Catalysts (Edexcel IGCSE Chemistry)

Catalysts & Rates

Spec point — know that a catalyst is a substance that increases the rate of a reaction, but is chemically unchanged at the end of the reaction know that a catalyst works by providing an alternative pathway with lower activation energy

The effect of a catalyst on rate of reaction

• Catalysts are substances which speed up the rate of a reaction without themselves being altered or consumed in the reaction

  • Normally only small amounts of catalysts are needed to have an effect on a reaction
  • The mass of a catalyst at the beginning and end of a reaction is the same

• Catalysts do not form part of the chemical equation but they are sometimes seen above or below the reaction arrow:

N₂ (g) + 3H₂ (g) rightwards harpoon over leftwards harpoon from 450 space degree straight C comma space 200 space atm to iron space catalyst of 2NH₃ (g)

SO₂ (g) + O₂ (g) → from straight V_ 2 straight O_ 5 catalyst to 450 degree straight C of SO₃ (g)

• Different processes require different types of catalysts but they all work on the same principle:

  • They provide an alternative pathway for the reaction to occur
  • The alternative pathway has a lower activation energy

Catalysts work by attracting reactant molecules on to the surface and so providing an alternate reaction pathway of lower 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

• The 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

• Enzymes act as catalysts in biological systems

Exam Hint The effect of a catalyst on activation energy can be shown on a reaction profile.