Chemical Kinetics

Syllabus Unit: Physical Chemistry

Why it matters

Chapter overview

Chemical kinetics answers the question thermodynamics ignores: not whether a reaction happens, but how fast, and why. This topic runs from measuring rate and writing experimentally-determined rate laws, through the sharp distinction between order and molecularity, to integrated rate laws that predict concentration over time, and finally the Arrhenius equation linking rate to temperature and the activation energy barrier every reaction must clear.

Chemistry Syllabus Structure Physical Chemistry ~33% weightage

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1Vary [A], Fix [B]Measure the initial rate

2Compare Rate RatiosRate ∝ [A]^m — find m

3Vary [B], Fix [A]Find order n in B

4Write the Rate LawRate = k[A]^m[B]^n

1Rate of Reaction & Rate Laws

A rate law can only be found by running the experiment — stoichiometry alone never tells you how fast a reaction actually goes.

Learning Objectives

Define rate of reaction and express it correctly using stoichiometric normalization.
Write a rate law, rate = k[A]^m[B]^n, and state that its exponents must be found experimentally.
Determine each reactant's order from initial rate data using the method of initial rates.
Distinguish average rate (over an interval) from instantaneous rate (a single moment).

Key Terms

Rate of ReactionThe change in concentration of a reactant or product per unit time, always expressed as a positive quantity.

Rate LawRate = k[A]^m[B]^n, an experimentally determined equation relating rate to reactant concentrations.

Order of Reaction (wrt a species)The power to which that species' concentration is raised in the rate law, found only by experiment.

Method of Initial RatesComparing initial rates across experiments with different starting concentrations to determine each reactant's order.

Kinetics asks 'how fast', a question entirely separate from thermodynamics's 'how far' — a thermodynamically favourable reaction (ΔG < 0) can still be too slow to observe without a catalyst or higher temperature.

Defining rate

Rate = -(1/a)Δ[A]/Δt = -(1/b)Δ[B]/Δt = +(1/c)Δ[C]/Δt, for aA + bB → cC

Average rate is measured over a finite time interval; instantaneous rate is the slope of the concentration-vs-time curve at one specific instant (as Δt → 0).

The rate law

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

Common MistakeThe exponents m and n are NOT simply the stoichiometric coefficients from the balanced equation — they must be determined experimentally. This is one of the most tested distinctions in all of chemical kinetics.

Method of initial rates

Run several experiments, varying one reactant's initial concentration while holding the others fixed.
Compare how the initial rate changes to deduce that reactant's order; repeat for each reactant.
Example: doubling [A] alone doubles the rate → order 1 in A. Doubling [B] alone quadruples the rate → order 2 in B. Overall order = 1 + 2 = 3.