Chemistry • Year 11 • Module 4 • Lesson 5

Activation Energy, Catalysts & Energy Diagrams

Build HSC Band 5–6 extended-response technique on activation energy, catalyst action, and the distinction between kinetic and thermodynamic quantities.

Master · Extended Response

1. Extended response — catalytic converters and industrial catalysis (Band 5–6)

7 marks Band 5–6

The graph below shows the energy profile for the oxidation of carbon monoxide in a modern Australian catalytic converter (Pt/Pd/Rh on ceramic honeycomb). The converter was introduced into Australian passenger vehicles under Australian Design Rule (ADR) 37 from the 1980s onwards.

Figure 1.1. Energy profile for CO + ½O₂ → CO₂ with and without Pt/Pd/Rh catalytic converter surface. Values representative of published surface-catalysis literature.

Q1. Using the graph and your knowledge of this topic, evaluate the claim that the catalytic converter improves the energetics of the CO oxidation reaction. In your response you must:

Define activation energy and explain how it is measured from the graph.
State what the catalyst changes AND what it does not change, with specific reference to the values shown.
Explain, using the concept of E_a, why the catalytic converter is effective at typical exhaust temperatures (400–600°C).
Justify why ΔH is the same for both pathways shown.
Classify the Pt/Pd/Rh catalyst and justify your classification using physical states.

Structure suggestion: paragraph 1 — define E_a and read values from graph; paragraph 2 — what the catalyst changes (E_a lowered 232→75) vs what it doesn't (ΔH = −283 both); paragraph 3 — mechanism at exhaust temperature (greater fraction of molecules have energy ≥ 75); paragraph 4 — why ΔH unchanged; paragraph 5 — catalyst classification + explicit judgement on the claim.

2. Source critique — a textbook claim about enzymes (Band 5–6)

7 marks Band 5–6

Source:

"Enzymes are biological catalysts that make chemical reactions possible inside living cells. Without enzymes, the reactions needed for life simply could not happen — they would have no energy source. Enzymes provide the energy needed for these reactions to proceed. They also permanently alter the energetics of a reaction — the final products that form in the presence of an enzyme are lower in energy than those produced without one, releasing more heat and making the reaction more exothermic."

— Adapted from a Year 10 science supplement, NSW, 2023 (names omitted)

Q2. This passage contains three distinct scientific errors about how enzymes (biological catalysts) work. For each error:

Identify and quote the specific claim from the source.
Explain why it is scientifically incorrect.
State the scientifically accurate version.

Your answer must use the terms: activation energy (E_a), ΔH, alternative pathway. Reference the energy profile diagram concept where appropriate.

The three errors involve: (1) what enzymes "provide" (energy vs a lower-E_a pathway); (2) whether reactions are impossible without enzymes; (3) whether enzymes change ΔH or the identity/energy of products.

Answers & Marking Guidelines — Do not peek before attempting

Q1 — Evaluate the claim (7 marks)

Marking notes:

[1 mark] Defines activation energy correctly: the minimum energy reactants must possess for a successful collision to occur, measured from the reactant energy level to the peak of the energy profile diagram. Correct graph reading: E_a(uncat) = 232 kJ mol⁻¹, E_a(cat) = 75 kJ mol⁻¹.

[1 mark] States clearly what the catalyst changes — lowers E_a from 232 to 75 kJ mol⁻¹ (a reduction of 157 kJ mol⁻¹) — and what it does not change: ΔH remains −283 kJ mol⁻¹, reactants and products are identical, and the catalyst is not consumed.

[1 mark] Explains effectiveness at exhaust temperatures: without converter, few CO molecules at 400–600°C possess ≥ 232 kJ mol⁻¹; with Pt/Pd/Rh surface, E_a = 75 kJ mol⁻¹, so a much larger proportion of molecules have sufficient energy, dramatically increasing rate.

