MOD 6 • RATE • CATALYSTS
Catalysts and Reaction Profiles
The full mechanism with reaction profiles. Spec-named catalysts only. And the "catalyst doesn't change yield" point that links forward to equilibrium.
What Is a Catalyst?
Catalyst — Spec 4.6.1.4 Verbatim
"Catalysts change the rate of chemical reactions but are not used up during the reaction. Different reactions need different catalysts. Enzymes act as catalysts in biological systems."
Three Key Features
Speeds up the rate — increases how fast a reaction proceeds.
Not used up — recovered unchanged at the end. Same mass before and after.
Specific — each catalyst works for particular reactions only, not universally.
Spec Note — Identifying a Catalyst
Spec requires you to "identify catalysts in reactions from their effect on the rate of reaction and because they are not included in the chemical equation for the reaction." If a substance speeds things up but doesn't appear in the balanced equation, it's a catalyst.
How a Catalyst Works
Mechanism — Spec Verbatim
"Catalysts increase the rate of reaction by providing a different pathway for the reaction that has a lower activation energy."
The Full Five-Link Chain
Catalyst provides a different pathway for the reaction.
This pathway has a lower activation energy (Eₐ).
More particles now have energy ≥ the new (lower) Eₐ.
A greater proportion of collisions have enough energy to react.
So there are more reactions per second → faster rate.
2-mark questions usually ask only for steps 1–2. 3-mark questions extend to step 4. Reading mark allocation is the trick.
Reaction Profile — With vs Without Catalyst
The spec explicitly states: "A reaction profile for a catalysed reaction can be drawn in the following form:" — meaning students should be able to draw and interpret one. The key features:
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What's the Same — and What's Different
FeatureSame or Different?Reactant energy levelSame — catalyst doesn't change reactantsProduct energy levelSame — catalyst doesn't change productsOverall energy change (ΔH)Same — exothermic/endothermic by the same amountActivation energy (peak height)LOWER with catalyst — this is the only change
Why Catalysts Matter Industrially
Reactions that would be too slow or too expensive without a catalyst become commercially viable with one. Three reasons matter for exam answers:
Lower Energy Cost
Catalysed reactions can run at lower temperatures — saves fuel and reduces CO₂ emissions.
Faster Production
Faster rate means more product made per hour — higher throughput from the same plant.
Reusable
Not used up → can be recovered and reused. Cost amortises over many reaction cycles.
Forward Link to 6.07 — Haber Process
The most-tested industrial catalyst on this spec is iron in the Haber process (4.10.4.1: "purified gases are passed over a catalyst of iron at a high temperature ... and a high pressure"). The full Haber-process trade-off appears in Lesson 6.07.
Catalysts You Need to Know
The spec says: "Students do not need to know the names of catalysts other than those specified in the subject content." So this is the complete list.
CatalystReactionSpec ref
Iron (Fe)Haber process: N₂ + 3H₂ ⇌ 2NH₃4.10.4.1
EnzymesBiological reactions (e.g. fermentation: glucose → ethanol + CO₂)4.6.1.4 / 4.7.2.3
Other Catalysts Appear in Past Papers
Past-paper questions sometimes use named catalysts that aren't formally on the spec list — e.g. manganese dioxide (MnO₂) for the decomposition of hydrogen peroxide (P2 Jun23 Q07.1). When this happens, the catalyst is given in the question stem — you don't need to recall its name independently. Focus on the mechanism, not memorising additional catalysts.
Spec Practical Opportunity (AT 5)
Spec 4.6.1.4 lists an AT 5 opportunity: "investigate the catalytic effect of adding different metal salts to a reaction such as the decomposition of hydrogen peroxide." This is exactly the experimental scenario in P2 Jun23 Q07 — different metal salts (or oxides) catalyse 2H₂O₂ → 2H₂O + O₂ at different rates. Be ready for "compare the effectiveness of different catalysts" questions.
What Examiners Won't Ask
You will not be asked to name a catalyst from memory beyond what the spec specifies. Vanadium(V) oxide for the Contact process, platinum/rhodium for catalytic converters, nickel for hydrogenation — these appear in textbooks and other specs but are not on AQA 8462.
Enzymes — Biological Catalysts
Enzyme
A biological catalyst — a protein that catalyses reactions in living organisms. Like all catalysts, enzymes provide a different pathway with lower activation energy and are not used up.
Key Spec Example — Fermentation
The spec covers enzymes in the context of fermentation (4.7.2.3, chemistry-only): "Aqueous solutions of ethanol are produced when sugar solutions are fermented using yeast." Yeast contains enzymes that catalyse the conversion of sugar to ethanol and carbon dioxide: sugar → yeast (enzymes) → ethanol + carbon dioxide The spec doesn't require you to write a balanced equation for fermentation — just to know that yeast is used and that enzymes catalyse the reaction.
Spec Conditions for Fermentation
Lesson 7.x covers fermentation in detail. For now, know that the spec requires you to "know the conditions used for fermentation of sugar using yeast" — typical conditions are around 25–35 °C (warm but not hot) and an absence of air (anaerobic). Hot conditions denature the enzymes; cold conditions slow them down too much.
Three Things Catalysts DON'T Do
This is where most exam marks are lost. Each of the following is a recurring distractor in examiner reports.
Wrong
"The catalyst starts the reaction."
Right
The reaction already occurs without a catalyst — just very slowly. The catalyst speeds it up; it doesn't initiate it.
Wrong
"The catalyst reduces the number of collisions."
Right
A catalyst doesn't change collision frequency — it lowers the activation energy, so a greater proportion of the existing collisions succeed.
Wrong
"The catalyst increases the yield" (in equilibrium reactions).
Right
A catalyst speeds up forward and reverse reactions equally. Equilibrium is reached faster, but the position — and the yield — are unchanged.
