Investigating Rate of Reaction
Recap Investigating Rate 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. The required practical measures rate two ways: the change in mass or gas volume, and the time for a precipitate to hide a cross (turbidity).
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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
This required practical investigates how a factor affects the rate of reaction, usually the reaction between sodium thiosulfate and hydrochloric acid, or between marble chips and acid.
In the disappearing cross method, sodium thiosulfate and acid are mixed over a printed cross. A precipitate of sulfur forms and clouds the solution, and the time taken for the cross to disappear is recorded. A shorter time means a faster reaction.
In the gas collection method, the volume of gas produced is measured at regular intervals using a gas syringe. Plotting volume against time gives a curve whose gradient is the rate. Control variables such as temperature, volume and concentration must be kept constant apart from the one being tested.
Revision notes
The disappearing cross method
Mix sodium thiosulfate and hydrochloric acid in a flask placed over a printed cross.
A pale yellow precipitate of sulfur forms, clouding the solution. Record the time taken for the cross to become invisible. A shorter time means a faster reaction, so rate is proportional to 1 ÷ time.
The gas collection method
Measure the volume of gas produced at regular time intervals using a gas syringe, or measure the mass lost on a balance.
Plot volume or mass against time. The gradient gives the rate, and the curve levels off when the reaction is complete.
Control variables
Keep the temperature, the volumes and the concentrations constant apart from the variable being investigated.
Without these controls, any change in rate could not be attributed to the factor being tested. The same person should judge the end point in the cross method, since judgement is subjective.
Key points
- A shorter time means a faster reaction.
- Rate is proportional to 1 ÷ time.
- Sulfur precipitate clouds the solution.
- A gas syringe measures gas volume over time.
- The gradient of the graph gives the rate.
- Control all variables except the one tested.
Worked examples
Example 1
A reaction takes 40 seconds for the cross to disappear. Calculate the rate as 1 ÷ time. [2 marks]
Model answer
Rate = 1 ÷ 40 (1 mark) = 0.025 per second (1 mark).
Example 2
Explain why the same person should judge when the cross disappears. [2 marks]
Model answer
Judging when the cross has disappeared is subjective (1 mark), so using the same person each time makes the comparison between results fairer and more consistent (1 mark).
Example 3
Name two variables that must be controlled when investigating the effect of concentration on rate. [2 marks]
Model answer
The temperature of the reactants (1 mark), and the total volume of the solution used (1 mark).
Common mistakes
Saying a longer time means a faster reaction.
A shorter time means faster. Rate is proportional to 1 ÷ time.
Not controlling temperature.
Temperature strongly affects rate, so it must be held constant.
Using different people to judge the end point.
Judgement is subjective, so the same person should judge every time.
Reading the rate from the height of the graph.
The gradient gives the rate, not the value on the axis.
Exam tips
- Use rate = 1 ÷ time for the disappearing cross method.
- Name specific control variables and explain why each matters.
- Mention the subjectivity of the end point judgement.
- Use the gradient for rate in the gas collection method.
Key terms
- Precipitate
- An insoluble solid formed in a solution.
- Gas syringe
- Apparatus for measuring the volume of gas produced.
- Control variable
- A variable kept constant for a fair test.
- Subjective
- Depending on personal judgement rather than measurement.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.