Module 5: Physical Chemistry and Transition Elements
5.1.1 How Fast?
Question Answer
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What is meant by the ‘rate of a reaction’? The change in concentration (of a reactant or a product) with tim
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Module 5: Physical Chemistry and Transition Elements
5.1.1 How Fast?
Question Answer
H
o
w
F
a
s
t
?
What is meant by the ‘rate of a reaction’? The change in concentration (of a reactant or a product) with time
What is meant by the ‘order of a reaction’? The power that a concentration is raised to in the rate equation
How is the overall order of a reaction determined? The overall order of a reaction is the sum of all the orders in the rate
equation.
Determine the order of reaction for each of the following
rate/concentration graphs:
A = first order
B = zero order
C = second order
Learning Grids for OCR A Level Year 2 Chemistry: Module 5 Page 1 of 4744 © ZigZag Education, 2017
Oxford: pp. 8–27
Pearson: pp. 10–22 1
A B
concentration
rate
rate
concentration
C
rate concentration
Question Answer
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d)
How would you determine the rate constant k for the first order
reaction represented in the graph above? k = gradient of rate/concentration graph
What is meant by the ‘rate determining step’ in a reaction? The slowest step in a multistep reaction mechanism
A chemical reaction was carried out using two different
concentrations of a reactant J.
Experiment
number
Initial [J]
(mol dm–3)
Initial rate of reaction
(mol dm–3 s
–1)
1 0.0200 2.70 × 10–3
2 0.0800 4.32 × 10–2
Using the experimental data above, determine the increase in
initial concentration of [J] between experiments 1 and 2 and the
effect this has on the initial rate. From this information, determine
the order for J.
0.0800/0.0200 = 4
Fourfold increase in initial concentration
4.32×10–2/2.70×10–3 = 16
Sixteenfold increase in rate = 42
increase in reaction rate
Therefore, reaction is second order with respect to J
Learning Grids for OCR A Level Year 2 Chemistry: Module 5 Page 2 of 4744 © ZigZag Education, 2017
Question Answer
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A reaction takes place between three reactants, W, V and X, to
make Y.
W + V + X → Y
Use the following information about the multistep reaction to
calculate the order for the three reactants involved.
Experiment
number
Initial
[W]
(mol dm–3)
Initial [V]
(mol dm–3)
Initial
[X]
(mol dm–3)
Initial rate of
reaction
(mol dm–3 s
–1)
1 0.00600 0.00600 0.00600 3.60 × 10–5
2 0.01200 0.00600 0.00600 7.20 × 10–5
3 0.00600 0.01800 0.00600 3.24 × 10–4
4 0.01800 0.00600 0.0180 1.08 × 10–4
Reaction is first order with respect to [W] as doubling the initial
concentration doubles the rate of the reaction.
The reaction is second order with respect to [V] as increasing the
concentration of [V] by three times increases the rate of reaction by
nine (i.e. 32
).
The reaction is zeroth order with respect to [X] as tripling the
concentration of W and tripling the concentration of X increases the
rate by three. We have already worked out that the reaction is first
order with respect to W, so the rate being tripled is due to W, and
tripling the [X] is having no effect.
What is the overall rate equation for this reaction? V ¿
2
rate=k [W ] ¿
Calculate the rate constant for experiment 1.
State your answer to 2 decimal places.
V ¿
2
¿
[W ] ¿
k=
rate
¿
What are the units for the rate constant for this reaction?
V ¿
2
¿
mol dm−3
¿
2
¿
[mol dm−3
] ¿
[W ] ¿
units=
rate
¿
Learning Grids for OCR A Level Year 2 Chemistry: Module 5 Page 3 of 4744 © ZigZag Education, 2017
Question Answer
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d)
Knowing that this is a multistep reaction, and using your
knowledge of the rate equation for this reaction, determine how
many molecules of each species are involved in the ratedetermining step.
From the rate equation, one molecule of W and two molecules of V.
Based on your knowledge of the rate-determining step, propose
a mechanism for this multistep reaction that involves an
intermediate, Z.
Rate-determining step must involve the species in the rate equation
Step 1: W +2V → Z
Compared to the equation
W +V +X →Y
We can see that the second step must:
Use up a molecule of X
Use up the intermediate, Z
Make a molecule of V to cancel out one of the molecules of V in the
first step so that only one is used overall
Make a molecule of Y
Step 2: X+Z→V +Y
Learning Grids for OCR A Level Year 2 Chemistry: Module 5 Page 4 of 4744 © ZigZag Education, 2017
Question Answer
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d)
Sketch the shape of the concentration–time graph you would
expect for a reactant [W] in the reaction above.
Graph should be a sketch of a constant half-life (first order reaction),
such as:
How can a concentration–time graph be used to determine the
rate of a reaction at different times?
Draw a tangent to the curve at a particular time.
Calculate the gradient of the tangent.
Learning Grids for OCR A Level Year 2 Chemistry: Module 5 Page 5 of 4744 © ZigZag Education, 2017
Concentration
Time
Question Answer
What shape would you expect the rate–concentration graph to be
for the reactant [V]? Sketch a graph to illustrate this.
A positive gradient with a curved line for second order reaction
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The results of this reaction were determined using the initial rates
method. Describe an experimental method that could have been
used to achieve these results.
A ‘clock reaction’ where the time (t) is measured for an amount of
product to form or for an amount of reactant to disappear.
The initial rate is proportional to 1/t.
This is carried out for each reactant in an experiment then a graph is
plotted of initial rate against concentration.
In the reaction above, the product of this reaction (Y) is a solid.
What observation could you look for when calculating the
initial rate?
The formation of a precipitate
In a different reaction, the product produced is a coloured
solution. How can this observation be monitored?
By use of colorimetry
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