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So far the factors that change a rate have all worked by changing how the particles collide — how often, or how energetically. A catalyst is different, and rather remarkable: it speeds a reaction up without being used up and without appearing in the overall equation, by making it easier for collisions to succeed. Nature has its own versions of these substances, called enzymes, which control the reactions inside every living cell. This lesson explains how a catalyst works, why it is not consumed, why catalysts are so valuable in industry, and how enzymes act as biological catalysts. It is part of Topic C5 of your OCR Gateway Combined Science course.
By the end of this lesson you should be able to describe what a catalyst does, explain in terms of activation energy how a catalyst speeds up a reaction, draw and interpret a reaction profile with and without a catalyst, and describe enzymes as biological catalysts.
This lesson combines AO1 (recalling what a catalyst is and does), AO2 (applying the activation-energy idea to explain the speed-up) and AO3 (reading and comparing reaction profiles drawn with and without a catalyst).
Catalysts are one of the most useful ideas in all of chemistry. Because a catalyst is not used up, a small amount can process a huge quantity of reactant, and because it lets reactions run faster and often at lower temperatures, it saves enormous amounts of energy and money in industry. The same principle, applied by living things, keeps you alive.
A catalyst is a substance that speeds up a reaction without being used up in the process. At the end of the reaction the catalyst is chemically unchanged, so it can be used again and again.
A catalyst works by providing a different reaction pathway that has a lower activation energy. Remember that only collisions with at least the activation energy are successful. If the activation energy is lower, then a greater proportion of the collisions already have enough energy to react — so more collisions are successful and the reaction goes faster. The catalyst does this without giving the particles any extra energy and without changing the products.
It is worth being clear about what a catalyst does not do:
All it changes is the rate, by lowering the barrier the particles must clear.
Exam Tip: The one-line definition to learn is: a catalyst speeds up a reaction by providing a pathway of lower activation energy, and is not used up. A very common misconception is that a catalyst "gives the particles more energy" — it does the opposite job, lowering the energy needed, so that more of the existing collisions succeed.
A reaction profile shows the effect of a catalyst clearly. The catalysed pathway has a lower hump (a smaller activation energy), while the reactant and product energy levels are unchanged — so the overall energy change of the reaction is exactly the same as before.
Reading the profile: the grey curve (no catalyst) has a high hump, so only the few collisions with that much energy can react. The green curve (with catalyst) has a lower hump, so many more collisions have enough energy — the reaction is faster. But both curves start and end at the same energy levels, which is why the catalyst changes only the speed, not the amount of energy released or absorbed overall.
Exam Tip: When you draw a catalysed reaction profile, keep the reactant and product levels exactly the same and draw only a lower hump. Drawing a lower product level (as if the catalyst changed the energy released) is a classic error — the catalyst changes only the activation energy.
A catalyst takes part in the reaction — it provides the alternative pathway — but it is regenerated by the end, so it emerges chemically unchanged. This has two important consequences.
First, only a small amount of catalyst is needed, however much reactant there is, because the same catalyst particles work over and over again. This is why catalysts are economical even when they are expensive: the cost is spread over a vast amount of product.
Second, because the catalyst is not consumed, it does not appear in the overall balanced equation for the reaction. It is present at the start and at the end in the same amount, so by convention it is written above the reaction arrow rather than as a reactant or product.
Exam Tip: "Not used up" is the phrase examiners look for. If asked how you could show a solid catalyst was not used up, the answer is that its mass would be unchanged at the end (you could filter it off, dry it and re-weigh it).
Catalysts are specific: a particular catalyst speeds up a particular reaction, or a particular type of reaction, and a catalyst that works well for one reaction may do nothing at all for another. This is why industry uses different catalysts for different processes. For example, an iron catalyst is used to make ammonia in the Haber process (which you will meet in the next lesson), while a catalyst based on platinum is used in the catalytic converters that clean up car exhaust gases.
Because a catalyst lets a reaction run fast at a lower temperature than would otherwise be needed, it saves the energy that would be spent on heating — and therefore saves money and reduces the environmental impact of a process. In modern industry, finding a good catalyst for a reaction is often the key to making the process economical at all.
flowchart LR
A["High activation energy<br/>reaction is slow"] --> B["Add a catalyst"]
B --> C["Alternative pathway<br/>lower activation energy"]
C --> D["More collisions succeed<br/>reaction is faster"]
D --> E["Catalyst unchanged<br/>and reused"]
Exam Tip: If an exam says a reaction is run "at a lower temperature using a catalyst", the point being tested is usually the saving of energy and cost. The catalyst lets the reaction go fast enough without the expense of a high temperature.
Living things carry out thousands of chemical reactions, and almost all of them would be far too slow at body temperature without help. That help comes from enzymes — biological catalysts made by living cells. An enzyme is a large protein molecule, and like any catalyst it speeds up a reaction by lowering the activation energy and is not used up, so the same enzyme molecule can be used repeatedly.
Enzymes are extremely specific — even more so than industrial catalysts. Each enzyme usually catalyses just one particular reaction, because its shape fits only one kind of reacting molecule. This lets a cell control its chemistry very precisely, switching individual reactions on by making the right enzyme.
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