JEE Chemistry — Chemical Kinetics Complete Chapter Guide

Chapter Overview & Weightage

Chemical Kinetics is one of those chapters where the JEE paper setters love to combine multiple concepts in a single question. The chapter carries consistent weightage — you can almost guarantee 1–2 questions every year.

Weightage Pattern: Chemical Kinetics contributes roughly 5–6% of the JEE Main Chemistry paper. In JEE Advanced, questions here tend to be multi-concept — linking rate law with Arrhenius or graphs with integrated rate equations.

Year	JEE Main (No. of Questions)	Marks	Key Topics Tested
2024	2	8	Half-life, Arrhenius equation
2023	1–2	4–8	Order determination, integrated rate law
2022	2	8	Rate law, activation energy
2021	1–2	4–8	Zero/first order, t₁/₂
2020	2	8	Rate constant units, collision theory
2019	1	4	Molecularity vs order

The chapter is highly predictable. The same concepts recycle — half-life, Arrhenius, order from graphs. If you master these core tools, you can solve 80% of the PYQs without seeing them before.

Key Concepts You Must Know

Prioritized by how frequently they appear in PYQs:

Tier 1 — Appears Almost Every Year

Rate law and rate constant — writing $r = k[A]^m[B]^n$ from experimental data
Integrated rate equations — zero order, first order (and how to identify which applies)
Half-life formulas — especially first-order $t_{1/2} = \frac{0.693}{k}$ (this is practically free marks)
Arrhenius equation — both the exponential form and the log form for calculating $E_a$

Tier 2 — Asked Frequently, Slightly More Nuanced

Order vs molecularity distinction — this is a favourite trick question source
Graph interpretation — [A] vs t, ln[A] vs t, 1/[A] vs t — identifying order from slope
Units of rate constant — deriving $k$ units from $\text{mol}^{1-n} \cdot L^{n-1} \cdot s^{-1}$

Tier 3 — Asked Occasionally in JEE Advanced

Collision theory — threshold energy, steric factor, orientation factor
Pseudo first-order reactions — why excess of one reactant simplifies the rate law
Temperature coefficient — the rule of thumb that rate doubles per 10°C rise

Important Formulas

r = k[A]^m[B]^n

When to use: Whenever you’re given concentration data at different times and asked for the rate constant or reaction order. The exponents $m$ and $n$ are determined experimentally — never assume they equal stoichiometric coefficients.

Zero Order:

[A] = [A]_0 - kt \qquad t_{1/2} = \frac{[A]_0}{2k}

First Order:

\ln[A] = \ln[A]_0 - kt \qquad t_{1/2} = \frac{0.693}{k}

Second Order:

\frac{1}{[A]} = \frac{1}{[A]_0} + kt \qquad t_{1/2} = \frac{1}{k[A]_0}

When to use: Match the linear graph — if $[A]$ vs $t$ is linear → zero order; if $\ln[A]$ vs $t$ is linear → first order; if $1/[A]$ vs $t$ is linear → second order.

k = Ae^{-E_a/RT}

Log form (used in numerical problems):

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

When to use: Any problem that gives you two rate constants at two temperatures and asks for activation energy, or vice versa. Always use $R = 8.314\ \text{J mol}^{-1}\text{K}^{-1}$ and convert temperature to Kelvin.

[k] = \text{mol}^{1-n} \cdot L^{n-1} \cdot s^{-1}

For zero order: $\text{mol L}^{-1} \text{s}^{-1}$

For first order: $\text{s}^{-1}$

For second order: $\text{L mol}^{-1} \text{s}^{-1}$

When to use: Unit-based questions that ask you to identify the reaction order from the units of $k$ alone. This is a 30-second question if you remember the pattern.

Solved Previous Year Questions

PYQ 1 — JEE Main 2024 (Shift 1)

Question: For a first-order reaction, the time required for 99% completion is how many times the time required for 50% completion?

Solution:

For first order, $t = \frac{2.303}{k} \log \frac{[A]_0}{[A]}$ .

For 99% completion: $[A] = 0.01[A]_0$

t_{99} = \frac{2.303}{k} \log \frac{[A]_0}{0.01[A]_0} = \frac{2.303}{k} \log 100 = \frac{2.303 \times 2}{k}

For 50% completion (half-life): $[A] = 0.5[A]_0$

t_{50} = \frac{2.303}{k} \log 2 = \frac{0.693}{k}

Ratio:

\frac{t_{99}}{t_{50}} = \frac{2.303 \times 2 / k}{0.693 / k} = \frac{4.606}{0.693} \approx 6.64 \approx \boxed{6.64}

Answer: ~6.64 times (often given as $\frac{2\log 100}{\log 2} = \frac{4}{0.301} \approx 6.64$ )

The ratio $t_{99}/t_{50}$ for first-order reactions is always $\approx 6.64$ . This specific ratio has appeared multiple times in different forms. Memorise it.

