(a) Reaction rates

(i) Following the course of a reaction

Reactions can be followed by measuring changes in concentration, mass and volume of reactants or products.

Measuring a change in mass Measuring a change in volume

g

Mas

s o

f b

eake

r an

d c

on

ten

ts

Time Time

Vo

lum

e o

f ga

s p

rod

uce

d

The rate is highest at the start of the reaction because the concentration of reactants is highest at this point.

The steepness (gradient) of the plotted line indicates the rate of the reaction.

reactant

product

Co

nce

ntr

atio

n

Time

We can also measure changes in concentration using a pH meter for reactions involving acids or alkalis, or by taking small samples and analysing them by titration or spectrophotometry.

Average rate = change in measured factor

change in time

The average rate of a reaction, or stage in a reaction, can be calculated from initial and final quantities and the time interval.

Calculating the average rate

Units of rate

The unit of rate is simply the unit in which the quantity of substance is measured divided by the unit of time used.

Using the accepted notation, ‘divided by’ is represented by unit-1.

For example, a change in volume measured in cm3 over a time measured in minuteswould give a rate with the units cm3min-1.

= 0.001moll-1s-1

Rate =0.05-0.00

50-0

0.05

50=

Rate =0.075-0.05

100-50

0.025

50=

= 0.0005moll-1s-1

Co

nce

ntr

atio

n m

oll-1

The rate of a reaction, or stage in a reaction, is proportional to the reciprocal of the time taken.

Rate 1

timeRate is inversely proportional to time.

i.e.Rate =

1

time

andTime =

1

rate

Units: s-1, min-1 etc. Units: s, min etc.

See Unit 1 PPA 1 and Unit 1 PPA 2 for examples of ‘clock reactions’ that use the relationship described above.

(ii) Factors affecting rate

The rates of reactions are affected by changes in concentration, particle size andtemperature.

The rate of reaction can be increased by- decreasing the particle size of a solid reactant- increasing the concentration of a reactant in solution- increasing the temperature at which the reaction occurs.

The factors which affect reaction rate can be understood more fully by examiningthe conditions needed for a reaction to take place.

For a chemical reaction to occur, reactant particles must collide –this is the basis for the Collision theory.

The collision theory can be used to explain the effects of concentration and surface areaon reaction rates.

Collision Theory

For a chemical reaction to occur reactants must come into contact (collide).

Not all collisions are successful.

There are two conditions that are required for a successful collision;

(i) The correct collision geometry i.e. reactants must collide with a particular orientation.

(ii) Enough energy to break existing bonds within the reactant molecules.

The first condition above can be used to help explain the affect of changing theconcentration or the particle size.

Any factor which increases the number of collisions per second between particles ofreactants is likely to increase the rate of reaction.

Concentration

Rate

For many reactions a graph of rate against concentration is a straight line showingthat the rate is directly proportional to concentration.

The Effect of Temperature on the Rate of Reaction

Rate

Temperature

We discovered that the rate of reaction is not directly proportional to the temperature, instead a 10°C rise in temperature roughly doubles the rate.

The increase in the number ofcollisions at a higher temperatureis not enough on its own toaccount for this increasein the rate.

Activation Energy and Energy Distribution

Unsuccessful collision:

reactantscollide not enough

energy tobreak bonds

reactantsmove apart

Successful collision:

reactantscollide

enoughenergy to

break bonds

productsmove apart

≥ Activation energy

The activation energy is the minimum kinetic energy required by colliding particlesbefore reaction will occur.

Temperature is a measure of the average kinetic energy of the particles of a substance.

Energy distribution diagrams can be used to explain the effect of changing temperature on the kinetic energy of particles.

T1 T2

The shaded areas represent the total number of moleculeswhich have sufficient energy to react.

The effect of temperature on reaction rate can be explained in terms of an increase in thenumber of particles with energy greater than the activation energy.

T1 T2

T2>T1

A small rise in temperature causes a significant increase in the number of moleculeswhich have energy greater than the activation energy.

Enthalpy

Potential energy diagrams

Exothermic changes cause heat to be released to the surroundings.

During an exothermic reaction potential energy possessed by the reactants is released to the surroundings. The products have less potential energy than the reactants.

Endothermic changes cause absorption of heat from the surroundings.

In an endothermic reaction the reactants absorb energy from the surroundings so that the products possess more energy than the reactants.

The enthalpy change is the energy difference between the reactants and the products.

The enthalpy change can be calculated from a potential energy diagram (if drawn to scale).

The enthalpy change has a negative value for exothermic reactions.

The enthalpy change has a positive value for endothermic reactions.

Enthalpy changes are usually quoted in kJmol-1.

A potential energy diagram can be used to show the energy pathway for a reaction.

products

The activated complex is an unstable arrangement of atoms formed at the maximum of the potential energy barrier, during a reaction.

If the total energy change for the bond breaking step is less than for the bond making step, the overall reaction will be exothermic.

If the reverse is true then the reaction will be endothermic.

The activation energy is the energy required by colliding molecules to form an activated complex.

The activation energy can be calculated from potential energy diagrams (if drawn to scale).

In potential energy diagrams the activation energy appears as an ‘energy barrier’.

The rate of a reaction will depend on the height of this barrier.

The higher the barrier the slower the reaction.

Note: the rate of reaction does not depend on the enthalpy change.

(iv) Catalysts

Catalysts can be classified as either heterogeneous or homogeneous.

A catalyst is a substance which alters the rate of a reaction without being used up in the reaction.

Heterogeneous – catalyst is in a different physical state from the reactants.

Homogeneous – catalyst is in the same physical state as the reactants.

Catalysts are used in many industrial processes.

Catalyst Process Reaction Manufacture Revisited in

Vanadium(V) oxide Contact 2SO2 + O2 2SO3 sulphuric acid

Iron Haber N2 + 3H2 2NH3 ammonia Unit 3

Platinum Ostwald 4NH3 + 5O2 4NO + 6H2O nitric acid

Nickel Hydrogenation Unsaturated oils + H2

saturated fatsmargarine Unit 2

Aluminium silicate Catalytic cracking

Breaking down long-chain hydrocarbon molecules

fuels and monomers for plastics industry

Unit 2

Heterogeneous catalysts work by the adsorption of reactant molecules.

The surface activity of a catalyst can be reduced by poisoning

Impurities in the reactants result in the industrial catalysts having to be regenerated orrenewed.

if certain substances are preferentially adsorbed or even permanently attached to thesurface of the catalyst.

Catalytic convertors are fitted to cars to catalyse the conversion of poisonous carbon monoxide and oxides of nitrogen to carbon dioxide and nitrogen.

Cars with catalytic converters only use ‘lead-free’ petrol to prevent poisoning of the catalyst.

The convertors contain honeycomb ceramic material covered with a metal catalyst such as platinum or rhodium.

The honeycomb structure is used to increase the surface area of the catalytic convertor.

Enzymes

Enzymes catalyse the chemical reactions which take place in the living cells of plants and animals.

For example, amylase catalyses the hydrolysis of starch and catalase catalyses the decomposition of hydrogen peroxide in the blood.

The molecular shape of an enzyme usually plays a vital role in its function.

substrate

enzyme

Products

Enzymes are usually highly specific.

Enzymes are used in many industrial processes.

Enzymes operate most effectively at a certain optimum temperature and within a narrow pH range.

Above this temperature or outwith this pH range the enzyme changes shape (the lock and key no longer fit together) and the enzyme is denatured.