Rate Equations and Orders

The rate equation is the mathematical relationship between the rate of a reaction and the concentrations of the reactants. It is one of the most important tools in chemical kinetics, and unlike a balanced equation, it can only be determined experimentally — it cannot be deduced from the stoichiometry of the reaction.

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The General Rate Equation

For a reaction involving reactants A and B, the rate equation takes the form:

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

Where:

• Rate is the rate of reaction (mol dm⁻³ s⁻¹)
• k is the rate constant — a proportionality constant that depends on temperature
• [A] and [B] are the concentrations of reactants (mol dm⁻³)
• m is the order with respect to A
• n is the order with respect to B
• (m + n) is the overall order of the reaction

The orders m and n are usually 0, 1, or 2 (though fractional and negative orders exist, they are beyond A-level).

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Understanding Orders of Reaction

Zero Order (m = 0)

If the reaction is zero order with respect to A, then changing [A] has no effect on the rate.

Rate = k[A]⁰ = k