AQA GCSE Combined Science Foundation (8464), Foundation tier · Chemistry › The rate and extent of chemical change › Rate of reaction
Practise Calculating rates of reactions. 8 exam-style questions plus unlimited generated ones on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
How to measure the rate of a reaction and calculate it from tables of results and from graphs of the amount of product formed (or reactant used) against time. Expect mean rate calculations and graph reading. These questions often link to the rates required practical.
Key facts
Mean rate = quantity of reactant used ÷ time taken
Mean rate = quantity of product formed ÷ time taken
Units of rate: g/s or cm3/s
Steeper curve = faster rate; horizontal line = reaction finished
1 minute = 60 s; 1 dm3 = 1000 cm3
Notes
Measuring the rate
diagram
The rate of reaction tells you how quickly a reactant is used up or a product is formed.
You follow a reaction by measuring the mass (in g) of a reactant or product, or the volume of a gas (in cm3), at regular time intervals.
Volume of gas: collect it in a gas syringe, or in an upside-down measuring cylinder filled with water.
Collecting a gas in a gas syringe: record the volume every 10 or 30 seconds.
Mass: if a gas escapes, stand the flask on a balance. The mass lost is the mass of gas given off.
Following mass loss: the flask stands on a balance while the gas escapes.
Mean rate
mean rate of reaction = quantity of reactant used ÷ time taken
mean rate of reaction = quantity of product formed ÷ time taken
The unit is the quantity unit ÷ the time unit: g/s or cm3/s (or g/min, cm3/min if the time is in minutes).
Between two times, use the change in quantity ÷ the change in time (e.g. 20 s to 60 s is 40 s).
Reading rate graphs
diagram
Plot the quantity (y-axis) against time (x-axis) and draw a smooth curve of best fit.
The steeper the curve, the faster the reaction. It is steepest at the start, when there are the most reactant particles.
A typical rate curve. The gradient (steepness) shows the rate.
The curve gets less steep as the reactants are used up, so the rate decreases.
When the line becomes horizontal, the reaction has stopped because one of the reactants has been used up.
A graph of the mass of the flask (as gas escapes) slopes down and then levels off. Here too, a steeper slope means a faster rate.
How to answer each type of question
Calculate the mean rate of reaction
2 to 3 marksGrade 4
Read the quantity at the start and at the end of the time interval from the table or graph.
Find the change in quantity and the time taken. Convert minutes to seconds if the unit asked for is per second.
Divide the quantity by the time and give the unit.
Example. A student reacted zinc with dilute sulfuric acid and collected the hydrogen in a gas syringe. After 2 minutes, 84 cm3 of hydrogen had been collected. Calculate the mean rate of reaction in the first 2 minutes. Give your answer in cm3/s.
Show the model answerHide the model answer
time = 2 × 60 = 120 s (1) mean rate = 84 ÷ 120 (1) = 0.70 cm3/s (1)
Explain the shape of a rate graph
2 to 3 marksGrade 5
Describe each part: steepest at the start, then less steep, then horizontal.
Explain the start: the concentration of reactants is highest, so the rate is fastest.
Explain the slowing down: reactants are being used up, so there are fewer collisions per second.
Explain the flat part: one reactant has been used up, so the reaction has stopped.
Example. A student added marble chips to excess hydrochloric acid and measured the volume of carbon dioxide produced. The graph of volume against time is steepest at the start, gradually becomes less steep, and is horizontal from 150 s onwards. Explain the shape of the graph.
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The rate is fastest at the start because there are the most reactant particles, so collisions are most frequent (1). The rate decreases because the marble chips are being used up, so there are fewer collisions per second (1). The line is horizontal after 150 s because all the marble chips have reacted, so the reaction has stopped (1).
Don’t lose marks
Dividing the time by the quantity (giving s/cm3) instead of the quantity by the time.
For a mean rate between two times, using just the final reading instead of the change during that interval.
Forgetting to convert minutes to seconds when the unit asked for is per second.
Leaving out the unit, or writing cm3 instead of cm3/s.
More tips
Memory tricks
'Per' means 'divide by': cm3/s means cm3 per second, so the calculation is volume ÷ time.
Sense check: the mean rate over the whole reaction must be lower than the rate at the very start.
In mass-loss experiments, the mass lost is the mass of gas made, so use it as the 'quantity of product formed'.
Exam technique
Always write the division out (e.g. 84 ÷ 120) so you can earn method marks even if the final answer slips.
Check the unit and the number of significant figures asked for in the question (g/s or cm3/s).
When asked how a graph shows the reaction has finished, say the line is horizontal (levels off), so no more product is being made.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 3
Say what the rate of reaction measuresRate is how quickly a reactant is used up or a product is formed.
Grade 4
Calculate a mean rate of reactionDivide the quantity of reactant used or product formed by the time taken, e.g. 30 cm3 ÷ 20 s = 1.5 cm3/s.
Grade 4
Give the correct unit for rateUse g/s for a mass or cm3/s for a volume of gas, matching the units of the quantity and the time.
Grade 5
Interpret graphs of product formed against timeThe steeper the curve, the faster the rate; a horizontal line means the reaction has stopped.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
A reaction produces 45 cm3 of gas in 30 s. Calculate the mean rate of reaction.
1.5 cm3/s
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy4 marks
The mean rate of a reaction can be calculated using: mean rate of reaction = quantity of reactant used or product formed ÷ time taken
(a) A reaction produces 45 cm3 of gas in 30 s. Calculate the mean rate of reaction.[1]
(b) In a different reaction, carbon dioxide is produced at a mean rate of 0.015 g/s. Calculate the mass of carbon dioxide produced in 120 s.[2]
(c) Which of these is a correct unit for the rate of a reaction? Tick (✓) one box.[1]
cm3
cm3/s
s/cm3
s
Show the answer and mark scheme
(a)Answer: 1.5 cm3/s
1.5 (cm3/s)
(b)Answer: 1.8 g
0.015 × 120
1.8 (g)
(c)Answer: cm3/s
Question 2Medium5 marks
A student added excess calcium carbonate powder to 50 cm3 of dilute nitric acid in a flask on a balance, and recorded the total mass at intervals. The mass of the flask and contents was 184.62 g at the start and 183.86 g after the reaction had finished.
(a) Explain why the mass decreased during the reaction.[1]
(b) The reaction took 240 s to finish. Calculate the mean rate of reaction over this time. Give your answer to 2 significant figures.[3]
(c) Suggest how the student could tell, without waiting and weighing, that carbon dioxide gas was still being produced.[1]
Show the answer and mark scheme
(a)Answer: Carbon dioxide gas was produced and escaped from the open flask.
carbon dioxide gas is produced and escapes from the (open) flask
(b)Answer: 0.0032 g/s
184.62 − 183.86 = 0.76 (g)
0.76 ÷ 240
0.0032 (g/s)
(c)Answer: Bubbles (effervescence) would still be seen in the flask.
bubbles / fizzing (effervescence) can still be seen in the flask