PYQ 2 — JEE Main 2022

Question: The activation energy of a reaction is 75 kJ mol⁻¹. The rate constant at 500 K is $1.0 \times 10^{-3}\ \text{s}^{-1}$ . What is the rate constant at 600 K? (R = 8.314 J mol⁻¹ K⁻¹)

Solution:

We use the two-temperature Arrhenius log form:

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

Substituting: $E_a = 75000\ \text{J mol}^{-1}$ , $T_1 = 500\ \text{K}$ , $T_2 = 600\ \text{K}$

\log\frac{k_2}{k_1} = \frac{75000}{2.303 \times 8.314} \times \frac{100}{500 \times 600}

= \frac{75000}{19.147} \times \frac{100}{300000}

= 3915.7 \times 3.33 \times 10^{-4} = 1.304

\frac{k_2}{k_1} = 10^{1.304} \approx 20.1

k_2 = 20.1 \times 1.0 \times 10^{-3} \approx \boxed{2.0 \times 10^{-2}\ \text{s}^{-1}}

Common error: Using $R = 8.314$ without converting $E_a$ from kJ to J first. Always check units — $E_a$ in J/mol, $R$ in J/mol/K. This unit mismatch costs marks every year.

PYQ 3 — JEE Advanced 2023

Question: For the reaction $A \to B$ , the rate doubles when concentration of A triples. What is the order of the reaction?

Solution:

Rate law: $r = k[A]^n$

Given: when $[A]$ becomes $3[A]$ , rate becomes $2r$ .

\frac{2r}{r} = \frac{k(3[A])^n}{k[A]^n} = 3^n

3^n = 2

n = \log_3 2 = \frac{\log 2}{\log 3} = \frac{0.301}{0.477} \approx 0.63

Answer: Order $\approx 0.63$ (fractional order)

Fractional order questions are more common in JEE Advanced than JEE Main. In JEE Main, orders are usually 0, 1, or 2. If you get a fractional answer in JEE Main, recheck your calculation.

Difficulty Distribution

For JEE Main, Chemical Kinetics questions break down roughly as:

Difficulty	Proportion	What it Tests
Easy	40%	Half-life formula, units of $k$ , identifying order from graphs
Medium	45%	Integrated rate law numerical, Arrhenius two-temperature problem
Hard	15%	Multi-step mechanisms, pseudo first-order, linking collision theory to rate

JEE Advanced flips this — expect 60% medium-hard, with graph analysis and multi-concept linking.

Expert Strategy

How toppers approach this chapter:

Step 1 — Lock down the graphs first. Before memorising formulas, understand why $\ln[A]$ vs $t$ being linear implies first order. When you understand the derivation, you never confuse the three graphs.

Step 2 — Practice Arrhenius numericals until they’re mechanical. There are only two types: find $E_a$ from two rate constants, or find $k$ at a new temperature. Drill 10 problems of each type — after that, 3 minutes per question is your target.

Step 3 — Molecularity vs order is a concept-clarity question, not a memory question. Molecularity is theoretical (mechanism-based, always a whole number). Order is experimental (can be fractional, zero, or negative). If you get confused, reread that sentence.

Time allocation in the exam: Chemical Kinetics questions are rarely ambiguous. Budget 3–4 minutes per question. If you’re crossing 5 minutes, mark and move — come back with fresh eyes.

Step 4 — PYQs from 2019–2024 are your goldmine. In JEE Main, the same question formats recycle with different numbers. Solve all 15–20 Chemical Kinetics PYQs from the last 6 years. You’ll notice 4–5 repeating templates.

Common Traps

Trap 1 — Confusing order with molecularity. Order is determined from the rate law (experimentally). Molecularity is the number of molecules in the rate-determining step. For $2A \to \text{products}$ , the order is NOT necessarily 2. The question “what is the molecularity of the following reaction?” is asking about mechanism, not kinetics.

Trap 2 — Half-life of zero-order reactions depends on initial concentration. For first order: $t_{1/2} = \frac{0.693}{k}$ (independent of $[A]_0$ ). For zero order: $t_{1/2} = \frac{[A]_0}{2k}$ (depends on $[A]_0$ ). Examiners frequently give you a zero-order reaction and let you apply the first-order half-life formula. Watch for the clue: if half-life changes as the reaction proceeds, you’re in zero-order territory.

Trap 3 — Arrhenius pre-exponential factor $A$ has the same units as $k$ . Students treat $A$ as dimensionless. It isn’t. $A$ has units of frequency (s⁻¹ for first order, L mol⁻¹ s⁻¹ for second order). Questions that ask for the ratio $k/A$ are asking for $e^{-E_a/RT}$ , which is always dimensionless and between 0 and 1.

Trap 4 — Pseudo first-order reactions look like first-order but aren’t. The reaction $A + B \to \text{products}$ can appear first order if $[B] \gg [A]$ . The observed rate constant $k' = k[B]$ is called the pseudo first-order rate constant. Questions sometimes give you $k'$ and $[B]$ and ask for the true $k$ — easy marks if you remember the definition.

The one formula that pays the most:

t_{1/2} = \frac{0.693}{k} \quad \text{(first order)}

Half-life problems appear in 70%+ of Chemical Kinetics PYQs in some form. If you can compute $k$ from $t_{1/2}$ and vice versa in under 30 seconds, you’ve secured easy marks before even reading the harder parts of the question.