1. Enzymes & Catalysts
C2.2 Rates of Reaction
20 November 2018

2. Starter
Draw enthalpy profiles for exothermic and endothermic reactions. In your diagrams label:
Activation energy
Energy change
Reactants
Products
Bond breaking energy
Bond making energy

3. Catalysts
A catalyst provides an alternative, lower energy pathway, for a reaction to take place.
The catalyst is not used up in the reaction
The catalyst may react with a reactant to form an intermediate or may provide a surface on which the reaction can take place.
At the end of the reaction the catalyst is regenerated.

4. Types of catalyst
A Homogenous catalyst has the same phase as the reactants.
The catalyst reacts with the reactants to form an intermediate.
The intermediate then breaks down to give the product and regenerates the catalyst.
e.g. Making esters with sulphuric acid as a catalyst:
C2H5OH (l) + CH3COOH (l)  CH3COOC2H5 (l) + H2O (l)
Catalyst: H2SO4 (l)
Different phases are things that are separated by a distinct boundary e.g. oil and water, a solid and a liquid etc.

5. Types of catalyst
A Heterogenous catalyst has a different phase (they are usually a solid) to the reactants.
Reactant molecules are absorbed onto the surface of the catalyst.
The reaction then takes place and the product is desorbed from the surface.
e.g. Hygrogenation of ethene using a nickel catalyst

6. Knowledge Check
State the difference between a homogenous catalyst and a heterogenous catalyst.
Describe the effect of a catalyst on the activation energy of a chemical reaction and on the enthalpy change of reaction.
Methanol can be manufactured by the reaction of carbon dioixide with hydrogen as shown in the equation: 3H2 (g) + CO2 (g) CH3OH (g) + H2O (g) ΔH = -49kJmol-1 The activation energy of the forward reaction is +225kJmol-1 a) Draw an enthalpy profile diagram for this reaction. b) Calculate the activation energy of the reverse reaction.

7. Enzymes These are known as ‘biological catalysts’.
They are complex globular proteins which act as homogeneous catalysts in living systems.
They typically work best at body temperature and catalyse specific reactions.
Their action generally increases with temperature until denaturing.
As well as temperature they are also sensitive to pH.
They are very effective and are used both in the body and in industry.

8. Uses and benefits of enzymes
Rennin in the dairy industry (making cheese)
Yeast and amylase in alcohol brewing
Lipase and protease in washing powders/detergents
Benefits:
Usually work at lower temperatures and pressures (energy saving)
Do not harm fabrics and food
Biodegradable
Often help in reactions that form ‘pure’ products – removes need for separation techniques

9. Economic importance of catalysts
Reduce temperature needed for reactions
Reduce energy needed
Reduce electricity needed
Reduce amount of fossil fuels used
Make product faster
Increase profitability
Obtain high atom economies
90% of chemical materials are produced using catalysts!!!

10. Knowledge check
Try the worksheet questions.