kinetics and equilibria

Question 1

A fixed mass of marble is reacted with dilute hydrochloric acid at a constant temperature.
Explain why the rate of the reaction is increased if the lumps of marble are reduced in size.

Answer 1

A fixed mass of marble is reacted with dilute hydrochloric acid at a constant temperature.
Explain why the rate of the reaction is increased if the lumps of marble are reduced in size.

Increased surface area (1)
more collisions (1)

Question 2

) Hydrogen peroxide decomposes more rapidly in the presence of aqueous hydrogen
bromide. The decomposition proceeds as shown by the following equations.
H2O2 + HBr → HBrO + H2O
HBrO + H2O2 → H2O + O2 + HBr
(i) Write an equation for the overall reaction.

Answer 2

) Hydrogen peroxide decomposes more rapidly in the presence of aqueous hydrogen
bromide. The decomposition proceeds as shown by the following equations.
H2O2 + HBr → HBrO + H2O
HBrO + H2O2 → H2O + O2 + HBr
(i) Write an equation for the overall reaction.

2H2O2 → 2H2O + O2

Question 3

Define the term catalyst

Answer 3

Speeds up chemical reaction

Remains unchanged

Question 4

Give two reasons, other than an increase in the reaction rate, why these equations
suggest that hydrogen bromide is behaving as a catalyst.

Reason 1
Reason 2

Answer 4

Give two reasons, other than an increase in the reaction rate, why these equations
suggest that hydrogen bromide is behaving as a catalyst.

Reason 1 : Remains unchanged
Reason 2 : Offers alternative reaction route

Question 5

(b) Define the term activation energy.

Answer 5

Minimum energy 1

To start a reaction

Question 6

At any time, most of the molecules of G have energy less than the activation energy.
Suggest why, at a constant temperature, most of G eventually decomposes.

Answer 6

At any time, most of the molecules of G have energy less than the activation energy.
Suggest why, at a constant temperature, most of G eventually decomposes.

Molecules gain energy Due to collisions

Question 7

State the effect, if any, of adding a catalyst on the time required for G to decompose,
compared with a similar sample without a catalyst. Explain in general terms how the
catalyst has this effect.

Time for decomposition :
Explanation :

Answer 7

State the effect, if any, of adding a catalyst on the time required for G to decompose,
compared with a similar sample without a catalyst. Explain in general terms how the
catalyst has this effect.

Time for decomposition : Decreases
Explanation : Ea lowered (1)
By alternative route (1)
So more molecules have energy > Ea

Question 8

What does the area under the curve represent?

Answer 8

What does the area under the curve represent?

the number of molecules

Question 9

Below is a Maxwell–Boltzmann curve showing the distribution of molecular energies for a
sample of gas at a temperature T.
(i) Label the axes on the diagram above

Answer 9

Below is a Maxwell–Boltzmann curve showing the distribution of molecular energies for a
sample of gas at a temperature T.
(i) Label the axes on the diagram above

y axis : number of molecules
x axis : molecular energy

Question 10

The rate of a chemical reaction may be increased by an increase in reactant
concentration, by an increase in temperature and by the addition of a catalyst.
State which, if any, of these changes involves a different activation energy. Explain
your answer.

Change(s) :
Explanation :

Answer 10

The rate of a chemical reaction may be increased by an increase in reactant
concentration, by an increase in temperature and by the addition of a catalyst.
State which, if any, of these changes involves a different activation energy. Explain
your answer.

Change(s) : catalyst
Explanation : Alternative route (1), with a lower activation energy

Question 11

The diagram below represents a Maxwell–Boltzmann distribution curve for the particles in a
sample of a gas at a given temperature. The questions below refer to this sample of particles.

(a) On the diagram draw a curve to show the distribution for this sample at a lower
temperature.

Answer 11

The diagram below represents a Maxwell–Boltzmann distribution curve for the particles in a
sample of a gas at a given temperature. The questions below refer to this sample of particles.

(a) On the diagram draw a curve to show the distribution for this sample at a lower
temperature.
higher + displaced to the left
steeper

Question 12

In order for two particles to react they must collide. Explain why most collisions do not
result in a reaction.

Answer 12

In order for two particles to react they must collide. Explain why most collisions do not
result in a reaction.

Energy must b < Ea

Question 13

State one way in which the collision frequency between particles in a gas can be increased
without changing the temperature.

Answer 13

State one way in which the collision frequency between particles in a gas can be increased
without changing the temperature.

Increase concentration (or pressure)

Question 14

Suggest why a small increase in temperature can lead to a large increase in the reaction rate
between colliding particles.

