AQA · A-Level · Chemistry
Course version: 7405 · Version from Sept 2016
Required practical 3 Investigation of how the rate of a reaction changes…
Full specification wording
Effect of temperature on reaction rate: Required practical 3 Investigation of how the rate of a reaction changes with temperature.
Required practical 3 follows one reaction at several temperatures.
Checked against the AQA 7405 specification (opens in a new tab) on .
- Point 4 of 4
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Required practical 3 follows one reaction at several temperatures. The usual choice is sodium thiosulfate with dilute hydrochloric acid, which slowly clouds with sulfur; the mixture is placed over a printed cross and the time for the cross to disappear is recorded. Since the same amount of sulfur forms each time, the rate is proportional to 1/t. Everything except temperature must be held constant: the concentrations, the total volume, the depth of liquid, the same cross, and ideally the same observer, because judging the moment the cross vanishes is subjective. The solutions are brought to temperature separately in a water bath before mixing, and the actual temperature of the mixture is measured rather than assumed. A plot of 1/t against temperature curves upwards.
What earns the marks
- Stating that the time for a fixed visible change is measured and that rate is proportional to 1/t.
- Naming the control variables, including concentration, total volume and depth of solution.
- Bringing the solutions to temperature in a water bath and measuring the temperature of the mixture.
- Describing the shape of the 1/t against temperature plot as an upward curve rather than a straight line.
Common mistakes and how to put them right
Plotting time against temperature and describing the falling curve as showing that rate decreases.
Heating the mixed solutions, so reaction occurs before timing starts.
Changing the total volume when varying a concentration, which alters the other concentration too.
Quoting the water bath setting as the reaction temperature instead of the measured value.
Treating the subjective endpoint as a systematic error when a different observer judges each run, where the inconsistency between observers makes it a random error - it is systematic only when one observer judges every run the same way.
Exam tips
- Name the specific control variables from this experiment rather than saying keep everything else the same.
- When asked for an improvement, target the subjective endpoint, for example by using a light sensor and data logger.
- Mention that sulfur dioxide is released, so the work is done in a well ventilated room or fume cupboard.
Try the lesson
The start of the Effect of temperature on reaction rate: Required practical 3 Investigation of how the rate of a reaction changes with temperature. lesson, made from checked teaching notes for this specification point.
Have a go first
In Required practical 3, a student measures how the rate of reaction changes with temperature. Which measurement is most suitable for comparing the rate between runs?
Listen
Required practical 3 follows one reaction at several temperatures. The usual choice is sodium thiosulfate with dilute hydrochloric acid, which slowly clouds with sulfur; the mixture is placed over a printed cross and the time for the cross to disappear is recorded. Since the same amount of sulfur forms each time, the rate is proportional to 1/t. Everything except temperature must be held constant: the concentrations, the total volume, the depth of liquid, the same cross, and ideally the same observer, because judging the moment the cross vanishes is subjective. The solutions are brought to temperature separately in a water bath before mixing, and the actual temperature of the mixture is measured rather than assumed. A plot of 1/t against temperature curves upwards. A strong answer earns marks for these points. stating that the time for a fixed visible change is measured and that rate is proportional to 1/t. naming the control variables, including concentration, total volume and depth of solution. bringing the solutions to temperature in a water bath and measuring the temperature of the mixture. describing the shape of the 1/t against temperature plot as an upward curve rather than a straight line. Exam tip: Name the specific control variables from this experiment rather than saying keep everything else the same.
Key words
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rate proportional to 1/t
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control variables
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water bath equilibration
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subjective endpoint
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See every definition
- rate proportional to 1/t
- Since the same amount of sulfur forms each time, the rate is proportional to 1/t.
- control variables
- Everything except temperature must be held constant: the concentrations, the total volume, the depth of liquid, the same cross, and ideally the same observer.
- water bath equilibration
- The solutions are brought to temperature separately in a water bath before mixing, and the actual temperature of the mixture is measured rather than assumed.
- subjective endpoint
- Judging the moment the cross vanishes is subjective.
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