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Rates of reaction

The rate of a reaction tells us how rapidly the products are made from the reactants.

Rate of reaction = amount of reactant used or amount of product madetime time

Fast reactions:

Slow reactions:

combustion, explosions

baking, corrosion/rusting, fermentation

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Measuring the rate of a reaction:

e.g.

1) Measure the volume

of gas produced in a reaction,at regular time intervals

2) Measure the decrease in mass of thereactants, at regular time intervals, as a

gas is produced and lost into the

atmosphere

3) Measure the time taken for a colour

change to occur, or the solution becoming

opaque.

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Rate Graphs:

If we measure the amount of product formed vs. time the results will look like

this:

time (s)

volumeofgas(cm3)

The gradient shows the rate of reaction:

It is initially steep because the reactions is fastest at the start.

The rate slows down as reactants are used up the gradient gets less steep.

Eventually the curve becomes flat as the reaction finishes, when one of the

reactants has been completely used up.

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Rate Graphs:

The initial gradient is still steepest, the reaction is fastest at the start

The curve becomes flat when the reaction stops because a reactant has

been completely used up.

time (s)

massofreactants(g)

If we measure the amount of reactants used up vs. time, the results will look

like this:

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Comparing rates:

time (s)

volumeofgas(cm3)

Practice questions: (answers at the end of the topic)

Which is faster, blue or green reaction ? Which reaction finishes first, blue or green ?

Which two curves could be the same reaction happening at two different

temperatures ? Which is hotter ?

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The reaction: methane + oxygen carbon dioxide + water

Our model:

= methane

The particles in a liquid or gas are moving around they have kinetic energy.

In order for a chemical reaction to occur the reactant particles must collide with each other.

There is an amount of energy called the Activation Energy for each reaction it is actually

the amount of energy needed to break the necessary bonds in the reactants.

If the particles are moving quickly and collide with more than the activation energy, then

they will react a successful collision a reaction occurs.

If the particles collide with less than the activation energy, then they will simply bounce off

one another and not react an unsuccessful collision.

= oxygen

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Reactions with a solid: e.g. zinc + hydrochloric acid

Imagine the particles in this reaction. A reaction can only take

place when the acid particles (blue) collide with the surface of

the solid zinc particles. A collision cannot take place with the

zinc particles inside a solid piece.Factors which affect the rate of reaction:

- temperature of reactants

- pressure (only applies to gases)

- concentration of reactants (applies to solutions)

- surface area (applies to solid reactants)

- use of a catalyst

ConcentrationThe concentration of a solution means the number of particles in a given volume of the

solvent (usually water). We measure concentration in moles per dm3. (1dm3 = 1 litre)

If we increase the concentration of a reactant, we have more particles per cm3

. This means that collisions will take place more frequently.

Therefore there will be more frequent successful collisions, and the rate of reaction will

increase.

Prediction: If we double the concentration of the acid, we double the number of acid

particles per cm3,we double frequency of successful collisions, therefore we double the

rate of the reaction. We can test this prediction experimentally.

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Pressure

Increasing the pressure of a gas makes theparticles closer together.

So we have more particles per cm3.

This means that collisions will take place more frequently.

Therefore there will be more frequent successful collisions, and the rate of reaction will

increase.

The same volume of different gases contain the same number of particles of

gas (when the gases are at the same pressure and temperature).

Temperature

If we increase the temperature of a gas or solution:

The particles will move more quickly (i.e. having more kinetic energy).

This means that the particles will collide more frequently.

Also the particles will collide with more energy.

This means that more of the collisions will be successful, because more often the

colliding particles have more energy than the activation energy for the reaction.\

Both of these factors will increase the rate of reaction.

Prediction: Because the particles are colliding more frequently AND they are colliding with

more energy, doubling the temperature should more than double the rate of reaction.

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Surface Area

Collision can only take place at the surface of a solid, so the larger the surface area, the

faster the rate of reaction.

For the same mass of solid reactant:

large lumps slow reaction; low surface area

small lumps faster reaction; more surface area

powder fast reaction; high surface area

Catalysts

A catalyst affects the rate of a reaction but is not used up during the reaction.

It therefore remains behind when the reaction is complete, ready to be used to catalyse

further reactions.

Manganese(IV) oxide (MnO2) is an example of a catalyst. It speeds up the breakdown

(decomposition) of hydrogen peroxide.MnO2

2 H2O2(aq) 2 H2O(l) + O2(g)Notice how the catalysts formula doesnt show up in the equation, because its not a

reactant or a product. We write it over the arrow, showing that it is one of the conditions

for the reaction.

