also detect isotopes of an element.

Mass spectroscopy can be used in radioactive dating and detecting drugs in sport.

Principles of Mass spectroscopy

The mass spectrometer consists of an ionisation chamber, a path along which the particles

travel and a detector.

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Mass spectrum sample

The most abundant peak on the spectrum is called the base peak and is given a relative abundance

of 100%.

Fragmentation Pattern

Organic molecules fragment into different parts and have many different possibilities of types of

fragments.

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Worked Example 15.1 p458 and Try These Yourself p459 and 15.3 p460 Q7

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Infrared Spectroscopy

Principles of Infrared Spectroscopy

Molecules are constantly moving (vibrating).

The two basic modes of vibration are stretching and bending.

The vibrations may be symmetrical or asymmetrical.

The molecule can absorb infrared energy and change to a higher energy vibration mode.

Types of vibration:

An infrared spectra records the transmittance of light against the energy of the frequency.

The x axis is the wave number (cm-1) which is the inverse of the wavelength.

Spectra fingerprint region: is the region from 1500 cm-1 – 500 cm-1 which is unioque to a

compound.

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Investigation 15.2 p472

Check Your Understanding15.5, 15.6 & 15.7 page 481 Q6, 7 & 9

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Chemical Synthesis and Design

Inquiry question: What are the implications for society of chemical synthesis and design?

Students:

● evaluate the factors that need to be considered when designing a chemical synthesis

process, including but not limited to:

– availability of reagents

– reaction conditions (ACSCH133)

– yield and purity (ACSCH134)

– industrial uses (eg pharmaceutical, cosmetics, cleaning products, fuels)

(ACSCH131)

– environmental, social and economic issues

Availability of Reagents

Chemical synthesis allows us to perform most of our daily substances.

Products such as medicines, food and food additives, plastics and biofuels as examples.

Chemical synthesis involves carrying out chemical reactions to purposely produce a specific

product.

The reactants are determined by the product required and the synthesis process.

Reactants must be commercially available, be cost effective and be conducive to safe

industrial processes.

Reaction Conditions

Reaction conditions are determined by the chemical reaction pathway.

Considerations include, temperature, required catalysts, pressure, acceptable yield

See notes on the Haber process as an example.

Reaction pathways may be single or multi-step reactions.

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Multi-step Process

The contact process is a multi-step process used to produce sulfuric acid.

The following process outlines the steps in the production of sulfuric acid.

Manufacture of Sulfuric Acid (H2SO4)

Most of the sulfuric acid manufactured is produced using the Contact Process.

Combustion Chamber

(combustion of sulfur)

-->

Converter(conversion of sulfur

dioxide)

-->

Absorption Tower(sulfur trioxide

absorbed into the sulfuric acid

mist

-->

Hydration of Oleum

to produce sulfuric acid

The Contact Process is a process involving the catalytic oxidation of sulfur dioxide, SO2, to sulfur trioxide, SO3.

I. Solid sulfur, S(s), is burned in air to form sulfur dioxide gas, SO2

S(s) + O2(g) → SO2(g)

The production of SO3 from SO2 takes place in a catalytic converter. It is an equilibrium

reaction and involves a compromise between reaction rate, equilibrium yield and

economic factors.

o At room temperature, the yield would be very high, but the reaction would

occur at an uneconomically slow rate. Increasing the temperature increases the

rate of reaction, however, the forward reaction is exothermic, so increasing the

temperature pushes the equilibrium to the left to absorb the heat, thus

decreasing the yield. A high temperature could also damage the catalyst,

making it less efficient.

450 - 600°C allows a fairly fast reaction rate plus good yield.

o A catalyst, vanadium pentoxide, is used to increase the reaction rate. This

reaction is called the Contact Process because sulfur dioxide and oxygen

molecules react in contact with the surface of the catalyst, which is arranged in

layers in towers.

o Increasing pressure pushes the equilibrium to the right (fewer particles), but

the equipment required is expensive, so a low pressure of only 1-2

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atmospheres is used. This pressure is sufficient to move gases through the

catalyst chamber.

o Excess oxygen is also used to push the equilibrium to the right and increase

yield. The stoichiometric mole ratio for the reaction shows the O2:SO2 ratio

needed is 1:2. In the industrial process, twice as much oxygen is used, the

O2:SO2 ratio used is 1:1.