Answer 14

Suggest why a small increase in temperature can lead to a large increase in the reaction rate
between colliding particles.

more molecules have E > Ea

Question 15

Explain in general terms how a catalyst works

Answer 15

Explain in general terms how a catalyst works

Question 16

The curve below shows the Maxwell–Boltzmann distribution of molecular energies, at a
constant temperature, in a gas at the start of a reaction. On this diagram the most probable
molecular energy at this temperature is indicated by the symbol Emp and the activation

Consider the following changes.
(i) The number of molecules is increased at constant temperature.

state how, if at all, the following would vary:
• the value of the most probable energy, Emp
• the number of molecules with the most probable energy, Emp
• the area under the molecular energy distribution curve
• the number of molecules with energy greater than the activation energy, Ea

Answer 16

The curve below shows the Maxwell–Boltzmann distribution of molecular energies, at a
constant temperature, in a gas at the start of a reaction. On this diagram the most probable
molecular energy at this temperature is indicated by the symbol Emp and the activation

Consider the following changes.
(i) The number of molecules is increased at constant temperature.

the value of the most probable energy, Emp is unchanged
• the number of molecules with the most probable energy, Emp increases
• the area under the molecular energy distribution curve increases
• the number of molecules with energy greater than the activation energy, Ea increased

Question 17

The curve below shows the Maxwell–Boltzmann distribution of molecular energies, at a
constant temperature, in a gas at the start of a reaction. On this diagram the most probable
molecular energy at this temperature is indicated by the symbol Emp and the activation
energy by the symbol Ea.

Consider the following changes.
(ii) The temperature is decreased without changing the number of molecules.

state how, if at all, the following would vary:
• the value of the most probable energy, Emp
• the number of molecules with the most probable energy, Emp
• the area under the molecular energy distribution curve
• the number of molecules with energy greater than the activation energy, Ea

Answer 17

The curve below shows the Maxwell–Boltzmann distribution of molecular energies, at a
constant temperature, in a gas at the start of a reaction. On this diagram the most probable
molecular energy at this temperature is indicated by the symbol Emp and the activation
energy by the symbol Ea.

Consider the following changes.
(ii) The temperature is decreased without changing the number of molecules.

state how, if at all, the following would vary:
• the position of Emp shifts to the left
• the number of molecules with the most probable energy, Emp increases
• the area under the molecular energy distribution curve is unchanged
• the number of molecules with energy greater than the activation energy, Ea decrease

Question 18

The curve below shows the Maxwell–Boltzmann distribution of molecular energies, at a
constant temperature, in a gas at the start of a reaction. On this diagram the most probable
molecular energy at this temperature is indicated by the symbol Emp and the activation
energy by the symbol Ea.

Consider the following change
(iii) A catalyst is introduced without changing the temperature or the number of
molecules.

state how, if at all, the following would vary:
• the value of the most probable energy, Emp
• the number of molecules with the most probable energy, Emp
• the area under the molecular energy distribution curve
• the number of molecules with energy greater than the activation energy, Ea

Answer 18

The curve below shows the Maxwell–Boltzmann distribution of molecular energies, at a
constant temperature, in a gas at the start of a reaction. On this diagram the most probable
molecular energy at this temperature is indicated by the symbol Emp and the activation
energy by the symbol Ea.

Consider the following change
(iii) A catalyst is introduced without changing the temperature or the number of
molecules.

state how, if at all, the following would vary:
• the position of Emp is unchanged
• the number of molecules with the most probable energy, Emp is unchanged
• the area under the curve is unchanged
• the number of molecules with energy greater than the activation energy, Ea increases

Question 19

4. Draw a Maxwell-Boltzman curve for a sample of gas at a temperature, T1. On the same axes draw
   a second curve for the same sample of gas at a higher temperature, T2. Label the curves and the
   axes clearly. Explain, using these curves, why an increase in temperature increases the rate of a
   gas phase reaction.
   Suggest how and why the first curve would change if a larger mass of gas were used at
   temperature T1

Answer 19

At T2: more molecules (1)
have sufficient energy (1)
plus reference to Ea or shaded area on graph (1)
Larger mass: more particles (1)
higher curve (1)
most probable energy is same

Question 20

) The graphs below show how, starting from A alone, the concentration of A varies with time
at temperatures of 300 K and 320 K for the reversible reaction given in part (b).
) Suggest why, as shown on the graphs, the concentration of A remains constant after
a time.

) Explain why, at 320 K, the concentration of A falls to a lower value compared with
the reaction at 300 K.

Answer 20

) The graphs below show how, starting from A alone, the concentration of A varies with time
at temperatures of 300 K and 320 K for the reversible reaction given in part (b).
) Suggest why, as shown on the graphs, the concentration of A remains constant after
a time.
equilibrium reached

) Explain why, at 320 K, the concentration of A falls to a lower value compared with
the reaction at 300 K.
) Reaction is endothermic
endothermic reaction favoured

Question 21

) The diagram below shows the distribution curve for the energies of molecules in a given
mass of methane.
If the graph were extended to higher energies, at what point would the curve meet the
horizontal axis? Explain your answer.

Answer 21

) The diagram below shows the distribution curve for the energies of molecules in a given
mass of methane.
If the graph were extended to higher energies, at what point would the curve meet the
horizontal axis? Explain your answer.

never as no maximum energy for molecules

Question 22

Sketch on the diagram a curve for a larger mass of methane at the same temperature

Answer 22

peak higher

max at same energy

Question 23

) A sample of ethane containing the same number of moles as the sample of methane
above is at the same temperature and pressure. Describe how the distribution curve
for ethane differs, if at all, from that above.

Answer 23

) A sample of ethane containing the same number of moles as the sample of methane
above is at the same temperature and pressure. Describe how the distribution curve
for ethane differs, if at all, from that above.