NEET · Chemistry

Chemical Kinetics for NEET: Rate Laws, Order and Arrhenius Equation

Own official Unit 8 as a model-selection chapter: rate, factors affecting rate, rate law, order and molecularity, zero- and first-order integrated relations, half-life, Arrhenius equation, activation energy and qualitative collision theory at current NEET depth.

Subject: Chemistry
Syllabus unit: Current official NEET UG 2026 scope

In short

Chemical Kinetics describes reaction rate. Do not choose an integrated equation until reaction order is established. Zero-order and first-order reactions have different concentration-time and half-life relations, while Arrhenius connects rate constant with absolute temperature and activation energy.

Syllabus mapping

Syllabus mapping
Unit	Topics
Current official NEET UG 2026 scope	Reaction rate, Rate law, order and molecularity, Units of rate constant, Zero order, First order, Arrhenius and collision theory

Unit
Current official NEET UG 2026 scope
Topics
Reaction rate, Rate law, order and molecularity, Units of rate constant, Zero order, First order, Arrhenius and collision theory

Before this chapter

Concepts in this chapter

1. Reaction rate

For aA + bB -> cC + dD, a stoichiometrically normalised rate may be written: rate = -(1/a)d[A]/dt = -(1/b)d[B]/dt = (1/c)d[C]/dt = (1/d)d[D]/dt. The sign convention reports positive forward rate while reactant concentrations fall.

2. Rate law, order and molecularity

An empirical rate law can be rate = k[A]^m[B]^n; overall order is m+n. These exponents are determined kinetically and need not match coefficients in the overall balanced equation. Molecularity belongs to an elementary step and counts the reacting species in that step; it is conceptually different from overall reaction order.

3. Units of rate constant

If rate uses mol L^-1 s^-1 and total order is N, then k units are (mol L^-1)^(1-N) s^-1. This is a high-value dimensional check.

4. Zero order

For rate=k, integration gives [A]=[A]0-kt and t1/2=[A]0/(2k). The half-life depends on starting concentration.

5. First order

For rate=k[A], ln([A]0/[A])=kt, or k=(2.303/t)log10([A]0/[A]). The half-life is t1/2=ln2/k, approximately 0.693/k, independent of initial concentration within the first-order model.

6. Arrhenius and collision theory

k=A exp(-Ea/RT). Between two temperatures, ln(k2/k1)=(Ea/R)(1/T1-1/T2). T must be kelvin and Ea units must match R. Collision theory remains qualitative at this level: effective collisions need sufficient energy and suitable orientation; no unnecessary advanced derivation is added.

7. Decision aids for this chapter

Order-to-Equation Decision Tree: order -> integrated law -> half-life relation -> k units.
Dimensional Order Checker: observed k units -> implied order.
Arrhenius Sign Sanity Check: for positive Ea, higher T should normally give larger k in the model.

Formula sheet

Normalised rate expression: concentration/time. Balanced stoichiometry
Normalised rate expression
Use when — Balanced stoichiometry
rate=k[A]^m[B]^n: k units depend on order. Experimentally specified rate law
rate=k[A]^m[B]^n
Use when — Experimentally specified rate law
k units=(mol L^-1)^(1-N)s^-1: N total order. Rate unit mol L^-1 s^-1
k units=(mol L^-1)^(1-N)s^-1
Use when — Rate unit mol L^-1 s^-1
[A]=[A]0-kt: concentration/time. Zero order
[A]=[A]0-kt
Use when — Zero order
t1/2=[A]0/(2k): time. Zero order
t1/2=[A]0/(2k)
Use when — Zero order
ln([A]0/[A])=kt: dimensionless ratio. First order
ln([A]0/[A])=kt
Use when — First order
t1/2=0.693/k: time. First order
t1/2=0.693/k
Use when — First order
k=A exp(-Ea/RT): T K; compatible Ea/R units. Arrhenius model
k=A exp(-Ea/RT)
Use when — Arrhenius model
ln(k2/k1)=(Ea/R)(1/T1-1/T2): dimensionless log. Same reaction/model
ln(k2/k1)=(Ea/R)(1/T1-1/T2)
Use when — Same reaction/model

Worked examples

A first-order reaction has a half-life of 20.0 min. Calculate k in min^-1.

Original NEET-style item: A first-order reaction has a half-life of 20.0 min. Calculate k in min^-1.

Reasoning: k=0.693/t1/2=0.693/20.0=0.03465 min^-1, approximately 3.47 x 10^-2 min^-1. This result is valid because the reaction is explicitly first order.

Common mistakes and what they actually indicate

Cannot distinguish order from molecularity
Knowledge gap
Why it happens
Preparation Intelligence classification: Knowledge Gap.
How it is corrected
Tie order to rate law and molecularity to elementary step.
Forgets zero/first-order relations
Recall gap
Why it happens
Preparation Intelligence classification: Recall Gap.
How it is corrected
Retrieve equation with model condition.
Uses Celsius in Arrhenius
Execution error
Why it happens
Preparation Intelligence classification: Execution Error.
How it is corrected
Convert to kelvin.
Chooses first-order half-life because problem mentions half-life
Decision / selection error
Why it happens
Preparation Intelligence classification: Decision / Selection Error.
How it is corrected
Determine order first.
Order/mechanism is not stated or evidenced
Needs review
Why it happens
Preparation Intelligence classification: Needs Review.
How it is corrected
Do not infer from overall stoichiometry.

FAQ