PYQ Vault

Playbook

Thermodynamics

Work of expansion, heat and the first law, Hess's law, then entropy and Gibbs energy. Signs decide most answers: who does work on whom, and which way heat flows.

Questions in the bank
128
q/paper in 2025–26
1.39
Calculation
65%
Notes pages
8

Strand: Calculate

When you’ll see it

Heat, work, ΔU, ΔH, ΔS or ΔG for a change, a bomb calorimeter, enthalpies to combine, or a temperature at which a reaction turns spontaneous.

How this chapter is tested

Most questions ask for a number, so the chapter rewards two habits above any formula. One sign convention: work done on the system is positive, so expansion work is −p_ext ΔV and ΔU = q + w. One unit check: ΔS and R come in joules, ΔH and ΔG in kilojoules.

The early pages settle heat, work and internal energy: reversible against irreversible work, free expansion, q = nCΔT, and ΔH = ΔU + Δn_g RT from a bomb calorimeter. The middle pages find a reaction enthalpy by whatever route the data allow: formation enthalpies, combustion enthalpies, Hess's law or bond enthalpies.

The last pages decide direction and extent. ΔG = ΔH − TΔS gives the sign cases and the crossover temperature T = ΔH/ΔS, and ΔG° = −2.303RT log K links the chapter to equilibrium. Distractors here are almost always the right size with the wrong sign, or a thousand times off.

The sub-skills

The distinct skills inside the chapter, in the order to learn them.

  • State functions and the first law

    ΔU = q + w with work on the system positive; state against path functions; intensive against extensive.

  • Work of expansion

    w = −p_ext ΔV; w_rev = −2.303 nRT log(V₂/V₁); zero into a vacuum; net work in a cycle is the enclosed area.

  • Heat capacity and calorimetry

    q = nCΔT with Cp − Cv = R; a bomb calorimeter gives ΔU; ΔH = ΔU + Δn_g RT counting gases only.

  • Hess's law

    Formation: products minus reactants; combustion: reactants minus products; or add, reverse and scale the given equations.

  • Phase change, solution and neutralisation

    ΔsubH = ΔfusH + ΔvapH; heat of neutralisation follows the limiting reagent and warms the whole mixed volume.

  • Bond enthalpies

    ΔrH = Σ bonds broken − Σ bonds formed, valid only when every species is a gas.

  • Entropy, Gibbs energy and spontaneity

    ΔG = ΔH − TΔS; ΔH < 0 with ΔS > 0 is spontaneous at all temperatures; ΔG = 0 at T = ΔH/ΔS.

  • Gibbs energy and K

    ΔG° = −2.303RT log K; the slope of log K against 1/T is −ΔH°/2.303R.

Traps to expect

Distractor shapes this chapter reuses. The Traps page covers the ones that cut across chapters.

  • Joules beside kilojoules

    With ΔH = 60 kJ and ΔS = 150 J K⁻¹, T = 60 000/150 = 400 K, not 0.4 K. The same slip makes a Δn_g RT correction a thousand times too big.

  • Signs reversed

    Broken minus formed, not formed minus broken; combustion data run reactants minus products. The sign-flipped value is always printed.

  • Liquid water counted as gas

    For C₂H₆(g) + 7/2 O₂(g) → 2CO₂(g) + 3H₂O(l), Δn_g = 2 − 4.5 = −2.5, not +0.5.

  • log for ln

    nRT log(V₂/V₁) without the 2.303 is 2.303 times too small, and the small value is always an option.

  • Bomb heat read as ΔH

    A bomb calorimeter runs at constant volume, so it measures ΔU. An enthalpy needs the Δn_g RT step, and the answer without it sits a few kJ away.

Learn it before you drill it

This chapter has full teaching notes — foundations, worked examples, self-checks and a mastery check for each page. Read the notes once, then drill page by page below.

Thermodynamics notes

Drill every Thermodynamics question

128 questions from the bank, across 8 subtopics.

Drill the 128 questions

Drill one subtopic at a time

The 8 subtopics, in teaching order.

Related playbooks

Often paired with this one — the technique or the trap overlaps. Drill these next.