Examiner Alert — P2 Jun23 Q09.6
Asked the effect of a catalyst on the equilibrium position, the mark scheme accepts: "no effect (on equilibrium position)" or "(equilibrium position) stays the same". Don't write "shifts left" or "shifts right" — neither.
Worked Example — Catalyst Mechanism (2 marks)
2 MARKS
Question (P2 Jun23 Q07.1 verbatim): Manganese dioxide catalyses the decomposition of hydrogen peroxide solution. Explain how a manganese dioxide catalyst increases the rate of decomposition of hydrogen peroxide.
M1
The catalyst provides a different pathway for the reaction.
M2
This pathway has a lower activation energy.
Mark scheme exactly. 2-mark catalyst questions reliably reward this exact pair: "different pathway" + "lower activation energy". Don't add filler — extra material can introduce wrong claims (e.g. "reduces collisions") that examiners can penalise. Stick to the verified phrases.
Worked Example — Full Mechanism (3 marks)
3 MARKS
Question: Explain, using collision theory, how a catalyst increases the rate of reaction.
M1
The catalyst provides a different pathway for the reaction.
M2
This pathway has a lower activation energy.
M3
A greater proportion of collisions now have enough energy to react (energy ≥ the new lower Eₐ) — so more reactions per second → faster rate.
Standard 3-mark structure. Build it as the same chain from Lesson 6.03's strategy: cause (lower Eₐ) → collision effect (greater proportion succeed) → rate consequence (faster). Avoid "successful" alone — write "have enough energy to react".
Spotting a Catalyst in an Equation
Spec 4.6.1.4 explicitly says students should "identify catalysts in reactions ... because they are not included in the chemical equation". This is a recurring 1-mark question type.
Two Telltale Signs
Substance speeds up the reaction but appears unchanged at the end.
Doesn't appear in the balanced chemical equation — it's often written above the arrow instead.
2H₂O₂(aq) → MnO₂ → 2H₂O(l) + O₂(g)
Manganese dioxide written above (or with) the arrow — not as a reactant or product.
Common Question Variant
"Which substance is the catalyst?" given a balanced equation and a reagent list — pick the one that's not in the equation but is involved in the experiment. "Which substance shows the catalyst is not used up?" — same mass at start and end. (P2 Jun23 Q07 uses 1.0 g MnO₂ as the catalyst — the experimental setup itself doesn't test recovery, but the principle that the mass should be unchanged is examinable.)
Practice 1 — Reaction Profile
3 MARKS
Question: The reaction profile of a reaction is drawn for both catalysed and uncatalysed pathways. State one difference and two similarities between the two profiles.
M1
Difference: the catalysed reaction has a lower activation energy (lower peak on the profile).
M2
Similarity: the reactants and products are at the same energy levels in both profiles (catalyst doesn't change reactants or products).
M3
Similarity: the overall energy change (ΔH) is the same — exothermic / endothermic by the same amount.
The "same overall energy change" point is the highest-yielding mark. If a question asks how a catalyst affects the energy released, the answer is "no effect" — only the activation energy changes, never the ΔH.
Practice 2 — Identifying a Catalyst
2 MARKS
Question: A student investigates the decomposition of hydrogen peroxide using manganese dioxide. Suggest two pieces of evidence the student could use to confirm that manganese dioxide is acting as a catalyst.
M1
The reaction is faster when manganese dioxide is added compared to the reaction without it (effect on rate).
M2
The mass of manganese dioxide is the same at the start and end of the reaction (it is not used up); or manganese dioxide does not appear in the balanced chemical equation.
Spec language: identify by "effect on the rate of reaction" AND "not included in the chemical equation". The mass-conservation evidence is a practical way to verify the second criterion.
Forward Link — Catalysts at Equilibrium
Lesson 6.05 introduces dynamic equilibrium. The catalyst rule there is simple but critical:
Catalysts and Equilibrium
A catalyst speeds up the forward reaction.
A catalyst also speeds up the reverse reaction by the same factor.
So equilibrium is reached faster, but the position is unchanged.
Yield is unchanged.
Examiner Alert — P2 Jun23 Q09.6
For "effect of catalyst on equilibrium position" — mark scheme: "no effect (on equilibrium position)" or "(equilibrium position) stays the same". This is a 1-mark gimme if you remember the rule. Lesson 6.06 covers which conditions do shift equilibrium — temperature, pressure, concentration. Catalyst is the only one that doesn't.
Lesson Recap
You should now be able to:
Define a catalyst as a substance that speeds up the rate of reaction without being used up; and an enzyme as a biological catalyst.
State the mechanism verbatim from the spec: "provides a different pathway for the reaction that has a lower activation energy".
Build the full chain for 3-mark questions: different pathway → lower Eₐ → greater proportion have enough energy → faster rate.
Draw and label a reaction profile with and without a catalyst — same reactants, same products, same ΔH, lower peak.
Identify a catalyst from its effect on rate and absence from the balanced equation.
State that catalysts have no effect on yield or equilibrium position — they only speed up reaching equilibrium.
Recall iron in the Haber process as the named industrial catalyst on this spec.
Avoid the wrong claims : catalysts don't start reactions, don't reduce collisions, don't change yield.
Top-Marks Habit
For any catalyst question, build the answer in spec language: "different pathway", "lower activation energy", "no effect on equilibrium position". These exact phrases appear in mark schemes year after year. Memorise them word-for-word.
UP NEXT • LESSON 6.05
Reversible Reactions and Dynamic Equilibrium
The ⇌ symbol explained. The three conditions for dynamic equilibrium. And the two phrases that examiners reject every year — "the reaction stops" and "the amounts are equal".
Next: What Is a Catalyst?