5.2.4 Catalysts & Enzymes

• Catalysts are substances which speed up the rate of a reaction without themselves being altered or consumed in the reaction
• The mass of a catalyst at the beginning and end of a reaction is the same and they do not form part of the equation
• Some important catalysts to know are:
  • Iron which is used to catalyse the Haber Process for the production of ammonia
  • Vanadium(V) oxide used to catalyse the Contact Process for the production of sulfuric acid 
  • Manganese(IV) oxide used to catalyse the decomposition of hydrogen peroxide 
• As observed above transition metals are used widely as catalysts as they have variable oxidation states allowing them to readily donate and accept different numbers of electrons. This is key to their catalytic activity
• Normally only small amounts of catalysts are needed to have an effect on a reaction
• Different processes require different types of catalysts but they all work on the same principle of providing an alternate route for the reaction to occur
• They do this by lowering the activation energy required, hence providing a reaction pathway requiring less energy
• Catalysis is a very important  branch of chemistry in commercial terms as catalysts increase the rate of reaction (hence the production rate) and they reduce energy costs

• Reaction profiles (sometimes called energy level diagrams) are graphical representations of the relative energies of the reactants and products in chemical reactions
• They also display the activation energy and overall energy change
• The diagram shows that when a catalyst is used, the activation energy is reduced as it creates an alternative pathway requiring lower activation energy
• Lowering the activation energy means more particles will have enough energy to react when they come together
• This shows that when a catalyst is used, the rate of reaction will increase

Graph showing the effect of using a catalyst on the activation energy of a reaction

• Enzymes are biological catalysts 
• They act as catalysts in biological systems, controlling many reactions which occur in cells 
• Some examples of their use:
  • Those involved in the digestive system e.g amylase in the salivary glands
  • Catalase, found in liver cells, which catalyses the decomposition of hydrogen peroxide, which, left un-catalysed can damage tissues
  • Enzymes in yeast produce alcohol through fermentation
• As well as being important for controlling reactions in cells, they are also important in industry
• Enzymes allow industrial reactions to happen at lower temperatures and pressures than usually needed, saving money and energy