These conditions produce a yield of about 99% sulfur trioxide.

The energy released from these exothermic reactions is used in the plant for melting

the sulfur or producing steam to generate electricity.

II. The gases are mixed with more air then cleaned by electrostatic precipitation to remove any particulate matter

III. The mixture of sulfur dioxide and air is heated to 450oC and subjected to a pressure of

101.3 - 202.6 kPa (1 -2 atmospheres) in the presence of a vanadium catalyst

(vanadium (V) oxide) to produce sulfur trioxide, SO3(g), with a yield of 98%.

2SO2(g) + O2(g) → 2SO3(g)

IV. Any unreacted gases from the above reaction are recylced back into the above reaction

V. Sulfur trioxide, SO3(g) is dissolved in 98% (18M) sulfuric acid, H2SO4, to produce

disulfuric acid or pyrosulfuric acid, also known as fuming sulfuric acid or oleum,

H2S2O7.

SO3(g) + H2SO4 → H2S2O7

This is done because when water is added directly to sulfur trioxide to produce sulfuric acid

SO3(g) + H2O(l) → H2SO4(l)

the reaction is slow and tends to form a mist in which the particles refuse to coalesce.

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VI. Water is added to the disulfuric acid, H2S2O7, to produce sulfuric acid, H2SO4

H2S2O7(l) + H2O(l) → 2H2SO4(l)

The oxidation of sulfur dioxide to sulfur trioxide in step III above is an exothermic reaction

(energy is released), so by Le Chatelier's Principle, higher temperatures will force the

equilibrium position to shift to the left hand side of the equation favouring the production of

sulfur dioxide.

Lower temperatures would favour the production of the product sulfur trioxide and result in a

higher yield.

However, the rate of reaching equilibrium at the lower temperatures is extremely low.

A higher temperature means equilibrium is established more rapidly but the yield of sulfur

trioxide is lower.

A temperature of 450oC is a compromise whereby a faster reaction rate results in a slightly

lower yield.

Similarly, at higher pressures, the equilibrium position shifts to the side of the equation in

which there are the least numbers of gaseous molecules.

2SO2(g) + O2(g) → 2SO3

On the left hand side of the reaction there are 3 moles of gaseous reactants, and the right hand

side there are 2 moles of gaseous products, so higher pressure favours the right hand side, by

Le Chatelier's Principle.

Higher pressure results in a higher yield of sulfur trioxide.

A vanadium catalyst (vanadium (V) oxide) is also used in this reaction in order to speed up

the rate of the reaction.

Linear and convergent pathways.

The production of ethyl butanoate is an example of a linear and convergent pathway as

outline in the following diagram.

Figure 16.9

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Yield and Purity

The yield of a reaction refers to the amount of product actually produced from the reactant.

Industrial processes always work to produce the maximum yield possible.

If the chemical process is reversible the reaction conditions must favour the forward reaction

and this requires applying Le Chatelier’s Principle.

More required

Industrial Uses

The industrial uses of ammonia.

- Ammonia is used to make solid and liquid fertilisers, explosives, nitric acid, sodium

carbonate, some pharmaceuticals and household cleaners. It is also used as a

refrigerant.

- To make solid fertilizer industrially, ammonia, which is a weak base, is reacted with

sulfuric acid to form ammonium sulfate fertiliser and with nitric acid to form

ammonium nitrate fertiliser.

Identify that ammonia can be synthesised from its component gases, nitrogen and hydrogen.

- Under pressure and heat, nitrogen and hydrogen react in the ratio of 1volume of nitrogen to 3 volumes of hydrogen to produce 2 volumes of ammonia.

Describe that synthesis of ammonia occurs as a reversible reaction that will reach equilibrium.

