Rate law, order and molecularity

The essentials

The rate law expresses the rate in terms of reactant concentrations raised to certain powers: for aA + bB → products, rate = k[A]ˣ[B]ʸ. The exponents x and y are found by experiment and need not equal a and b. The proportionality constant k is the rate constant; it equals the rate when all concentrations are 1 mol L⁻¹ and depends on temperature but not on concentration.

Key points

• Order = sum of the powers in the rate law (x + y). It can be 0, a whole number or a fraction, and is determined experimentally.
• Molecularity = number of reacting species that collide simultaneously in an elementary step. It is always a whole number (1, 2 or rarely 3) and cannot be zero.
• For a complex reaction, the slowest step (rate-determining step) controls the overall rate; molecularity has no meaning for the overall complex reaction.
• Units of k: (mol L⁻¹)¹⁻ⁿ s⁻¹. Zero order: mol L⁻¹ s⁻¹; first order: s⁻¹; second order: L mol⁻¹ s⁻¹.
• Examples: 2NO + O₂ → 2NO₂ has rate = k[NO]²[O₂] (third order); CHCl₃ + Cl₂ → CCl₄ + HCl has rate = k[CHCl₃][Cl₂]^½ (order 1.5).

Exam tips

• To find order from data, compare two experiments in which only one concentration changes: if doubling [A] doubles the rate, order in A is 1; if it quadruples the rate, order is 2; if no change, order is 0.
• You can identify the order from the units of k.

Common mistakes

• Writing the rate law directly from the balanced equation.
• Saying molecularity can be fractional or zero.