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On a reaction profile, a catalyst gives a lower activation energy peak, with the same reactant and product energy levels. Free notes and 6 questions answered.
Identifying catalysts and reaction profiles is part of Rate of reaction in GCSE AQA Higher Chemistry.
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On a reaction profile, a catalyst gives a lower activation energy peak, with the same reactant and product energy levels. Free notes and 6 questions answered.
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Use this lesson to stay inside Rate of reaction while connecting the detail back to the wider Topic 6: The rate and extent of chemical change topic.
Read the core explanations from Identifying catalysts and reaction profiles before testing yourself from memory.
You've probably noticed that some reactions happen instantly while others take forever. Drop sodium in water and it explodes immediately. Leave iron in air and it rusts slowly over months. What if there was a way to speed up the slow reactions without changing what you end up with?
That's exactly what catalysts do. They're like the ultimate chemical cheat code - they make reactions happen faster without getting used up themselves. Think of them as the friend who knows a shortcut through town that gets you there quicker.
How do you know if a catalyst is involved in a reaction? There are two dead giveaways that examiners love to test you on.
First, the reaction rate increases dramatically when the catalyst is added. Same reactants, same conditions, but suddenly everything happens much faster. Second, the catalyst doesn't appear in the balanced chemical equation because it's not consumed.
Worked example: Identifying a Catalyst from Experimental Data
A student adds substance X to a reaction between A and B. The reaction time drops from 120 seconds to 30 seconds. After the reaction, substance X is still present and unchanged. Is X a catalyst?
Answer: Yes, substance X is a catalyst because it increases the reaction rate without being consumed.
This is why you'll never see a catalyst written in the main chemical equation. For example, if manganese dioxide catalyses the decomposition of hydrogen peroxide, you write:
$2H_2O_2 \rightarrow 2H_2O + O_2$
The $MnO_2$ catalyst might be written above the arrow, but never as a reactant or product because it's still there at the end, unchanged.
To understand how catalysts work, we need to think about energy. Every reaction has an energy barrier that particles must overcome to react - like climbing over a hill to get to the other side.
A reaction profile is like a map of this energy journey. The reactants start at one energy level, climb up to the activation energy peak, then drop down to the products' energy level.
Here's where catalysts work their key scientific idea: they provide a different route with a lower hill to climb. Same starting point, same destination, but an easier path.
Worked example: Drawing a Catalysed Reaction Profile
Draw a reaction profile showing both catalysed and uncatalysed pathways for an exothermic reaction.
Answer: Two curves with same start/end points but different activation energy peaks - catalysed pathway has lower peak.
Key Point: The catalyst doesn't change the overall energy change of the reaction. If it's exothermic without a catalyst, it's still exothermic with one. The catalyst just makes it happen faster by lowering the activation energy barrier.
While you don't need to memorise long lists of catalysts, there are a few that appear regularly in GCSE questions. These are the ones that could save you marks in the exam.
Manganese dioxide ($MnO_2$) catalyses the decomposition of hydrogen peroxide - you've probably seen this in practicals where it makes hydrogen peroxide fizz violently. Iron is used as a catalyst in the Haber process for making ammonia. Enzymes are biological catalysts that speed up reactions in living things.
The key thing isn't memorising every catalyst, but recognising the pattern: substance present at start and end, reaction goes faster, not written in the equation.
Catalyst questions are mark-friendly if you know the key phrases examiners want to hear. They're looking for specific scientific language, not just general understanding.
When asked to identify a catalyst, always mention both criteria: increases reaction rate AND is not consumed. When drawing reaction profiles, label your axes clearly and show that the catalysed pathway has lower activation energy but the same overall energy change.
Remember, catalyst questions often combine with other topics like rates of reaction or energy changes. The examiners love to test whether you can link these concepts together in longer answers.
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Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
Iron is used as a catalyst in the Haber process for making ammonia. Like all catalysts, it increases the rate of the reaction without being consumed.
Manganese dioxide (MnO₂) is used as a catalyst for the decomposition of hydrogen peroxide, making it fizz violently. It is not consumed, so it is still present at the end of the reaction.
On a reaction profile, a catalyst gives a lower activation energy peak, with the same reactant and product energy levels. The overall energy change is unchanged, so an exothermic reaction stays exothermic.
A substance is a catalyst if it increases the reaction rate and is not consumed, so it is still present and unchanged at the end. For example, if substance X cuts a reaction time from 120 s to 30 s and is still present and unchanged, X is a catalyst.
Catalysts are not written as reactants or products in chemical equations because they are not consumed and are still there at the end, unchanged. For example, the decomposition of hydrogen peroxide is written 2H₂O₂ → 2H₂O + O₂, with MnO₂ at most written over the arrow.
A catalyst does not change the overall energy change of a reaction. If a reaction is exothermic without a catalyst, it is still exothermic with one; on a reaction profile the catalysed pathway has the same start and end points but a lower activation energy peak.