GCSE Combined Science revision · Chemistry: reactions and energy

Higher: rates from tangents and in mol/s

Higher only

The idea

On a graph of product against time, the rate at a particular time is the gradient of a tangent drawn to the curve at that time. Rates can also be given in mol/s: mean rate = moles of product formed ÷ time. The curve is steepest at the start, when the concentration of reactants is highest.

Draw a straight tangent touching the curve at the time.
Pick two far-apart points on the tangent.
Gradient = change in y ÷ change in x.

Watch out Use the tangent's gradient, not the coordinates of the point on the curve.

See it worked

Worked example

A tangent to a gas volume curve passes through (0 s, 10 cm³) and (40 s, 50 cm³). Find the rate at that point.

Gradient = (50 − 10) ÷ (40 − 0).
= 40 ÷ 40 = 1 cm³/s.

Answer 1 cm³/s

Your turn

Question 1Calculator allowed2 marks

0.06 mol of product forms in 30 s. What is the mean rate in mol/s?

Question 2Calculator allowed2 marks

A tangent passes through (10 s, 0.20 g) and (50 s, 0.60 g). What is the rate in g/s?

Question 3Calculator allowed1 mark

Why is the reaction fastest at the start?

In past papers

10 official questions in 10 papers since 2018. The latest:

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