Edexcel IGCSE Chemistry · Spec 3.12-3.14C
Catalysts & Reaction Profiles
How catalysts speed up reactions, shown on reaction profile diagrams.
Chemistry revision video
Catalysts & Reaction Profiles
Prefer to watch on YouTube? Open this video on YouTube.
Explained
Catalysts, activation energy, and what the profile actually shows
A catalyst speeds up a chemical reaction without being used up. It takes part, but it is chemically unchanged at the end, so you can recover all of it and use it again. That is why a small quantity can process an enormous amount of reactant.
Activation energy
Colliding particles only react if they collide with at least a certain amount of energy. That minimum is the activation energy. Collisions with less energy simply bounce apart.
A catalyst provides an alternative route with a lower activation energy. Lowering the bar means a greater proportion of collisions have enough energy to succeed, so more collisions are successful and the reaction goes faster.
Note what a catalyst does not do. It does not make particles move faster, it does not increase the number of collisions, and it does not change how much energy the reaction releases or absorbs overall.
Reading a reaction profile
A reaction profile plots energy against the progress of the reaction. The reactants sit at one level on the left, the products at another on the right, and there is a hump between them.
The height of the hump above the reactants is the activation energy. The difference in height between reactants and products is the overall energy change. In an exothermic reaction the products are lower than the reactants, and in an endothermic reaction they are higher.
Adding a catalyst draws a second, lower hump between the same two levels. The start and finish do not move. Only the peak comes down, and being able to say that in those terms is what the question is testing.
Enzymes
Enzymes are biological catalysts. They are proteins made by living cells, and they lower activation energy in the same way. What is different is that they are highly specific, working on one substrate or one type of reaction, and they only function within a narrow range of temperature and pH.
What examiners say about this topic
Principal examiner reports for Edexcel International GCSE Chemistry are exact about the language of collisions. On a question about temperature, four marks required the rate of reaction increasing, particles moving faster, more frequent collisions and more successful collisions. One response scored three marks with no mention of more successful collisions or of more particles having the activation energy. Another mentioned kinetic energy and rate but neither frequency nor success, and scored two. The report states plainly that candidates must talk about more frequent and successful collisions.
Frequent and successful are not interchangeable, and both are needed. A catalyst works on the success side of that pair, not on the frequency side, which is exactly what separates it from raising the temperature.
A separate report notes that phosphoric or sulfuric acid is used as a catalyst in the hydration of ethene to make ethanol, but that some learners wrongly listed the acid as a reactant. A catalyst never appears in the equation.
Why industry cares
A catalyst lets a reaction run fast enough at a lower temperature and pressure, which cuts the energy used and therefore the cost and the carbon dioxide released. Because it is not consumed, the same charge keeps working for a long time. Iron in the Haber process is the standard example to quote.
Spec 3.12-3.14C
What you need to know
- Say what a catalyst does
- Show a catalyst on a reaction profile
- Recognise enzymes as biological catalysts
Active recall
Quick check
Answer each question before opening the answer.
How does a catalyst speed up a reaction?
It provides an alternative reaction pathway with a lower activation energy.
Is a catalyst used up in the reaction?
No — it is chemically unchanged at the end.
Chemistry revision app
Take this topic further in the app
275 guided topics with questions marked as you answer them, section checkpoints and timed practice papers. Fifteen topics are free to try, with no card needed.