Calculating rates of reactions: draw, and interpret, graphs showing the quantity of product formed or quantity of reactant used up against time

The idea

When a reaction happens, the amount of product increases or the amount of reactant decreases as time passes. Plotting quantity on the y-axis against time on the x-axis gives a curve whose steepness shows how fast the reaction is going. A steep section means a fast reaction; a flat section means the reaction has stopped because a reactant has run out. For example, adding magnesium to dilute hydrochloric acid and collecting the hydrogen produced gives a curve that rises quickly then levels off. If you plot the mass of reactant remaining, the curve falls instead. However, a graph of 'reactant used up' will rise and level off, just like a product-formed graph. Always label both axes with quantity and unit, choose a sensible scale, plot points accurately and join them with a smooth curve.

What earns the marks

• Axes are labelled with quantity (for example volume of gas in cm³ or mass in g) and time in s, with units stated.
• A sensible scale is chosen so the plotted points fill most of the grid, and points are plotted accurately.
• Points are joined with a smooth curve of best fit, not a series of straight lines or a dot-to-dot pattern.
• The gradient of the curve is described as the rate: a steeper curve means a faster reaction at that time.
• The plateau shows the reaction has finished because a limiting reactant has been used up, so the quantity no longer changes.
• A 'reactant used up' graph rises and flattens at a high value, whereas a 'reactant remaining' graph falls and flattens at a low value.

Common mistakes and how to put them right

• Joining the points with straight lines instead of a smooth curve.

  Put it right by drawing one continuous curve that passes as close as possible to all the points.

• Forgetting units on the axes.

  Put it right by writing, for example, 'volume of gas / cm³' and 'time / s' on the labels.

• Reading the plateau as the reaction speeding up.

  Put it right by explaining that a flat line means no further change, so the rate is zero because a reactant is exhausted.

Exam tips

• Check the axis labels carefully: a rising curve means product formed or reactant used up, while a falling curve means reactant remaining.
• Quote values from the graph with units when asked to interpret, such as the time taken to reach a certain volume.
• If asked to compare two curves, refer to which is steeper and what that shows about rate, not just which is higher.