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6.1 Measuring the rate of reaction

How to measure the rate of a reaction from the amount of reactant used or product formed over time, using gas volume, loss of mass or a colour change, how to calculate a mean rate, and (Higher) how to find the rate at a moment from the gradient of a tangent.

AQA Edexcel OCR 2 min read

What rate means

Key terms

  • Rate of reaction: how quickly reactants are used up or products are made.
  • Mean rate: the average rate over a period of time.
  • Tangent: a straight line that touches a curve at one point, with the same gradient as the curve there.
mean rate of reaction = quantity of reactant used / time taken
mean rate of reaction = quantity of product formed / time taken

The quantity can be a mass in grams or a volume in cm3, so the units of rate are g/s or cm3/s. At Higher tier it can also be an amount in moles, giving mol/s.

Ways to measure it

Two ways to measure rate: collecting gas in a gas syringe, and recording the loss of mass on a balance as gas escapes through cotton wool
Two ways to measure rate: collecting gas in a gas syringe, and recording the loss of mass on a balance as gas escapes through cotton wool
  • Volume of gas given off: collect it in a gas syringe (or an upturned measuring cylinder full of water) and read the volume at regular intervals.
  • Loss in mass: put the flask on a balance with a cotton wool plug. As the gas escapes, the mass falls.
  • Colour change or precipitate: time how long it takes for a cross under the flask to disappear as a cloudy precipitate forms (sodium thiosulfate and acid), or for a colour to change.

Reading rate graphs

A graph of volume of gas against time that is steep at first and levels off, with a tangent drawn at 20 seconds to find the rate at that moment
A graph of volume of gas against time that is steep at first and levels off, with a tangent drawn at 20 seconds to find the rate at that moment
  • The graph is steepest at the start, when there are the most reactant particles, so the rate is fastest.
  • It gets less steep as reactants are used up.
  • It becomes flat when one reactant has run out: the reaction has stopped.
  • A steeper line means a faster reaction.

Example: 30 cm3 of gas is made in the first 20 s. Mean rate = 30 ÷ 20 = 1.5 cm3/s.

Rate at a particular moment (Higher)

Draw a tangent to the curve at that time and calculate its gradient: change in y ÷ change in x. On a steep part of the curve the tangent is steep, so the rate is high.

Test yourself on this lesson

6 exam questions · 12 marks · with mark schemes and model answers

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