Enthalpy & Energy Profile Diagrams

Q1 — Sample Band 6 response (8 marks), annotated

Enthalpy change (ΔH) is the heat energy exchanged between the system and surroundings at constant pressure, equal to H(products) − H(reactants). A negative ΔH indicates that energy flows from the system to the surroundings (exothermic). [1 — definition + sign interpretation]

Both combustions are exothermic (ΔH < 0). Propane releases more energy per mole (−2220 kJ mol⁻¹) than methane (−890 kJ mol⁻¹), so more heat is available per mole of propane burned — an advantage for a generator needing sustained output. [1 — ΔH comparison criterion] The activation energies are similar: methane Ea = 420 kJ mol⁻¹; propane Ea = 440 kJ mol⁻¹. Both fuels require comparable energy input to initiate combustion — ignition difficulty is not a significant differentiating factor. [1 — E_a comparison criterion]

E_a(rev) for methane: E_a(rev) = E_a(fwd) − ΔH = 420 − (−890) = 1310 kJ mol⁻¹. E_a(rev) for propane: 440 − (−2220) = 2660 kJ mol⁻¹. The very large reverse Ea values confirm both combustions are highly thermodynamically favoured in the forward direction — neither reaction will spontaneously reverse under generator operating conditions. [1 — E_a(rev) calculations, both correct] [1 — interpretation of reverse Ea]

In the Karratha LNG context, methane (natural gas) is the fuel produced on-site and already piped to facility infrastructure. Using propane would require separate transport and storage, adding logistical complexity and cost in a remote coastal environment. [1 — Australian context applied]

Recommendation: methane is the preferred fuel for the Karratha emergency generator. Although propane releases more energy per mole, methane is produced and stored on-site, has essentially the same activation energy (only 20 kJ mol⁻¹ lower Ea, a negligible ignition difference), and avoids introducing a second fuel type into an already safety-critical facility. A practical limitation is that methane’s lower energy density per mole means a larger volume of gas must be burned to deliver the same energy output as propane — storage tank sizing must account for this. [1 — justified recommendation] [1 — practical limitation identified]

Marking criteria (8 marks):

• 1 mark — Defines ΔH correctly and states that its sign indicates direction of energy flow (negative = exothermic).
• 1 mark — Compares ΔH per mole: propane releases more energy per mole (−2220 vs −890 kJ mol⁻¹).
• 1 mark — Compares E_a(fwd): values are similar (420 vs 440); ignition difficulty is comparable.
• 1 mark — Calculates E_a(rev) for methane correctly (1310 kJ mol⁻¹).
• 1 mark — Calculates E_a(rev) for propane correctly (2660 kJ mol⁻¹).
• 1 mark — Interprets the large reverse E_a values: combustion is thermodynamically irreversible under operating conditions; the high reverse E_a confirms forward reaction strongly favoured.
• 1 mark — Names the Karratha LNG context and applies it to the recommendation (on-site availability, logistical argument).
• 1 mark — Reaches a justified, evidence-based recommendation that identifies a practical limitation (e.g. lower energy density of methane means larger gas volume needed).

Q2 — Source critique sample response (7 marks), annotated

Error 1: “exothermic reaction” — cold packs use endothermic reactions (dissolution of NH₄NO₃). ΔH > 0. [1 — correct identification & correction; Card 02]

Error 2: “The reaction gets cold” — the reaction does not “get cold”; the surroundings (including your hand) cool down because heat flows from the surroundings into the endothermic system. Precise language: system and surroundings must be distinguished. [1 — system/surroundings language; Card 02 common error callout]

Error 3: “E_a is the total height of the peak from the bottom of the graph” — E_a is measured from the reactant enthalpy level to the peak, not from the x-axis or absolute zero. The position of the reactant level on the y-axis is arbitrary; only differences between levels are meaningful. [1 — E_a measurement rule; Card 02 diagram section]

Error 4: “A higher peak means a faster reaction” — a higher E_a means a slower reaction (fewer collisions have sufficient energy to overcome a larger barrier). Higher E_a = slower, not faster. [1 — E_a and rate relationship; Card 02 + Module 3 connection]

Error 5: “only applies if you double the coefficients” — the sign reversal when reversing ΔH applies to any reversal of the equation, regardless of coefficients. The coefficients affect the magnitude (scaling rule), but the sign reversal is simply a consequence of reversing the reaction direction. [1 — thermochemical equation rules; Card 03]

Corrected paragraph (2 marks for accuracy and precision): An instant cold pack works because it contains ammonium nitrate, which undergoes an endothermic dissolution when mixed with water (ΔH > 0). In an endothermic reaction, energy flows from the surroundings into the system, so the surroundings (including the injured area) cool down — the reaction absorbs heat from your hand. On an energy profile diagram, the activation energy is measured from the reactant enthalpy level up to the transition state peak — not from the bottom of the graph. A larger activation energy means fewer collisions have sufficient energy to react, so the reaction is slower. Reversing a thermochemical equation always flips the sign of ΔH; scaling the coefficients changes the magnitude. These two rules are independent.

Marking criteria (7 marks):

• 1 mark — Identifies and corrects Error 1: cold packs use endothermic, not exothermic, reactions.
• 1 mark — Identifies and corrects Error 2: the surroundings cool down (not “the reaction gets cold”); precise system/surroundings language.
• 1 mark — Identifies and corrects Error 3: E_a is from the reactant level to the peak, not from the x-axis.
• 1 mark — Identifies and corrects Error 4: higher E_a = slower reaction, not faster.
• 1 mark — Identifies and corrects Error 5: sign reversal applies to any reversal of the equation, not only when coefficients are doubled.
• 2 marks — Corrected paragraph: 1 mark for scientific accuracy (all corrections integrated); 1 mark for precision and appropriate lesson terminology (ΔH, endothermic, activation energy, surroundings, thermochemical equation).