NEET Chemistry — Thermodynamics Complete Chapter Guide

Chapter Overview & Weightage

Thermodynamics connects energy changes to chemical reactions. NEET focuses on the first law, Hess’s law, and Gibbs energy. The good news: the mathematics is simpler than in physics thermodynamics — mostly algebra with enthalpy and entropy.

Thermodynamics carries 4-5% weightage in NEET with 2-3 questions. Hess’s law calculations and Gibbs energy spontaneity predictions are the most tested.

Year	NEET Q Count	Key Topics Tested
2025	2	Hess’s law, entropy change
2024	3	Gibbs energy, bond energy
2023	2	Enthalpy of formation, spontaneity
2022	2	First law, Hess’s law
2021	3	Entropy, Gibbs energy, work

graph TD
    A[Thermodynamics] --> B[First Law]
    A --> C[Enthalpy]
    A --> D[Entropy]
    A --> E[Gibbs Energy]
    B --> F[q = ΔU + W]
    C --> G[Hess's Law]
    C --> H[Bond Enthalpy]
    C --> I[Enthalpy of Formation]
    D --> J[ΔS universe > 0]
    E --> K[ΔG = ΔH - TΔS]
    E --> L[Spontaneity Criteria]

Key Concepts You Must Know

Tier 1 (Always asked)

First law: $\Delta U = q + W$ (IUPAC sign convention)
Hess’s law calculations using formation enthalpies
Gibbs energy: $\Delta G = \Delta H - T\Delta S$
Spontaneity: $\Delta G < 0$ means spontaneous

Tier 2 (Frequently asked)

Bond enthalpy calculations: $\Delta H = \Sigma(\text{bonds broken}) - \Sigma(\text{bonds formed})$
Entropy and second law
Standard enthalpy of formation, combustion, neutralization
Kirchhoff’s equation (variation of $\Delta H$ with temperature)

Tier 3 (Occasional)

Work in isothermal reversible expansion: $W = -nRT\ln(V_2/V_1)$
Coupled reactions and Gibbs energy additivity

Important Formulas

\Delta U = q + W

Work done by system (expansion): $W = -P_{ext}\Delta V$

For reversible isothermal: $W = -nRT\ln\dfrac{V_2}{V_1} = -2.303nRT\log\dfrac{V_2}{V_1}$

For adiabatic: $q = 0$ , so $\Delta U = W$

\Delta H_{rxn} = \sum \Delta H_f(\text{products}) - \sum \Delta H_f(\text{reactants})

Bond enthalpy method:

\Delta H = \sum BE(\text{reactants}) - \sum BE(\text{products})

Note: bonds broken need energy (positive), bonds formed release energy (negative).

\Delta G = \Delta H - T\Delta S

$\Delta H$	$\Delta S$	Spontaneity
$-$	$+$	Always spontaneous
$+$	$-$	Never spontaneous
$-$	$-$	Spontaneous at low T
$+$	$+$	Spontaneous at high T

Equilibrium temperature: $T = \Delta H / \Delta S$ (when $\Delta G = 0$ )

For the spontaneity table, use the mnemonic: “Exothermic + disorder = always yes, Endothermic + order = always no.” The other two depend on temperature.

Solved Previous Year Questions

PYQ 1 — NEET 2024

Problem: For a reaction, $\Delta H = -40$ kJ/mol and $\Delta S = -100$ J/mol/K. At what temperature will the reaction become non-spontaneous?

Solution:

$\Delta G = 0$ at the transition temperature:

T = \frac{\Delta H}{\Delta S} = \frac{-40000}{-100} = \mathbf{400 \text{ K}}

Below 400 K: $\Delta G < 0$ (spontaneous). Above 400 K: $\Delta G > 0$ (non-spontaneous).

Watch the units. $\Delta H$ is often in kJ while $\Delta S$ is in J/K. Convert to the same units before dividing. This unit mismatch is the most common calculation error in this chapter.

PYQ 2 — NEET 2023

Problem: Using Hess’s law, find $\Delta H$ for the reaction: C(s) + $\frac{1}{2}$ O $_2$ (g) $\to$ CO(g), given: C(s) + O $_2$ (g) $\to$ CO $_2$ (g), $\Delta H_1 = -393$ kJ CO(g) + $\frac{1}{2}$ O $_2$ (g) $\to$ CO $_2$ (g), $\Delta H_2 = -283$ kJ

Solution:

Target: C + $\frac{1}{2}$ O $_2$ $\to$ CO

Use: Reaction 1 minus Reaction 2:

\Delta H = \Delta H_1 - \Delta H_2 = -393 - (-283) = \mathbf{-110 \text{ kJ}}

Difficulty Distribution

Difficulty	% of Questions	What to Expect
Easy	35%	Spontaneity prediction, definition-based
Medium	50%	Hess’s law, bond enthalpy calculation
Hard	15%	Multi-step Hess’s law, coupled reactions

Expert Strategy

Week 1: Master Hess’s law. The skill is algebraic manipulation of thermochemical equations. Practise reversing equations (change sign of $\Delta H$ ) and multiplying (scale $\Delta H$ ).

Week 2: Gibbs energy and the spontaneity table. Memorise the four cases and practise finding the transition temperature.

Week 3: PYQs. Thermodynamics questions in NEET follow 4-5 clear patterns. After 20 PYQs, you will recognise every question type.

Common Traps

Trap 1 — Sign convention for work. In chemistry (IUPAC), $W = -P_{ext}\Delta V$ . Work done BY the system is negative. This is opposite to the physics convention. Do not mix them up in NEET.

Trap 2 — Bond enthalpy formula direction. $\Delta H = \text{bonds broken} - \text{bonds formed}$ . Breaking bonds needs energy (positive). Forming bonds releases energy (negative when subtracted, it becomes negative on the result). Getting the order wrong flips the sign of $\Delta H$ .

Trap 3 — $\Delta H$ of formation of elements in standard state is zero. C(graphite), O $_2$ (g), N $_2$ (g), H $_2$ (g) all have $\Delta H_f = 0$ . Students sometimes include these in Hess’s law sums — that is an error.

Trap 4 — Entropy of the universe, not just the system. A reaction can have $\Delta S_{system} < 0$ and still be spontaneous if $\Delta S_{surroundings}$ is positive enough. The criterion is $\Delta G < 0$ (which accounts for both).