6.4 Required practical: rates of reaction
6 exam questions worth 16 marks. Write your answer, open the mark scheme, then give yourself the marks you earned. Your answers are saved on this device.
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In the disappearing cross experiment, why does the cross disappear?
Mark scheme
a (sulfur) precipitate forms making the solution cloudy
Model answer
A sulfur precipitate forms, so the solution becomes cloudy and the cross cannot be seen.
Give the independent variable and two control variables in this investigation.
Mark scheme
- independent: concentration (of sodium thiosulfate / acid) (1)
- control, any two from: temperature; volume of solutions; same cross / paper; same observer (2)
Model answer
The independent variable is the concentration of sodium thiosulfate. The temperature and the total volume of solution must be kept the same.
The cross disappeared after 40 seconds.
Calculate the rate using rate = 1 / time.
Mark scheme
- 1 ÷ 40 (1)
- = 0.025 (s-1) (1)
Model answer
rate = 1 ÷ 40 = 0.025 s-1.
Suggest how the timing of the end point could be made more accurate.
Mark scheme
- use a light sensor / data logger (1)
- so the end point is not judged by eye / not subjective (1)
Model answer
Use a light sensor and data logger to detect when the light falls to a set level, so the end point is not judged by eye.
Describe how to use the gas collection method to find how the concentration of hydrochloric acid affects the rate of its reaction with magnesium.
Mark scheme
Level 3 (5–6 marks): a clear method with control variables, measurements and how the results show the rate. Level 2 (3–4 marks): most steps with some control of variables. Level 1 (1–2 marks): some relevant steps.
Indicative content:
- measured volume of acid of known concentration in a conical flask
- add a set length / mass of magnesium ribbon
- bung connected to gas syringe / upturned measuring cylinder
- start a stopwatch and record volume every 10 seconds
- repeat with different concentrations
- control temperature, volume of acid, mass of magnesium
- plot volume against time; steeper initial gradient means faster rate
- repeat and find means
Model answer
Put 50 cm3 of hydrochloric acid of known concentration in a conical flask. Add a 3 cm length of magnesium ribbon and quickly fit a bung connected to a gas syringe. Start a stopwatch and record the volume of gas every 10 seconds until it stops increasing. Repeat with different concentrations of acid, keeping the volume of acid, the length of magnesium and the temperature the same. Plot volume against time for each concentration: the steeper the initial gradient, the faster the rate. Repeat each concentration and take means.
Explain why the reaction is faster with a higher concentration of sodium thiosulfate.
Mark scheme
- more particles in the same volume (1)
- more frequent collisions (1)
Model answer
There are more thiosulfate particles in the same volume, so collisions with acid particles are more frequent.
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