Teacher's notes and answers to questions in the book - Hodder Plus ...

WJEC GCSE Additional Science Teacher’s Notes
11
Rates of reaction and
chemical calculations
_Measuring the rate of a reaction (pages 111–15)_____
PRACTICAL Measuring rates of reaction (pages 112–15)
Safety: make sure you consult the relevant CLEAPSS guidance before conducting this practical
work.
Measuring reactions involving gases
1. What is the shape of your graph
The graph will be a positive curve with a decreasing positive gradient.
2. What does your graph tell you Where is the rate of reaction fastest
The rate of reaction is highest at the start of the reaction and it gradually decreases with
time.
3. Can you tell when the reaction is complete
The reaction is complete when the volume of gas produced stays constant for a period of
time.
4. Calculate the rate of reaction at the steepest part of your graph by drawing a tangent line and
measuring the gradient (the units will be cm 3 /s).
Student measurements from their own graph.
5. Repeat the rate of reaction calculations for both methods. Are the rates of reaction the same
Should they be the same Why might the two rates be different
The rates of reaction are likely to be similar – they should be the same, but there may be
differences due to more or less ambient air in the tubes, friction in the gas syringe, leaks,
etc.
Measuring reactions involving changes of mass
6. What is the shape of your graph
The graph will be a negative curve with decreasing negative gradient.
7. What does your graph tell you Where is the rate of reaction fastest
The rate of reaction is highest at the start of the reaction and it gradually decreases with
time.
8. Can you tell when the reaction is complete
The reaction is complete when the mass of the system stays stays constant for a period of
time.
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