NEET Chemistry — Chemical Kinetics Complete Chapter Guide

Chapter Overview & Weightage

Chemical Kinetics is about HOW FAST reactions happen. While thermodynamics tells us whether a reaction is possible, kinetics tells us whether it will actually occur at a meaningful rate. NEET loves first-order kinetics and the Arrhenius equation.

Chemical Kinetics carries 3-4% weightage in NEET with 1-2 questions. First-order rate law and Arrhenius equation are tested most frequently.

Year	NEET Q Count	Key Topics Tested
2025	2	Half-life, Arrhenius equation
2024	1	First-order integrated rate law
2023	2	Order determination, activation energy
2022	2	Rate law, half-life
2021	1	Arrhenius, temperature dependence

graph TD
    A[Chemical Kinetics] --> B[Rate of Reaction]
    A --> C[Rate Law]
    A --> D[Integrated Rate Laws]
    A --> E[Temperature Dependence]
    C --> F[Order of Reaction]
    C --> G[Rate Constant k]
    D --> H[Zero Order]
    D --> I[First Order]
    D --> J[Second Order]
    E --> K[Arrhenius Equation]
    E --> L[Activation Energy]
    E --> M[Collision Theory]

Key Concepts You Must Know

Tier 1 (Always asked)

Rate law: $r = k[A]^m[B]^n$ (order = $m + n$ )
First-order: $k = \dfrac{2.303}{t}\log\dfrac{[A]_0}{[A]_t}$ , $t_{1/2} = 0.693/k$
Arrhenius equation: $k = Ae^{-E_a/RT}$
Half-life of first-order is independent of initial concentration

Tier 2 (Frequently asked)

Zero order: $[A] = [A]_0 - kt$ , $t_{1/2} = [A]_0/(2k)$
Determining order from experimental data
Units of rate constant for different orders
Effect of temperature: rate roughly doubles for every 10 K rise

Tier 3 (Occasional)

Collision theory and steric factor
Pseudo first-order reactions
Molecularity vs order distinction

Important Formulas

Order	Rate Law	Integrated Form	Half-life	Units of k
0	$r = k$	$[A] = [A]_0 - kt$	$[A]_0/(2k)$	mol L $^{-1}$ s $^{-1}$
1	$r = k[A]$	$\ln[A] = \ln[A]_0 - kt$	$0.693/k$	s $^{-1}$
2	$r = k[A]^2$	$1/[A] = 1/[A]_0 + kt$	$1/(k[A]_0)$	L mol $^{-1}$ s $^{-1}$

k = Ae^{-E_a/RT}

Log form: $\log k = \log A - \dfrac{E_a}{2.303RT}$

Two-temperature form:

\log\frac{k_2}{k_1} = \frac{E_a}{2.303R}\left(\frac{1}{T_1} - \frac{1}{T_2}\right)

For first-order reactions, the fraction remaining after $n$ half-lives is $(1/2)^n$ . After 3 half-lives, only 12.5% remains. After 10 half-lives, less than 0.1% remains. This shortcut eliminates the need for log calculations in many NEET problems.

Solved Previous Year Questions

PYQ 1 — NEET 2024

Problem: A first-order reaction has a rate constant of $0.0693$ min $^{-1}$ . How long will it take for 75% of the reactant to decompose?

Solution:

75% decomposed means 25% remains, so $[A]/[A]_0 = 0.25 = 1/4 = (1/2)^2$ .

This means 2 half-lives have passed.

t_{1/2} = \frac{0.693}{0.0693} = 10 \text{ min}

t = 2 \times 10 = \mathbf{20 \text{ min}}

PYQ 2 — NEET 2023

Problem: If the activation energy of a reaction is 50 kJ/mol, by what factor does the rate increase when temperature is raised from 300 K to 310 K? ( $R = 8.314$ J/mol/K)

Solution:

\log\frac{k_2}{k_1} = \frac{E_a}{2.303R}\left(\frac{1}{T_1} - \frac{1}{T_2}\right)

= \frac{50000}{2.303 \times 8.314}\left(\frac{1}{300} - \frac{1}{310}\right)

= \frac{50000}{19.147}\left(\frac{10}{93000}\right) = 2612 \times 1.075 \times 10^{-4} = 0.281

\frac{k_2}{k_1} = 10^{0.281} \approx \mathbf{1.91}

Rate approximately doubles — consistent with the general rule.

Difficulty Distribution

Difficulty	% of Questions	What to Expect
Easy	40%	Half-life, order from units
Medium	45%	Arrhenius two-temperature, integrated rate law
Hard	15%	Order determination from data, pseudo reactions

Expert Strategy

Week 1: First-order kinetics is the most tested. Master the integrated rate law, half-life, and the “fraction remaining” shortcut.

Week 2: Arrhenius equation — both the log form and the two-temperature form. Most NEET problems give you two temperatures and two rate constants and ask for $E_a$ .

Week 3: Distinguishing order from experimental data. If $t_{1/2}$ is constant, it is first order. If $t_{1/2}$ changes with concentration, check zero or second order.

Common Traps

Trap 1 — Molecularity and order are different. Molecularity is a theoretical concept (number of molecules in the rate-determining step) and is always a whole number. Order is experimental and can be fractional. A reaction can be bimolecular but first order overall.

Trap 2 — Half-life of zero-order depends on concentration. $t_{1/2} = [A]_0/(2k)$ . Unlike first order, if you double the initial concentration, the half-life doubles. Students assume all half-lives are constant — only first-order has this property.

Trap 3 — Units of $E_a$ in Arrhenius. Use J/mol when $R = 8.314$ J/mol/K. If $E_a$ is given in kJ/mol, convert to J/mol first. This is the same unit trap as in thermodynamics.