[1 mark] Explains mechanism: Pt/Pd/Rh provides an alternative pathway (CO and O₂ adsorb onto metal surface, bonds weaken, reaction proceeds, CO₂ desorbs) with lower-energy transition state at each step.

[1 mark] Justifies why ΔH is unchanged: ΔH = E(products) − E(reactants) — these levels are identical in both pathways (same reactants CO+O₂, same products CO₂). The catalyst only changes the peak height, not the start or end levels.

[1 mark] Classifies correctly as heterogeneous: Pt/Pd/Rh = solid; exhaust gases = gaseous; solid ≠ gaseous → heterogeneous. Full justification referring to different phases.

[1 mark] Reaches an explicit evaluative judgement: the claim is false. The catalytic converter does not improve the energetics of the reaction — ΔH is unchanged. It improves the kinetics by lowering E_a, making the reaction proceed at a useful rate at exhaust temperatures.

Sample high-band response (abbreviated): Activation energy is the minimum energy required for colliding reactants to form the transition state and proceed to products; on the graph it is the vertical distance from the reactant energy level (0 kJ mol⁻¹) to the peak. The Pt/Pd/Rh catalyst lowers E_a from 232 to 75 kJ mol⁻¹ by providing an alternative mechanism in which CO and O₂ adsorb onto the metal surface, a lower-energy transition state forms at the surface, and CO₂ desorbs as product. ΔH = −283 kJ mol⁻¹ for both pathways — this is unchanged because reactants (CO + O₂) and products (CO₂) are identical, and ΔH depends only on their energy difference, not on the pathway. At 400–600°C, without the converter, only an extremely small fraction of CO molecules possess ≥ 232 kJ mol⁻¹ of kinetic energy, so uncatalysed oxidation is negligibly slow. With the converter, E_a = 75 kJ mol⁻¹: a much larger fraction of molecules at the same temperature now meet this lower threshold, so the rate increases dramatically and near-complete conversion occurs. The Pt/Pd/Rh is heterogeneous (solid catalyst; gaseous reactants; solid ≠ gas). Therefore the claim is false: the catalyst improves the rate (kinetics) — not the energetics (ΔH) — of CO oxidation.

Q2 — Source critique (7 marks)

Error 1 — "Enzymes provide the energy needed for these reactions to proceed." [2 marks]

This is incorrect. Enzymes do not supply energy to reactions. [1 mark] An enzyme lowers the activation energy (E_a) by providing an alternative reaction pathway — its active site stabilises the transition state, reducing the energy barrier. Reactant molecules still require thermal energy from their environment to reach the (now lower) transition state; the enzyme simply reduces how much energy is required. On an energy profile diagram, the enzyme lowers the peak, but does not add energy to the system. [1 mark]

Error 2 — "Without enzymes, the reactions needed for life simply could not happen." [2 marks]

This overstates the case. Enzymatically catalysed reactions are thermodynamically spontaneous (ΔG < 0) and can occur without the enzyme — they proceed uncatalysed, just at an extremely slow rate that is incompatible with sustaining life on a biological timescale. [1 mark] The enzyme increases rate by lowering E_a; it does not make an otherwise thermodynamically impossible reaction occur. The claim confuses kinetics (rate, E_a) with thermodynamics (whether a reaction can occur, ΔG). [1 mark]

Error 3 — "Enzymes permanently alter the energetics … products are lower in energy … more exothermic." [3 marks]

This is incorrect. An enzyme does not change ΔH (the enthalpy change) of a reaction. [1 mark] The products formed in the presence of an enzyme are chemically identical to those formed without one — they are at the same energy level. Because reactants and products are unchanged, the energy difference between them (ΔH) is identical with or without the enzyme. On an energy profile diagram, the reactant level and product level are the same in both pathways; only the peak (transition state) is lower in the catalysed pathway. [1 mark] The correct statement is: enzymes lower E_a only; ΔH, and therefore whether the reaction is exothermic or endothermic, is entirely unchanged by the presence of the enzyme. [1 mark]