Measuring Rates of Reaction

Before we can write rate equations or determine reaction orders, we need to be able to measure how fast a reaction proceeds. The rate of a chemical reaction is defined as the change in concentration of a reactant or product per unit time. In practice, we measure a physical quantity that is proportional to concentration and track how it changes.

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Defining Rate

The rate of reaction can be expressed as:

Rate = change in concentration / change in time

For a reactant: Rate = −Δ[A] / Δt (negative because concentration decreases)

For a product: Rate = +Δ[B] / Δt (positive because concentration increases)

Units of rate are typically mol dm⁻³ s⁻¹, though mol dm⁻³ min⁻¹ is also used for slower reactions.

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Methods for Measuring Rate

1. Gas Collection

If a reaction produces a gas, you can collect it using a gas syringe or by collecting it over water using an inverted measuring cylinder. Measuring the volume of gas produced at regular time intervals gives a volume-time graph, which can be converted to a concentration-time graph if the total volume is known.

Example: The reaction of magnesium with hydrochloric acid: