Chapter 3 - Chemical Kinetics

Chemical Kinetics explains how fast a chemical reaction occurs, how its rate is measured and which factors can change it. Thermodynamics can indicate whether a reaction is feasible, but kinetics determines whether it proceeds in a fraction of a second or takes years. This distinction is essential for understanding combustion, food spoilage, medicines, industrial synthesis and enzyme-controlled processes.

The chapter develops rate expressions, rate laws, reaction order, integrated equations, half-life, activation energy and collision theory. It is highly scoring when students understand the logic behind graphs, units and equations.

Rate of a chemical reaction

The rate of reaction is the change in concentration of a reactant or product per unit time. Reactant concentration decreases, so a negative sign is used when rate is written in terms of reactant disappearance. Product concentration increases, so its rate expression is positive.

For a balanced reaction, stoichiometric coefficients must be included to obtain a single consistent reaction rate. Average rate is measured over a finite time interval, whereas instantaneous rate is the slope of the concentration-time curve at a particular moment.

Factors affecting reaction rate

Reaction rate may depend on:

• concentration of reactants;

• pressure for gaseous reactions;

• temperature;

• the physical state and surface area of reactants;

• the nature of the reactants; and

• the presence of a catalyst.

Greater concentration generally increases collision frequency. Higher temperature increases both collision frequency and the fraction of molecules with sufficient energy to react. A catalyst supplies an alternative pathway with lower activation energy without changing the overall thermodynamic equilibrium.

Rate law, rate constant and reaction order

The rate law expresses the experimentally observed dependence of rate on reactant concentrations. The powers of concentration terms are not generally taken from the balanced overall equation; they must be determined experimentally unless the reaction is an elementary step.

The sum of these powers is the overall order of the reaction. Order may be zero, fractional or integral. The rate constant k is the proportionality constant in the rate law, and its units depend on the overall order. Molecularity, in contrast, describes the number of reacting species in an elementary step and is always a positive integer.

Integrated rate equations

Integrated rate equations relate concentration to time. Class 12 mainly focuses on zero-order and first-order reactions.

For a zero-order reaction, concentration decreases linearly with time. A plot of concentration against time is a straight line, and the half-life depends directly on initial concentration.

For a first-order reaction, the logarithm of concentration changes linearly with time. Its half-life is independent of initial concentration and equals 0.693/k. Radioactive decay is a familiar first-order example.

Questions may provide concentration, pressure or another proportional measurable quantity. Students should first identify the correct integrated form and then keep logarithm conventions and units consistent.

Pseudo-first-order reactions

A reaction involving more than one reactant can behave as first order when one reactant is present in such large excess that its concentration remains effectively constant. The constant concentration becomes part of the observed rate constant. Hydrolysis reactions carried out in excess water often illustrate this idea.

Temperature dependence and the Arrhenius equation

Reaction rates usually increase significantly with temperature. The Arrhenius equation relates the rate constant to temperature and activation energy. Its logarithmic forms make it possible to calculate activation energy from rate constants at two temperatures or from the slope of a graph.

Activation energy is the energy barrier separating reactants from products. A catalyst lowers this barrier by providing a different mechanism. It speeds both forward and reverse reactions and helps the system reach equilibrium faster, but it does not change the equilibrium constant.

Collision theory

According to collision theory, reacting particles must collide to react, but not every collision is effective. A successful collision needs sufficient energy and an appropriate orientation. This explains why the rate depends on more than the total number of collisions.

What can students study on Teachoo?

• average and instantaneous reaction rates;

• stoichiometric rate expressions;

• factors affecting reaction rate;

• experimental rate laws;

• order, molecularity and rate-constant units;

• zero-order and first-order integrated equations;

• half-life and concentration-time graphs;

• pseudo-first-order reactions;

• Arrhenius-equation numericals;

• activation energy and catalysts; and

• collision theory.

Common mistakes to avoid

• Do not take rate-law powers from the overall equation without experimental evidence.

• Do not use order and molecularity as synonyms.

• Include stoichiometric coefficients in the general reaction-rate expression.

• Check the unit of the rate constant; it changes with reaction order.

• Keep natural logarithm and common logarithm forms separate.

• A catalyst changes the pathway and rate, not ΔG° or the equilibrium constant.

Best way to study Chemical Kinetics

Build a comparison table for zero- and first-order reactions containing the rate law, integrated equation, graph, slope, unit of k and half-life. Practise determining order from experimental data before doing integrated-equation numericals. Finish with Arrhenius questions, where unit consistency and careful use of temperatures in kelvin are essential.

Frequently asked questions

What is the difference between rate and rate constant?

Reaction rate can change as concentrations change. The rate constant is fixed for a specified reaction at a specified temperature, though it changes with temperature and catalyst.

Can reaction order be zero or fractional?

Yes. Order is determined experimentally and may be zero, fractional or an integer.

Can molecularity be zero or fractional?

No. Molecularity refers to the number of participating species in an elementary step and is a positive integer.

Why is the half-life of a first-order reaction constant?

Its half-life depends only on the rate constant, not on the initial reactant concentration.

What is a pseudo-first-order reaction?

It is a higher-order reaction that behaves kinetically as first order because the concentration of one reactant is effectively constant, usually due to large excess.

Does a catalyst change the equilibrium composition?

No. It accelerates both forward and reverse reactions and reduces the time needed to reach equilibrium without changing the equilibrium constant.

Why does temperature increase reaction rate?

At higher temperature, a larger fraction of molecules has energy equal to or greater than the activation energy, so the number of effective collisions rises.

Learn Chemical Kinetics with Teachoo through comparison tables, graph-based reasoning and fully worked NCERT numericals.