Pattern 1: Work Calculation (appears nearly every year)

Question type: Calculate work done in isothermal reversible expansion of $n$ mol ideal gas. Key formula: $w = -nRT\ln(V_2/V_1)$ Trap: Sign confusion — work BY system is negative. Practice: NEET 2016, 2018, 2021 — all tested this formula.

Pattern 2: $\Delta H$ vs $\Delta U$ Distinction

Question type: Given $\Delta H$ or $\Delta U$, calculate the other for a specific reaction. Key formula: $\Delta H = \Delta U + \Delta n_g RT$ Trap: Including solids/liquids in $\Delta n_g$. Practice: Appears in NEET 2013, 2015, 2019.

Pattern 3: $\Delta G$ and Spontaneity Prediction

Question type: Given $\Delta H$ and $\Delta S$, predict spontaneity or find crossover temperature. Key formula: $\Delta G = \Delta H - T\Delta S = 0$ at crossover → $T = \Delta H/\Delta S$ Trap: Using $\Delta G^\circ = 0$ instead of $\Delta G = 0$ for equilibrium. Practice: NEET 2017, 2020, 2022 — direct application questions.

Pattern 4: Hess's Law and Bond Enthalpies

Question type: Calculate $\Delta H$ using formation enthalpies or bond energies. Key formula: $\Delta H = \Sigma(BE_{broken}) - \Sigma(BE_{formed})$ Trap: Wrong bond counting (especially in ethane, ethylene, benzene). Practice: NEET 2014, 2016, 2023.

Pattern 5: Assertion-Reason on Equilibrium vs Standard

Question type: A: "$\Delta G = 0$ at equilibrium"; R: "because $\Delta G^\circ = 0$" Answer: A is true, R is false. Trap: This exact question appeared NEET 2019 and similar versions in NEET 2021.

Key Statistics

• Thermodynamics contributes 2-3 questions per year (5-7.5% of chemistry marks)
• Most questions test concepts from Gibbs equation and Hess's law
• Numerical questions always involve $\Delta G$ calculation or work done in expansion