NEET Chemistry — Physical Chemistry Complete Chapter Guide

Chapter Overview & Weightage

Physical Chemistry covers atomic structure, states of matter, thermodynamics, equilibrium, electrochemistry, kinetics, and solutions. This unit is calculation-heavy — strong numerical skills directly translate to high marks.

Physical Chemistry carries 25-30% weightage in NEET Chemistry — 12-15 questions. Thermodynamics, equilibrium (pH problems), and kinetics (rate laws) contribute the most questions.

Key Concepts You Must Know

Tier 1 (Core)

Atomic structure: quantum numbers, electronic configuration, Aufbau/Hund’s/Pauli principles
States of matter: ideal gas law $PV = nRT$ , kinetic theory, real gases (van der Waals)
Thermodynamics: $\Delta G = \Delta H - T\Delta S$ , spontaneity criteria, Hess’s law
Chemical equilibrium: $K_c$ , $K_p$ , Le Chatelier’s principle, relation $K_p = K_c(RT)^{\Delta n}$
Ionic equilibrium: pH = $-\log[H^+]$ , buffer solutions, common ion effect, solubility product $K_{sp}$
Electrochemistry: Nernst equation, Faraday’s laws, conductance
Chemical kinetics: rate law, order, integrated rate equations, half-life, Arrhenius equation
Solutions: Raoult’s law, colligative properties (boiling point elevation, freezing point depression, osmotic pressure)

Important Formulas

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

Spontaneous: $\Delta G < 0$

\Delta G° = -RT\ln K = -2.303RT\log K

Hess’s law: $\Delta H_{rxn} = \sum \Delta H_f(\text{products}) - \sum \Delta H_f(\text{reactants})$

Zero order: $[A] = [A]_0 - kt$ , $t_{1/2} = [A]_0/(2k)$

First order: $\ln[A] = \ln[A]_0 - kt$ , $t_{1/2} = 0.693/k$ (independent of concentration)

Second order: $1/[A] = 1/[A]_0 + kt$ , $t_{1/2} = 1/(k[A]_0)$

Arrhenius: $k = Ae^{-E_a/RT}$ , $\ln(k_2/k_1) = \frac{E_a}{R}\left(\frac{1}{T_1} - \frac{1}{T_2}\right)$

Relative lowering of vapour pressure: $\frac{P° - P}{P°} = x_{\text{solute}}$

Boiling point elevation: $\Delta T_b = K_b \cdot m \cdot i$

Freezing point depression: $\Delta T_f = K_f \cdot m \cdot i$

Osmotic pressure: $\pi = iCRT$

Where $i$ = van’t Hoff factor (1 for non-electrolytes, 2 for NaCl, 3 for CaCl $_2$ )

For first-order kinetics, $t_{1/2}$ is independent of initial concentration — this fact alone identifies a reaction as first order. NEET tests this: “if half-life doesn’t change with concentration, the order is ___.” Answer: first order.

Solved Previous Year Questions

PYQ 1 — NEET 2024

Problem: For a reaction at 25 degrees C, $\Delta H = -10$ kJ and $\Delta S = +30$ J/K. Is the reaction spontaneous?

Solution:

$\Delta G = \Delta H - T\Delta S = -10000 - (298)(30) = -10000 - 8940 = -18940$ J = $-18.94$ kJ

$\Delta G < 0$ → spontaneous

Note: both favourable enthalpy ( $\Delta H < 0$ ) and entropy ( $\Delta S > 0$ ) — spontaneous at all temperatures.

PYQ 2 — NEET 2023

Problem: The half-life of a first-order reaction is 10 minutes. What fraction remains after 30 minutes?

Solution:

After $n$ half-lives: fraction = $(1/2)^n = (1/2)^3 = \mathbf{1/8}$

PYQ 3 — NEET 2022

Problem: 0.1 M NaCl solution has a van’t Hoff factor $i = 1.8$ . The osmotic pressure at 27 degrees C is:

Solution:

$\pi = iCRT = 1.8 \times 0.1 \times 0.0821 \times 300 = \mathbf{4.43 \text{ atm}}$

Expert Strategy

Weeks 1-2: Thermodynamics and equilibrium — connected topics. Understand $\Delta G$ , $K$ , and Le Chatelier as different ways to predict reaction direction.

Weeks 3-4: Kinetics and electrochemistry. Kinetics is formula-driven. Electrochemistry connects to thermodynamics via $\Delta G = -nFE$ .

Week 5: Solutions and atomic structure. Colligative properties use the same formula template (property = constant $\times$ molality $\times$ $i$ ). Atomic structure is NCERT-based.

Common Traps

Trap 1 — Convert $\Delta S$ units to match $\Delta H$ . $\Delta H$ is often in kJ, $\Delta S$ in J/K. Convert to the same unit before computing $\Delta G$ . Missing this gives answers off by a factor of 1000.

Trap 2 — $K_p = K_c(RT)^{\Delta n}$ only when $\Delta n \neq 0$ . If $\Delta n = 0$ (same moles of gas on both sides), $K_p = K_c$ . Students often apply the correction even when it’s unnecessary.

Trap 3 — Van’t Hoff factor $i$ is NOT always the theoretical value. NaCl theoretically gives $i = 2$ , but actual $i$ may be 1.8 due to ion pairing. NEET sometimes gives the actual $i$ value — use it, don’t assume the theoretical.

Trap 4 — pH of a strong acid is simply $-\log C$ (no $K_a$ needed). For weak acids: $pH = -\log\sqrt{K_a \cdot C}$ . Students use the weak acid formula for strong acids or vice versa.