5.2 Introduction to Rate Law

What is a rate law?

A rate law expresses reaction rate as a function of reactant concentrations: rate = k[A]^m[B]^n. Here k is the rate constant, and m, n are the orders with respect to each reactant, determined experimentally (NOT from stoichiometric coefficients). The sum m + n gives the overall reaction order.

Reaction orders

Order describes how rate depends on concentration. Zero order: changing [A] has no effect on rate. First order: rate is directly proportional to [A]. Second order: rate is proportional to [A]². Orders are usually 0, 1, or 2, and must be found from experimental data, not guessed from the balanced equation.

Finding orders from data

Using the method of initial rates, compare experiments where one reactant's concentration changes while others stay constant. If doubling [A] doubles the rate, it's first order in A; if it quadruples the rate, it's second order; if rate is unchanged, it's zero order in A. The rate constant k is then solved using any trial's data.

Key terms

Rate law

An equation relating reaction rate to reactant concentrations: rate = k[A]^m[B]^n, determined experimentally.

Reaction order

The exponent on a reactant's concentration in the rate law, showing how rate depends on that concentration.

Overall reaction order

The sum of all individual reactant orders in the rate law.

Rate constant (k)

A proportionality constant in the rate law; its value and units depend on the overall reaction order and temperature.

Method of initial rates

A technique using multiple experimental trials with different starting concentrations to determine reaction orders.

Try an AP-style question

rate = k[A][B]². What is the overall order?

1. 4
2. 1
3. 3
4. 2