Catalysts Do Not Provide Energy

What students typically write

“The catalyst gives energy to the particles so they can react.” Or: “Catalysts provide the activation energy.” Or: “A catalyst adds energy to the reaction so it goes faster.”

Students know that reactions need activation energy and that catalysts speed reactions up, so they assume catalysts must be supplying that energy. This confuses lowering a barrier with providing energy to overcome it.

Why it is wrong

A catalyst does not add energy to the system. It provides an alternative reaction pathway that has a lower activation energy. This means more particles already have enough energy to react at the same temperature, so the rate increases.

Think of it as a mountain pass. The reactants need to get over a mountain (activation energy) to become products. A catalyst does not give the hikers jet packs (energy). It finds a lower pass through the mountains (alternative pathway) so more hikers can make it over with the energy they already have.

Key facts about catalysts that mark schemes test:

• A catalyst provides an alternative pathway with lower activation energy
• A catalyst is not used up / is chemically unchanged at the end
• A catalyst does not change the overall energy change of the reaction
• A catalyst does not change the products
• A catalyst does not change the position of equilibrium (it speeds up forward and reverse reactions equally)

What the mark scheme actually wants

A typical question: “Explain how a catalyst increases the rate of a reaction.” [2]

Marking points:

1. Provides an alternative reaction pathway (1 mark)
2. With a lower activation energy (1 mark)

An answer that says “the catalyst gives energy to the reactants” scores 0 because it contradicts how catalysts work.

Worked example: wrong vs right

Question: Manganese dioxide is used as a catalyst in the decomposition of hydrogen peroxide. Explain its effect on the reaction. [3]

Wrong answer: “The manganese dioxide provides energy to the hydrogen peroxide molecules so they can decompose faster. It makes the reaction release more heat.” This scores 0/3. Catalysts do not provide energy, and they do not change the overall energy released.

Right answer: “Manganese dioxide provides an alternative reaction pathway (1) with a lower activation energy (1). More hydrogen peroxide molecules have energy greater than or equal to the new, lower activation energy, so more successful collisions occur per unit time and the rate increases. The MnO2 is chemically unchanged at the end (1).” This scores 3/3.

How to avoid this mistake

Draw the energy-level diagram with and without a catalyst. The only difference is the height of the activation energy hump. The starting energy (reactants) and the finishing energy (products) are the same. The overall energy change is the same. Only the barrier is lower.

If you draw this diagram correctly, it is physically impossible to make the mistake of saying catalysts “provide energy” because the diagram shows they do not change the energy of anything. They just lower the hill in between.

Remember: catalysts change the route, not the energy supply.

For full coverage of activation energy and energy-level diagrams, see exothermic and endothermic reactions and energy level diagrams.