Edexcel IAL Chemistry AS Topic 9 Kinetics and Equilibria

[Maximum number: 11]

Ammonium nitrate, $\mathrm{NH}_{4} \mathrm{NO}_{3}$ , is used in the manufacture of fertilisers and explosives. It is produced on a large scale using only methane, water and air.
The process has four stages.

(a)

The first two reactions in Stage 1 involve the production of hydrogen.

At temperature $T_{1}$ , methane reacts with excess steam to give hydrogen.

\mathrm{CH}_{4}(\mathrm{~g})+\mathrm{H}_{2} \mathrm{O}(\mathrm{~g}) \rightleftharpoons 3 \mathrm{H}_{2}(\mathrm{~g})+\mathrm{CO}(\mathrm{~g}) \quad \Delta H=+206 \mathrm{~kJ} \mathrm{~mol}^{-1}

At a different temperature, $T_{2}$ , the carbon monoxide reacts with more steam.

\mathrm{CO}(\mathrm{~g})+\mathrm{H}_{2} \mathrm{O}(\mathrm{~g}) \rightleftharpoons \mathrm{H}_{2}(\mathrm{~g})+\mathrm{CO}_{2}(\mathrm{~g}) \quad \Delta H=-42 \mathrm{~kJ} \mathrm{~mol}^{-1}

[ 2 ]

(i)

Give the reason why excess steam is used in the first reaction.

[ 1 ]

An answer that makes reference to one of the following points:

- shifts position of equilibrium to the right

OR

increases the (equilibrium) yield (of $\mathrm{H}_{2}$ )

Ignore to increase rate (of forward reaction) Ignore cheaper to have steam in excess Ignore to react with most of the $\mathrm{CH}_{4}$

Allow to increase yield (of CO / products)

Do not award so all of the $\mathrm{CH}_{4}$ reacts / so reaction goes to completion

Do not award to increase the moles of gas/pressure

(ii)

Predict which of $T_{1}$ and $T_{2}$ is the higher temperature.

Justify your answer.

[ 1 ]

An answer that makes reference to the following points:

- $T_{1}$ (is higher)

and

(first reaction is) endothermic

Accept reverse argument

Allow positive enthalpy change for endothermic Allow (first reaction) absorbs (heat) energy for endothermic

Ignore just +206 for endothermic

Ignore correct reference to effect of temperature on equilibrium yields

Do not award absorbs more energy to break (reactant) bonds

(b)

In Stage 2, the hydrogen from Stage 1 reacts with nitrogen (from the air) to produce ammonia. The conditions for this reaction are:
- a temperature of 700 K
- a pressure in the range $100-200 \mathrm{~atm}$
- an iron catalyst

\mathrm{N}_{2}(\mathrm{~g})+3 \mathrm{H}_{2}(\mathrm{~g}) \rightleftharpoons 2 \mathrm{NH}_{3}(\mathrm{~g}) \quad \Delta H=-92 \mathrm{~kJ} \mathrm{~mol}^{-1}

Give one advantage and one disadvantage of using a pressure of 200 atm, compared to a pressure of 100 atm , in Stage 2.

[ 2 ]

An answer that makes reference to the following points:

- advantage of using high pressure

- disadvantage of using high pressure

Examples of advantage: shifts position of equilibrium to right / products

OR increases (equilibrium) yield (of $\mathrm{NH}_{3}$ )

OR increases rate

OR increases occupation of catalyst active sites

Ignore any reference to collisions

(c)

The reaction in Stage 2 has an activation energy, $E_{\text {cat }}=+70 \mathrm{~kJ} \mathrm{~mol}^{-1}$ .

The uncatalysed reaction between $\mathrm{N}_{2}$ and $\mathrm{H}_{2}$ has an activation energy, $E_{\mathrm{a}}=+290 \mathrm{~kJ} \mathrm{~mol}^{-1}$ .

[ 4 ]

(i)

Complete the profile for the catalysed and uncatalysed reactions.

Label the activation energies and the enthalpy change of reaction, $\Delta H$ .
Your diagram must match the scale shown for the production of $\mathrm{NH}_{3}$ .

Figure for Question (i) — Edexcel A-Level Chemistry AS

[ 3 ]

An answer that makes reference to the following
points:

- $\Delta H$ labelled

and

arrow pointing downwards

(1)

- labelled reaction profiles for uncatalysed and catalysed reactions

(1)

- correct scale for activation energies

(1)

Example of labelled reaction profile:
$\mathrm{H} / \mathrm{kJ} \mathrm{mol}^{-1}$
reaction progress
Allow arrows to start/end within one small square of correct
placement and penalise incorrect placement once only
Allow -92 / 'enthalpy change' for $\Delta H$
Do not award double headed arrow
Allow any form of unambiguous labelling, eg values
Allow double headed arrows
Do not award downward arrows
Do not award $E_{\text {cat }}>E_{\text {a }}$
Accept accuracy of ± one small square
Ignore scale shown on y-axis

Figure for Answer to Question (i) — Edexcel A-Level Chemistry AS

(ii)

Suggest why the use of the catalyst makes Stage 2 more sustainable.

