6
© 2021 Royal Society of Chemistry Rates of reaction | Supporting resources
Additional resources
Pause-and-think questions
Teacher version
Timestamp(s) Question Answer/discussion points
00:11/00:20 Define the rate of reaction. Rate of reaction: the change in the
concentration of the reactant or product per
unit time.
01:06 Can you think of any other reactions where
we could use the initial rate method to
measure the rates of reaction?
The thiosulfate–hydrochloric acid reaction,
otherwise known as the disappearing cross.
The catalytic decomposition of hydrogen
peroxide, where a fixed amount of oxygen
gas is collected.
Or another reaction where a fixed amount
of gas is collected, eg the reaction between
magnesium and hydrochloric acid.
01:24/01:35 Volume of gas is one property that can be
continuously monitored. Can you think of
any others?
Other examples include: colour change by
colourimetry; pH by using a pH meter or
sensor; temperature using a temperature
sensor or probe; monitor concentration by
periodically taking a sample to determine
the concentration, eg by titration.
01:48 Initial rate method: the iodine clock
02:12 The starch doesn’t actually take part in the
chemical reaction, so why do you think we
are adding it?
As an indicator to show where the end point
is. (Reaction is shown in detail at 06:33)
02:46 What other variables could we change in this
experiment?
The concentration of potassium iodide or
the temperature at which the reaction is
carried out.
02:50 What are the other variables in this
experiment that must be kept constant?
Examples: volume of all reactants,
concentration of the other solutions,
temperature, experimental method.
06:10 Why do you think it is important to use a
different labelled measuring cylinder for
each solution?
To avoid contamination and to make sure
that the samples don’t get mixed up.
07:18 Why do you think this is called a clock
reaction?
The time for the blue/black colour to appear
can be adjusted by varying the amount of
thiosulfate in solution x, so a ‘clock’ of any
desired time interval can be produced.
07:37/08:28 Draw a table suitable for recording the results
of this experiment – and record the results.
Solution Time (s)
A (30 cm
3
H
2
O
2
) 15
B (25 cm
3
H
2
O
2
) 20
C (20 cm
3
H
2
O
2
) 25
D (15 cm
3
H
2
O
2
) 32
E (10 cm
3
H
2
O
2
) 46