Meaning of the term rate of reaction.

The idea

The rate of reaction is the change in the concentration of a reactant or product per unit time, with units mol dm⁻³ s⁻¹. On a concentration against time graph the rate at any moment is the gradient of the tangent at that point, steepest at the start when the reactants are most concentrated and falling to zero when the reaction stops. Because concentration is often awkward to follow directly, experiments track something proportional to it: the volume of gas released, the loss of mass, the intensity of a colour, or the time taken for a fixed visible change. In that last case the rate is proportional to 1/t, so a plot of 1/t against temperature or concentration behaves like a rate plot even though it is not in absolute units.

What earns the marks

• Defining rate as the change in concentration of a reactant or product per unit time.
• The units mol dm⁻³ s⁻¹.
• Identifying the rate at a given moment as the gradient of the tangent to a concentration against time curve.
• Recognising 1/t as a measure of rate when a fixed extent of reaction is timed.

Common mistakes and how to put them right

• Defining rate as how fast a reaction goes, with no reference to concentration or time.

• Quoting the units as mol dm⁻³ or s⁻¹ alone.

• Calculating a gradient from two widely spaced points on a curve instead of drawing a tangent.

• Treating the time taken as the rate, so a longer time is reported as a faster reaction.

• Assuming the rate is constant because the graph looks nearly straight over a short interval.

Exam tips

• Draw tangents with a ruler over a wide span so the gradient can be read accurately.
• State the units every time you quote a rate; they are frequently a mark in their own right.
• If the experiment times a disappearing cross or a colour change, say explicitly that rate is proportional to 1/t.