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How does a catalyst work ?

Reactions require successful collisions between the reactant particles. The reactant particles

must collide with enough energy to break the bonds in the reactants. This is called the

Activation Energy for the reaction.

When a catalyst is added to a reaction it provides an alternative reaction pathway - a place

for the reaction to happen where the Activation Energy is lower, so more of the collisions

between the reactants are successful so the rate is faster.

Catalysts need a high surface area so

that there are lots of places for thereactants to bind onto the catalyst

surface ready to react. A powdered

catalyst, or a rough gauze provides a

high surface area.

Close-up of a Pt-Rh

catalyst used in industry

Is being a catalyst different from being a reactant ?

A reactant is used up during the reaction as it is turned into the products.

A catalyst is not used up.

We could remove it after the reaction (e.g. by filtering it then washing and drying it) and

weigh it to show it is all still there. We could use it again and again.

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Examples of catalysts:

The catalytic converter in the exhaust system of cars contains a catalyst made from

platinum, rhodium and palladium. This catalyst removes toxic carbon

monoxide and nitrogen oxides (which cause

acid rain) from the exhaust gases from

the car.

Catalytic converter

The Haber process is used to make ammonia. An iron

catalyst is used to speed up the reaction so a lowertemperature can be used in the reactor, (lower energy

costs). It also means less fuel is burnt, conserving finite

fossil fuel resources and minimising greenhouse gas

emissions.

Ammonia reactor

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Reversible reactions

Some reactions do not go to completionwe dont get 100% yield because not all of the

reactants react to form products.

One of the reasons for this is that some reactions are reversible the products can react toform the reactants !

e.g. 3 H2 + N2 2 NH3but also 2 NH3 N2 + 3 H2

We use a special symbol to denote a reaction which is reversible:

3 H2 + N2 2 NH3A single arrow in an equation means that the reactants form the products but the products

cant react to form the original reactants again.

e.g. CaCO3 CaO + CO2 (thermal decomposition of calcium carbonate)

The double-headed arrow means the reaction can go in either direction

e.g. NH4Cl NH3 + HCl(ammonium chloride reacts to form ammonia and hydrogen chloride

and also

ammonia and hydrogen chloride react to form ammonium chloride)

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Heat

NH4Cl NH3 + HCl

NH3 + HCl NH4Cl

thermal decomposition

neutralisation

The forward reaction is a thermal decompostition ammonium chloride breaks up when

heated into two simpler substances: ammonia and hydrogen chloride.

The reverse reaction is a neutralisation the hydrogen chloride is acting as an acid (H+

donor) and the ammonia as a base (H+ acceptor).

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gjuPMMfZQ&ust=1371919630844243http://www.google.co.uk/url?sa=i&rct=j&q=sublimation+ammonium+chloride&source=images&cd=&docid=4acpIQrPUFaTHM&tbnid=t9seUsZbXoIwUM:&ved=0CAUQjRw&url=http://www.labyrinthdesigners.org/alchemy-laboratory/aqua-regia-fulminating-gold/&ei=b4TEUeaxNI2M0wWAlYDwDQ&psig=AFQjCNGZg2n5DP6hHmGA0GzrLgjuPMMfZQ&ust=1371919630844243http://www.google.co.uk/url?sa=i&rct=j&q=sublimation+ammonium+chloride&source=images&cd=&docid=4acpIQrPUFaTHM&tbnid=t9seUsZbXoIwUM:&ved=0CAUQjRw&url=http://www.labyrinthdesigners.org/alchemy-laboratory/aqua-regia-fulminating-gold/&ei=b4TEUeaxNI2M0wWAlYDwDQ&psig=AFQjCNGZg2n5DP6hHmGA0GzrLgjuPMMfZQ&ust=1371919630844243http://www.google.co.uk/url?sa=i&rct=j&q=ammonium+chloride&source=images&cd=&cad=rja&docid=8X0_FeKG1jj71M&tbnid=n6pYG2Z3J2yHqM:&ved=0CAUQjRw&url=http://www.sciencephoto.com/media/4312/view&ei=I4TEUa25HbKa0QW3xoGYBw&psig=AFQjCNEeDyADM3oqMgS6fKTZv404zQkU-w&ust=1371919735021692

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A reversible reaction the test for water:

White, anhydrous copper sulphate forms blue hydrated copper

sulphate if water is added. The reaction is reversible. If the crystals

are heated they turn white as anhydrous copper sulphate is formed.