[ 1 ]

(d)

In Stage 3, nitrogen monoxide, NO, is produced in the reaction between $\mathrm{NH}_{3}$ (from Stage 2) and $\mathrm{O}_{2}$ (from the air). The conditions used are a temperature of 1100 K in the presence of a platinum-rhodium catalyst.

4 \mathrm{NH}_{3}(\mathrm{~g})+5 \mathrm{O}_{2}(\mathrm{~g}) \rightleftharpoons 4 \mathrm{NO}(\mathrm{~g})+6 \mathrm{H}_{2} \mathrm{O}(\mathrm{~g}) \quad \Delta H=-905 \mathrm{~kJ} \mathrm{~mol}^{-1}

[ 1 ]

(i)

Give one reason why a high temperature is needed in this reaction.

[ 1 ]

An answer that makes reference to one of the following points:

- increase rate

OR

rate is slow at low temperature

OR

catalyst does not work at low temperature

OR

so more reactants/collisions have $E \geq E_{\mathrm{a}}$

OR

to break O=O / N-H bonds

Do not award to increase yield

Do not award to shift position of equilibrium (to left / right)

Do not award reverse reaction is endothermic

Allow to increase the number of successful collisions Ignore to increase collision frequency

Allow catalyst more efficient at high temperature

Allow to activate the catalyst

Accept (to reach) high activation energy

Allow to break bonds in oxygen/ammonia/reactants

(e)

The NO from the first reaction in Stage 3 is cooled and then converted to nitrogen dioxide, $\mathrm{NO}_{2}$ , by reaction with more $\mathrm{O}_{2}$ .

2 \mathrm{NO}(\mathrm{~g})+\mathrm{O}_{2}(\mathrm{~g}) \rightleftharpoons 2 \mathrm{NO}_{2}(\mathrm{~g})

Nitric acid, $\mathrm{HNO}_{3}(\mathrm{aq})$ , is produced by the addition of water.

3 \mathrm{NO}_{2}(\mathrm{~g})+\mathrm{H}_{2} \mathrm{O}(\mathrm{l}) \rightarrow 2 \mathrm{HNO}_{3}(\mathrm{aq})+\mathrm{NO}(\mathrm{~g})

Explain how adding water in the second reaction affects the yield of $\mathrm{NO}_{2}$ in the first reaction.

[ 2 ]

An explanation that makes reference to the following points:

- $\mathrm{NO}_{2}$ removed (in second reaction)

- shifting position of equilibrium (in first reaction) to right and increasing the yield (of $\mathrm{NO}_{2}$ )

(1)

Allow (as) NO formed (in second reaction)

Ignore $\mathrm{HNO}_{3}$ is formed (in second reaction) Ignore reaction is irreversible Ignore $\mathrm{NO}_{2}$ dissolves

[Maximum number: 2]

A Maxwell-Boltzmann distribution curve for the particles present in a reaction mixture is shown. Number of particles with energy, E

Figure for Question 2 — Edexcel A-Level Chemistry AS

(a)

Which statement is correct?

A

position X represents the mean energy of the particles

B

activation energy, $E_{\mathrm{a}}$ , is the mean energy of the particles that react

C

the area under the curve to the right of the activation energy, $E_{\mathrm{a}}$ , represents the number of particles with enough energy to react

D

adding a catalyst moves the activation energy, $E_{\mathrm{a}}$ , to the right

[ 1 ]

(b)

What happens to the distribution curve when the temperature of the gas is decreased?

Position of the peak

Height of the peak

shifts to the right

lower

shifts to the right

higher

shifts to the left

lower

shifts to the left

higher

[ 1 ]

[Maximum number: 7]

Rhubarb stems contain ethanedioic acid. Ethanedioic acid reacts with acidified potassium manganate(VII), $\mathrm{KMnO}_{4}$ , and decolorises the solution.

(a)

A student carried out an experiment using rhubarb juice as a source of ethanedioic acid.

The student used different volumes of rhubarb juice and a constant volume of manganate(VII) as shown. An indication of rate was calculated using $1 \div \mathrm{t}$ .

[ 3 ]

(i)

Complete the table.

Table for Question (i) — Edexcel A-Level Chemistry AS

[ 1 ]

807

υ.υσο-й

0.40

(1)

- boxes completed to 3SF

261

0.00383

3.83

, 40

ก กค.A.T

$1{ }^{m}$

(ii)

Explain why a rate of reaction is greater at higher concentrations.

[ 2 ]

(b)

The Maxwell-Boltzmann distribution for the particles in a reaction is shown.

[ 4 ]

(i)

Draw a curve to represent the distribution at a higher temperature.

