Catalysts Explained For A Level Chemists

If all you can remember from your GCSE Chemistry course is that “catalysts increase the rate of reactions” then you’d better get ready to level-up your knowledge!

Catalysts are incredibly important in organic and physical chemistry, and so of course AS and A Level students are expected to learn about them in quite extensive detail. 

But if you lost your class notes, have gaps in your understanding or just want to get ahead of the game, you’re in the right place. 

This blog post will provide an introduction to catalysts (a subtopic in the reaction kinetics topic) for the CIE exam specification. Written by our in-house Chemistry tutor, it is a great way to prepare to tackle our NEW collection of multiple choice Topic Questions

Ready to increase the rate of your progress in class?  Let’s get started…

Speeding up reactions

Chemists may want to increase the rate of a chemical reaction for a number of reasons.  

The ways to achieve this are: 

  1. Raise the temperature of the reactants (raising the temperature will give the molecules more energy, bringing them closer to their activation energy
  2. Increase the pressure or concentration of the reactants (this will increase the number of successful collisions between reactant molecules)
  3. Catalysis (this means adding a catalyst, and we’ll be explaining this one in more detail below)

Why does a catalyst help to increase the reaction rate?

A catalyst offers an alternative reaction pathway with a lower activation energy. This means that a higher proportion of the molecules will have energy exceeding the reaction activation energy, so there will be more successful collisions (causing the reaction rate to increase). 

This can be represented on a Maxwell-Boltzmann distribution like this: 

  • The green area under the curve represents the particles which had sufficient energy to react prior to the addition of the catalyst
  • The orange and green areas represent the particles which had the sufficient energy to react after the addition of the catalyst
  • The catalyst provides an alternative reaction pathway which demands a lower activation energy

Be careful: a catalyst does not “lower the activation energy of the reaction”, so don’t write this on your exam paper! It provides an alternative reaction pathway with lower activation energy. 

Remember that catalysts are unchanged themselves by the reactions – therefore they must accept an equal number of electrons to those they donate (and vice versa).  

Heterogenous and homogenous catalysts

There are two main types of catalyst. Heterogeneous catalysts are in a different phase to the reactants, and homogeneous catalysts are in the same phase as the reactants. 

Remember – if two components of a mixture have a visible ‘boundary’ between them, they are in different ‘phases’. This could mean that one is a solid and one is a liquid, but it could also mean that they are both liquids which don’t dissolve in one another. 

Homogeneous catalysts will either be oxidised or reduced by one reactant (i.e they will either lose or gain electrons), then the opposite by the other reactant. Both of these actions have a lower activation energy than the uncatalysed reaction – meaning that the rate of the reaction is accelerated WITH a catalyst. 

Heterogeneous catalysts adsorb one or more reactant at active sites on their surface. There is an interaction which increases the reactivity of the reactant molecules. The reaction takes place, and the product molecules are desorbed.

The Contact Process

An example of a reaction which makes use of a heterogeneous catalyst is the Contact Process. This (reversible) reaction is used to convert sulphur dioxide into sulfur trioxide (which produces sulfuric acid when added to water). 

2SO_{2}+O_{2}=2SO_{3}

SO_{3}+H_{2}O=H_{2}SO_{4}

The catalyst is V_{2}O_{5} (vanadium oxide).

The conditions are 450 degrees celsius and 1 or 2 atm pressure. 

Remember: for your CIE A Level exam, you will also need to revise the details of the Haber Process. 

Catalytic Converters

Vehicle exhaust fumes contain a number of toxic molecules, including carbon monoxide and nitrogen oxides. To prevent these fumes entering the atmosphere, catalytic converters are fitted to cars, buses and vans. 

Catalytic converters contain metals like platinum, rhodium and palladium. These act as heterogeneous catalysts, breaking down the poisonous molecules into harmless ones. 

For example, the reaction which converts carbon monoxide and nitrogen monoxide into carbon dioxide and nitrogen can be catalysed by platinum, deposited as a thin layer on a ceramic honeycomb.

Remember: Enzymes are examples of biological catalysts, built from proteins. They catalyse reactions in biological organisms. Because their Active Sites have specialised shapes, each enzyme will only catalyse one specific reaction.

Your introduction to catalysts is now complete! 

Are you ready to test yourself? Head straight to our CIE Topic Questions on Reaction Kinetics.  

CIE AS and A Level Chemistry 2019-2021 Topic Questions 

CIE AS and A Level Chemistry 2021 and beyond Topic Questions 

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