Syllabus Edition
First teaching 2023
First exams 2025
Which of the following factors can affect the value of the activation energy of a reaction?
|
1 |
The presence of a catalyst |
|
2 |
Changes in temperature |
|
3 |
Changes in the concentration of the reactants |
1 only
1 and 2
3 only
1, 2 and 3
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The Maxwell-Boltzmann energy distribution curve below describes a mixture of two gases at a given temperature. For a reaction to occur between the gaseous molecules, they must collide with sufficient energy.
Of the two activation energy (Ea) values shown, one is for a catalysed reaction, the other for an uncatalysed one.
When a catalyst is used, which pair of statements is correct?
| A |
Ea1 uncatalysed reaction fewer effective collisions |
Ea2 catalysed reaction more effective collisions |
| B |
Ea1 catalysed reaction fewer effective collisions |
Ea2 uncatalysed reaction more effective collisions |
| C |
Ea1 uncatalysed reaction more effective collisions |
Ea2 catalysed reaction fewer effective collisions |
| D |
Ea1 catalysed reaction more effective collisions |
Ea2 uncatalysed reaction fewer effective collisions |
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The diagram represents the reaction pathway for the following reaction:
Z (g) + Y (g) → X (g) + W (g)
What statement is true about the reverse reaction, W (g) + X (g) → Y (g) + Z (g)?
It will have a smaller activation energy and a negative ∆H
It will have a smaller activation energy and a positive ∆H
It will have a larger activation energy and a negative ∆H
It will have a larger activation energy and a positive ∆H
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Boltzmann distributions are shown in the diagrams.
In diagram 1, one line, P or Q, corresponds to the activation energy in the presence of a catalyst and the other line corresponds to the activation energy of the same reaction in the absence of a catalyst.
In diagram 2, one curve, X or Y, corresponds to a temperature higher than that of the other curve.
Which combination gives the correct curve and line?
|
Presence of catalyst |
Higher temperature |
|
|
A B C D |
P P Q Q |
X Y X Y |
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1 dm3 of gas X and 1 dm3 of gas Y are kept at the same temperature and pressure.
The speed of the molecules is plotted against the fraction of molecules with that speed. From the graph what can be said about the two gases?
The molar mass of X is larger than the molar mass of Y
The molar mass of Y is larger than the molar mass of X
The average kinetic energy of X is higher than that of Y
The average kinetic energy of Y is higher than that of X
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The Boltzmann distribution shows the number of molecules that have particular kinetic energy at a constant temperature.
If the temperature is decreased by 10 °C, what happens to the size of the areas labelled H, J and K?
|
H |
J |
K |
|
|
A B C D |
Decreases Decreases Increases Increases |
Decreases Increases Decreases Decreases |
Decreases Decreases Decreases Increases |
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The distribution of molecular energies in a sample of a gas at a given temperature is shown by the Boltzmann distribution graph below.
If the temperature is increased, what will happen to the position of point X?
Fewer molecules possess the most probable energy value so X will shift to the right.
Fewer molecules possess the most probable energy value so X will shift to the left.
More molecules possess the most probable energy value so X will shift to the left.
The position of X will stay the same but the area under the distribution curve increases.
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The diagram shows a reaction pathway for an endothermic reaction.
Which arrow represents the activation energy for the forward reaction?
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The dotted-line curve on each graph below represents the corresponding distribution for a gas at 300 K.
Which solid-line curve most accurately represents the distribution of molecular energies in the same gas at 500 K?
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The energies of the reactants, the products and the transition state of a reaction are shown in the reaction pathway diagram below.
Which expression correctly represents how to calculate the activation energy of the forward reaction?
E1 - E2
E2 - E1
E2 - E3
E3 - E2
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The diagram below represents, for a given temperature, the Boltzmann distribution of the kinetic energy of the molecules in a mixture of two gases that react slowly together without a catalyst.
The activation energy for the reaction, Ea, is marked for the uncatalysed reaction.
What would the position of Ea be if the reaction took place with an effective catalyst?
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Four possible reactions (A, B, C and D) of the equation below are measured at the same temperature.
X (g) + Y (g) → Z (g)
Which reaction pathway diagram shows the reaction that would proceed most rapidly and with good yield?
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The diagram shows a Boltzmann distribution of molecular energies for a gaseous mixture. The distribution has a peak, labelled Q on the diagram.
What happens when an effective catalyst is added to the mixture?
The height of the peak remains the same, and the activation energy moves to the left.
The height of the peak decreases and the activation energy moves to the left.
The height of the peak remains the same, and the activation energy moves to the right.
The height of the peak decreases and the activation energy moves to the right.
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