Calculating Rates of Reaction
Practise Calculating Rates of Reaction for GCSE Chemistry with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA, Edexcel and OCR. Mean rate = quantity of reactant used or product formed ÷ time; find the rate at a point from the gradient of a tangent to the curve.
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These worksheets and mark schemes are original, written for Virtus Academy and checked against the current AQA, Edexcel and OCR specifications. Every worksheet comes with a full mark scheme.
Topic overview
The rate of a reaction measures how quickly reactants are used up or products are formed.
The mean rate is calculated as the quantity of reactant used divided by the time taken, or the quantity of product formed divided by the time taken. Units are typically g/s or cm³/s, depending on what is measured.
On a graph of product formed against time, the gradient gives the rate. A steeper gradient means a faster reaction. The rate is fastest at the start, because the concentration of reactants is highest, and the line levels off when the reaction is complete.
Revision notes
Calculating the mean rate
Mean rate = quantity of reactant used ÷ time taken, or quantity of product formed ÷ time taken.
Units are usually g/s if mass is measured, or cm³/s if gas volume is measured. Always include the unit in your answer.
Using graphs
On a graph of product against time, the gradient gives the rate at that moment.
A steeper gradient means a faster rate. The gradient is steepest at the start because reactant concentration is highest, and falls to zero when the line becomes horizontal and the reaction is complete.
Rate at a specific time
To find the rate at a particular moment on a curve, draw a tangent to the curve at that point and calculate its gradient.
This gives the instantaneous rate, as opposed to the mean rate over an interval.
Key points
- Rate measures how quickly reactants are used or products formed.
- Mean rate = quantity ÷ time.
- Units are typically g/s or cm³/s.
- The gradient of a graph gives the rate.
- The rate is fastest at the start.
- A horizontal line means the reaction has finished.
Worked examples
Example 1
48 cm³ of gas is produced in 24 seconds. Calculate the mean rate of reaction. [2 marks]
Model answer
Mean rate = volume ÷ time = 48 ÷ 24 (1 mark) = 2 cm³/s (1 mark).
Example 2
Explain why the rate of reaction is fastest at the start. [2 marks]
Model answer
The concentration of the reactants is highest at the start (1 mark), so there are more frequent collisions between particles and the reaction proceeds more quickly (1 mark).
Example 3
A student's graph becomes horizontal after 60 seconds. Explain what this shows. [2 marks]
Model answer
No more product is being formed (1 mark), so the reaction has finished because at least one of the reactants has been completely used up (1 mark).
Common mistakes
Forgetting the units.
Rate needs a compound unit such as cm³/s or g/s.
Reading the rate from the height rather than the gradient.
It is the gradient that gives the rate, not the value on the axis.
Saying the reaction stops because it runs out of time.
It stops because a reactant has been used up.
Using a chord when a tangent is needed.
A tangent gives the rate at a specific moment; a chord gives a mean.
Exam tips
- Always include the unit in a rate answer.
- Use the gradient, not the height, to find rate from a graph.
- Draw a tangent for the rate at a specific time.
- Explain the falling rate through decreasing concentration.
Key terms
- Rate of reaction
- How quickly reactants are used or products formed.
- Mean rate
- The average rate over a period of time.
- Gradient
- The steepness of a graph, giving the rate.
- Tangent
- A line touching a curve, used to find the rate at a moment.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.