Chemistry • Year 11 • Module 4 • Lesson 5
Activation Energy, Catalysts & Energy Diagrams
Lock in the core vocabulary — activation energy, transition state, catalyst types — and the fundamental rules for reading energy profile diagrams.
1. Term–definition match
In the right-hand column write the matching term from this list: activation energy (Ea), transition state, catalyst, homogeneous catalyst, heterogeneous catalyst, energy profile diagram, enthalpy change (ΔH), enzyme, activated complex, reaction coordinate. 10 marks
| # | Definition | Matching term |
|---|---|---|
| 1.1 | The minimum energy that colliding reactant molecules must possess for a reaction to occur and products to form. | |
| 1.2 | The high-energy, unstable species that exists momentarily at the peak of the energy profile; also called the transition state. | |
| 1.3 | A substance that provides an alternative reaction pathway with lower activation energy; is regenerated and not consumed overall. | |
| 1.4 | A catalyst that is in the same physical state (phase) as the reactants — for example, H+(aq) in an aqueous esterification reaction. | |
| 1.5 | A catalyst that is in a different physical state from the reactants — for example, solid Pt with gaseous exhaust gases in a catalytic converter. | |
| 1.6 | A graph of potential energy versus reaction progress that shows Ea, the transition state peak, the reactant and product energy levels, and ΔH. | |
| 1.7 | The difference between the enthalpy of products and the enthalpy of reactants; shown as the vertical gap between the reactant level and the product level on an energy profile diagram. | |
| 1.8 | A protein that acts as a biological catalyst; its active site stabilises the transition state and lowers Ea for biochemical reactions. | |
| 1.9 | The unstable, high-energy species at the peak of the energy profile; same as activated complex. | |
| 1.10 | The horizontal axis of an energy profile diagram, representing how far the reaction has proceeded from reactants to products. |
2. Fill in the blanks
Complete the passage below using the word bank provided. Each word or phrase is used once. 8 marks
Word bank: activation energy | catalyst | enthalpy change (ΔH) | regenerated | alternative pathway | heterogeneous | lower | transition state
A _______________ speeds up a reaction by providing an _______________ with a lower _______________. The peak of the energy profile diagram represents the _______________, the highest-energy point of the reaction. A catalyst causes this peak to be _______________, meaning a greater proportion of molecular collisions now have sufficient energy to react. Crucially, the _______________ remains the same because the identities of the reactants and products are unchanged. The catalyst itself is _______________ in the mechanism. In a catalytic converter, the solid platinum metal is classified as a _______________ catalyst because it is in a different phase from the gaseous exhaust molecules.
3. True or false — with correction
For each statement, circle T or F. If the statement is false, write the corrected version on the line provided. 10 marks (1 T/F + 1 correction where needed)
3.1 A catalyst lowers the activation energy of the forward reaction only; the activation energy of the reverse reaction remains unchanged. T / F
3.2 A catalyst causes the ΔH of a reaction to become more negative, making the reaction release more energy. T / F
3.3 Ea is measured from the reactant energy level to the peak of the energy profile diagram. T / F
3.4 The iron catalyst used in the Haber process (N2(g) + 3H2(g) → 2NH3(g)) is a homogeneous catalyst. T / F
3.5 Enzymes are biological catalysts that lower Ea by providing a favourable environment for the transition state in their active site. T / F
4. Function recall
Answer each prompt in 1–2 sentences using precise lesson terms. 8 marks (2 each)
4.1 What is the function of activation energy (Ea) in a chemical reaction?
4.2 What is the function of a catalytic converter in a modern car? Include the physical states of the catalyst and the reactants in your answer.
4.3 Why does a catalyst NOT change ΔH, even though it changes the rate of reaction?
4.4 What is the function of the transition state (activated complex) in a reaction mechanism?
5. Build a concept map
Draw labelled arrows between the six terms below to show how they are connected. Each arrow must carry a linking phrase. Aim for at least 6 labelled arrows. 6 marks
Supplied terms: activation energy (Ea) · catalyst · transition state · reaction rate · ΔH · energy profile diagram.
Q1 — Term–definition matches
1.1 activation energy (Ea) • 1.2 activated complex • 1.3 catalyst • 1.4 homogeneous catalyst • 1.5 heterogeneous catalyst • 1.6 energy profile diagram • 1.7 enthalpy change (ΔH) • 1.8 enzyme • 1.9 transition state • 1.10 reaction coordinate.
Q2 — Cloze answers (in order)
catalyst · alternative pathway · activation energy · transition state · lower · enthalpy change (ΔH) · regenerated · heterogeneous.
Q3 — True / false with corrections
3.1 False. A catalyst lowers Ea of both the forward and reverse reactions by the same amount; both pathways share the same transition state peak, which is lowered equally.
3.2 False. A catalyst does not change ΔH. ΔH depends only on the energy levels of reactants and products, which are identical with or without a catalyst. The reaction releases the same total energy; it just does so at a faster rate.
3.3 True. Ea is the vertical distance from the reactant energy level to the peak of the energy profile — not from the x-axis (zero).
3.4 False. Iron (Fe) is a solid while N2 and H2 are gases — different phases — so Fe is a heterogeneous catalyst in the Haber process.
3.5 True. Enzymes are protein catalysts whose active site geometry stabilises the transition state, lowering Ea and increasing the rate of biochemical reactions.
Q4.1 — Function of activation energy
Ea is the minimum energy that colliding molecules must have for a reaction to proceed and products to form. It sets the energy threshold; only collisions with energy ≥ Ea are successful. Ea is a kinetic quantity — it determines rate, not whether the reaction is energetically favourable.
Q4.2 — Function of catalytic converter
A catalytic converter converts toxic exhaust gases (CO, unburnt hydrocarbons, NOx — all gaseous) into less harmful products (CO2, H2O, N2) by adsorbing the gases onto a solid platinum/palladium/rhodium surface. The solid metal catalyst (different phase from gaseous reactants) is heterogeneous; it lowers Ea for the conversion reactions so they proceed at typical exhaust temperatures. The metals are not consumed.
Q4.3 — Why ΔH is unchanged by a catalyst
ΔH depends on the energy of the reactants and the energy of the products (the difference between start and end energy levels). A catalyst only changes the pathway — the height of the peak — without altering what the reactants are or what the products are. Because the start and end levels remain the same, the energy difference (ΔH) is identical with or without the catalyst.
Q4.4 — Function of the transition state
The transition state (activated complex) is the highest-energy, partially bonded arrangement of atoms at the peak of the energy profile. It represents the point at which old bonds are partly broken and new bonds are partly formed. A molecule must reach this state for a reaction to proceed to products. Its energy relative to the reactant level is Ea.
Q5 — Sample concept map
A correct map should include arrows such as:
- catalyst — lowers → activation energy (Ea)
- activation energy (Ea) — is height from reactant level to → transition state
- lower Ea — increases proportion of successful collisions, increasing → reaction rate
- catalyst — does NOT change → ΔH
- energy profile diagram — displays both → Ea and ΔH
- energy profile diagram — shows peak representing → transition state
Award 1 mark per correct labelled arrow with accurate causal direction. ΔH and Ea must be shown as independent quantities for full marks.