CuSO4.5H2O(s) CuSO4(s) + 5H2O(l)hydrated anhydrous + water

copper sulphate copper sulphate

An alternative test for water:

Blue, anhydrous cobalt chloride is also used as a test for

water it turns to pink hydrated cobalt chloride if water is

added. If the crystals are heated they turn blue again.

CoCl2.6H2O(s) CoCl2(s) + 6H2O(l)hydrated anhydrous + water

cobalt chloride cobalt chloride

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Equilibrium

When reactants are turning into products, and products are

turning into reactants, a dynamic equilibrium can be set up.

(Think of this as being like a balance-pointthe forward

and reverse reactions becoming balanced. Running up an

escalator which is going down at a rate causing you to stay

in the same place would be one example of a dynamic

equilibrium. Adjusting the flow into and out of the pot so

that the volume of liquid remains the same even though the

actual liquid is constantly changing, would be another

example)

At equilibrium:

There will be both products and reactants present in

a mixture. The rate of products turning into reactants will be

the same as the rate of reactants turning into

products.

To get an equilibrium, none of the reactants or

products must escape you need a closed system.

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How it works:

The reactants begin to react forming

products. The initial rate of forward

reaction will be fast.

The forward reaction slows down as

reactants are used up.

Products have now been formed, so

they begin to react to form

reactants. The initial rate of the

reverse reaction is very slow as onlya few product particles are present.

As more products are formed, the

rate of the reverse reaction

increases.

Eventually the rates of forward

reaction and backward reactions

become the same. THE REACTIONS

HAVENT FINISHED BUT AN

EQUILIBRIUM IS SET UP.

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How much is there at equilibrium ?

The amounts of the products and the reactants at equilibrium depends on the conditions

(e.g. temperature, pressure).

Changing the conditions will change the amounts of reactants and products present in

the mixture - a new equilibrium gets set up.

We call this changing the POSITION of the equilibrium.

- if changing the conditions causes the position of equilibrium to move in the forwarddirection (to the right) we get more products, less reactants in the equilibrium mixture

- if changing the conditions causes the position of equilibrium to move in the backwards

direction (to the left) we get less products, more reactants in the equilibrium mixture

N2 + 3H2 2NH3

N2 + 3H2 2NH3

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Making more product at equilibrium

Whenever we change the conditions, the position of the equilibrium will shift to try and

oppose the change.

N.B. The effect of pressure only

applies to GASES in the balanced

equation !

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Example:

CH3OH (g) + H2O(g) CO2(g) + 3H2(g)This reaction is called steam reforming of methanol. It is used to produce hydrogen, avaluable fuel, from an alcohol.

What would be the best conditions of temperature and pressure to get the best yield of

hydrogen?

Temperature: A high temperature would move the position of equilibrium in theendothermic direction, which is the forward direction, so more products

would be made.

Pressure: There are four moles of gases on the right side of the equation and only

two moles of gases on the left side. A low pressure would move the

position of equilibrium in the forward direction, making more product.

Catalyst: A catalyst would speed up the reaction (in both directions) so products

would be made sooner. It would have no effect on the position of

equilibrium, and hence the yield, only how quickly the product is made.

endothermic in the forward

direction

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True or False ? (answers at the end of the topic)

All chemical reactions are reversible.

The mass can change if a solid product is formed in a closed

system.

Equilibrium can only happen in closed systems.

In a reversible reaction, reactants can form products and

products can form reactants.

A

symbol shows that an equilibrium has been set up.

Once at equilibrium no more products are formed.

Once at equilibrium, the forward and backward reactions stop.

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Which is faster, blue or green reaction ? BLUE steeper gradient.

Which reaction finishes first, blue or green ? BLUE flat after shortest time.

Which two curves could be the same reaction happening at two different

temperatures ? Which is hotter ? BLUE (HOTTER) , GREEN because bothproduce the same volume of gas, just at different rates.

time (s)

volumeofgas(cm3)

Answers:

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True or False ?

All chemical reactions are reversible.

FALSEburning, and rusting, for example, cant be reversed

The mass can change if a solid product is formed in a closed system

FALSE all the same atoms are still there, mass is conserved

Equilibrium can only happen in closed systems.

TRUE a closed system is a requirement for an equilibrium

In a reversible reaction, reactants can form products and products can form

reactants. TRUE this is exactly what does happen

A symbol shows that an equilibrium has been set up.

FALSE it only shows that the reaction is reversible

Once at equilibrium no more products are formed.

FALSE the reactions happen continuously, so product is still beingmade, but it is being used up by the reverse reaction at the same rate.

Once at equilibrium, the forward and backward reactions stop.

FALSE these reactions continue, but at the same rate