Catalysts

A catalyst is a substance that speeds up a chemical reaction but is not used up. It makes reactions happen faster and can be recovered unchanged at the end.

Key ideas

• Faster reactions: A catalyst increases the rate of reaction.
• Unchanged: It is not consumed; the same catalyst can be used again.
• Lower activation energy: A catalyst provides an alternative pathway with lower activation energy, $E_a$.
• Collision theory link: With a lower $E_a$, more particle collisions have enough energy to be successful each second.
• No change to yield or position of equilibrium: In reversible reactions a catalyst speeds up both forward and reverse reactions equally, so the final amounts do not change.

How a catalyst works

Imagine reactant particles climbing a hill of energy to react. The height of this hill is the activation energy, $E_a$. A catalyst lowers the hill, so more particles can get over it at the same temperature. This increases the number of successful collisions per second, so the reaction is faster.

Some solid catalysts also offer a surface for particles to stick to, helping them meet in the right way to react.

Types and examples

• Enzymes: Biological catalysts in living things. They are specific (like a key fitting a lock) and work best at certain temperatures and pH.
• Transition metal catalysts: Many transition metals and their compounds are effective catalysts.
  • Haber process: iron catalyst for ammonia production $$N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g)$$
  • Contact process: vanadium(V) oxide, V$_2$O$_5$, for making sulfur trioxide $$2SO_2(g) + O_2(g) \rightleftharpoons 2SO_3(g)$$
  • Car catalytic converters: platinum, palladium, rhodium change harmful gases (e.g. carbon monoxide and nitrogen oxides) into less harmful gases.

Investigating the effect of a catalyst

Compare the rate with and without a catalyst using: volume of gas produced over time, change in mass when a gas leaves, or the time for a visible change (e.g. cloudiness to appear). The catalysed reaction will reach the same final amount, but faster.

Common misconceptions

• A catalyst is not used up.
• A catalyst does not change what products form; it only changes how fast they form.
• A catalyst does not increase temperature; it lowers $E_a$ so more collisions succeed at the same temperature.
• In equilibrium, a catalyst does not shift the position; it helps reach equilibrium faster.