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- The synthesis of ammonia occurs as a reversible reaction. This means that ammonia is

formed from nitrogen and hydrogen (the forward reaction) and once some ammonia is

produced, some nitrogen and hydrogen are formed from the ammonia (the reverse

reaction). When nitrogen and hydrogen are initially added to a reaction vessel, the

reaction is slow. Equilibrium is reached when the rate of the forward reaction is the

same as the rate of the reverse reaction.

 

- To ensure that sufficient ammonia is produced, conditions need to be established that

shift the equilibrium position to the right.

Explain that the use of a catalyst will lower the reaction temperature required and identify the catalyst(s) used in the Haber process.

- With the use of catalyst, the activation energy for the reaction is lowered. A finely

ground iron catalyst, with large surface area, is used in the Haber process. The

gaseous nitrogen and hydrogen molecules are adsorbed on to the solid catalyst surface

and rearrange forming the ammonia molecules. By lowering the activation energy, a

catalyst enables a more rapid reaction at lower temperatures.

Identify the reaction of hydrogen with nitrogen as exothermic.

- The forward reaction, to produce ammonia, releases 46 kJ of energy for each mole of ammonia formed.

Explain why the rate of reaction is increased by higher temperatures.

- As the temperature rises, the particles move more quickly and have higher kinetic

energy. This increases the frequency of collisions between particles that can react and

also increases the amount of energy available for the reaction. Most of the increased

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rate of reaction comes from more of the colliding particles exceeding the activation

energy needed for the reaction to occur. The increased frequency of collisions is less

important in increasing the rate of reaction. The rate of both the forward and reverse

reactions is increased.

Explain why the yield of product in the Haber process is reduced at higher temperatures using Le Chatelier's principle.

The forward reaction in which ammonia is formed is exothermic. Le Chatelier's

principle states that if a system in equilibrium is disturbed, the system will adjust

itself to minimise the disturbance. In this case, Le Chatelier's principle indicates that

with high temperature providing more heat, the reverse reaction is favoured and the

decomposition of ammonia occurs.

Another way to view itIf the reaction is written like that following, heat is like a product.

As temperature, and therefore the heat available increases, the equilibrium position shifts to the left and the yield of ammonia is reduced.

Analyse the impact of increased pressure on the system involved in the Haber process. 

- In accordance with Le Chatelier's principle, increasing the pressure favours the

production of ammonia because two molecules of gaseous ammonia occupy a

smaller volume than the four molecules of gaseous reactants.

- High pressure also increases the reaction rate because the gas molecules are closer

and at higher concentrations. However, high-pressure equipment is expensive and

requires considerable energy to operate.

- To achieve an economic yield of about 30%, a pressure of 35 000 kPa (35 MPa or 345

atm) is used.

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Explain why the Haber process is based on a delicate balancing act involving reaction energy, reaction rate and equilibrium. 

- As the temperature is increased more energy is available to exceed the reaction

activation energy and thus the reaction rate between nitrogen and hydrogen to

form ammonia increases. However increasing temperature favours decomposition

of the ammonia product. A compromise temperature providing a satisfactory

reaction rate and satisfactory yield of ammonia is selected.

 

- To achieve an economic yield of about 30%, the temperature is raised to 525°C

and pressure of 35 000 kPa (35 MPa or 345 atm) is used.

Increasing yieldLiquefying and removing the ammonia as it is produced also increase the yield of ammonia.

Explain why monitoring of the reaction vessel used in the Haber process is crucial and discuss the monitoring required. 

- The raw materials must be monitored to ensure they are clean. Any carbon dioxide

detected must be removed. It is often separated and diverted to use for the

production of urea at a nearby fertiliser manufacturing plant. Any oxygen present

could cause an explosion with the hydrogen. 

- A chemical engineer or technician monitoring the reaction vessel needs to ensure

that the appropriate temperature and pressure conditions are maintained, within an

acceptable range, so that about 30% yield is achieved. 

- The quality of the catalyst surface needs to be monitored to ensure good

adsorption of the nitrogen and hydrogen gases. The system must be kept free of

contaminants to ensure maximum surface of the catalyst is available for

adsorption of nitrogen and hydrogen. 

- Temperature needs to be monitored, as too high a temperature can permanently

damage the catalyst. Ammonia synthesis SchoolScience, UK

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