[ 1 ]

a line that has a similar shape with peak that is lower and to the right

Example of a drawing

Ignore shading

Do not award if the line cut the curve more once

Do not award if the line plateaus above the height of the $E_{\mathrm{a}}$ line
Do not award if line does not start at the origin
Ignore changes to the $E_{\mathrm{a}}$ line

(ii)

Explain why increasing the temperature increases the rate of reaction.

[ 3 ]

[Maximum number: 1]

Ammonium nitrate, $\mathrm{NH}_{4} \mathrm{NO}_{3}$ , is used in the manufacture of fertilisers and explosives. It is produced on a large scale using only methane, water and air.
The process has four stages.

In Stage 3, nitrogen monoxide, NO, is produced in the reaction between $\mathrm{NH}_{3}$ (from Stage 2) and $\mathrm{O}_{2}$ (from the air). The conditions used are a temperature of 1100 K in the presence of a platinum-rhodium catalyst.

4 \mathrm{NH}_{3}(\mathrm{~g})+5 \mathrm{O}_{2}(\mathrm{~g}) \rightleftharpoons 4 \mathrm{NO}(\mathrm{~g})+6 \mathrm{H}_{2} \mathrm{O}(\mathrm{~g}) \quad \Delta H=-905 \mathrm{~kJ} \mathrm{~mol}^{-1}

Give one reason why a high temperature is needed in this reaction.

An answer that makes reference to one of the following points:

- increase rate

OR

rate is slow at low temperature

OR

catalyst does not work at low temperature

OR

so more reactants/collisions have $E \geq E_{\mathrm{a}}$

OR

to break O=O / N-H bonds

Do not award to increase yield

Do not award to shift position of equilibrium (to left / right)

Do not award reverse reaction is endothermic

Allow to increase the number of successful collisions Ignore to increase collision frequency

Allow catalyst more efficient at high temperature

Allow to activate the catalyst

Accept (to reach) high activation energy

Allow to break bonds in oxygen/ammonia/reactants

[Maximum number: 1]

A Maxwell-Boltzmann distribution curve for the particles present in a reaction mixture is shown. Number of particles with energy, E

Figure for Question 5 — Edexcel A-Level Chemistry AS

Which statement is correct?

A

position X represents the mean energy of the particles

B

activation energy, $E_{\mathrm{a}}$ , is the mean energy of the particles that react

C

the area under the curve to the right of the activation energy, $E_{\mathrm{a}}$ , represents the number of particles with enough energy to react

D

adding a catalyst moves the activation energy, $E_{\mathrm{a}}$ , to the right

[Maximum number: 1]

Rhubarb stems contain ethanedioic acid. Ethanedioic acid reacts with acidified potassium manganate(VII), $\mathrm{KMnO}_{4}$ , and decolorises the solution.

A student carried out an experiment using rhubarb juice as a source of ethanedioic acid.

The student used different volumes of rhubarb juice and a constant volume of manganate(VII) as shown. An indication of rate was calculated using $1 \div \mathrm{t}$ .

Complete the table.

Table for Question 6 — Edexcel A-Level Chemistry AS

807

υ.υσο-й

0.40

(1)

- boxes completed to 3SF

261

0.00383

3.83

, 40

ก กค.A.T

$1{ }^{m}$

[Maximum number: 1]

A Maxwell-Boltzmann distribution curve for the particles present in a reaction mixture is shown. Number of particles with energy, E

Figure for Question 7 — Edexcel A-Level Chemistry AS

What happens to the distribution curve when the temperature of the gas is decreased?

Position of the peak

Height of the peak

shifts to the right

lower

shifts to the right

higher

shifts to the left

lower

shifts to the left

higher

[Maximum number: 1]

The Sabatier process uses carbon dioxide and hydrogen to produce methane and water.

\mathrm{CO}_{2}(\mathrm{~g})+4 \mathrm{H}_{2}(\mathrm{~g}) \rightleftharpoons \mathrm{CH}_{4}(\mathrm{~g})+2 \mathrm{H}_{2} \mathrm{O}(\mathrm{~g}) \quad \Delta_{\mathrm{r}} H=-165 \mathrm{~kJ} \mathrm{~mol}^{-1}

The process is carried out at a temperature of $400^{\circ} \mathrm{C}$ and a pressure of 30 atm , in the presence of a nickel catalyst.

State how a catalyst increases the rate of a reaction.

[Maximum number: 3]

Ammonium nitrate, $\mathrm{NH}_{4} \mathrm{NO}_{3}$ , is used in the manufacture of fertilisers and explosives. It is produced on a large scale using only methane, water and air.
The process has four stages.

The reaction in Stage 2 has an activation energy, $E_{\text {cat }}=+70 \mathrm{~kJ} \mathrm{~mol}^{-1}$ .

The uncatalysed reaction between $\mathrm{N}_{2}$ and $\mathrm{H}_{2}$ has an activation energy, $E_{\mathrm{a}}=+290 \mathrm{~kJ} \mathrm{~mol}^{-1}$ .

Complete the profile for the catalysed and uncatalysed reactions.

Label the activation energies and the enthalpy change of reaction, $\Delta H$ .
Your diagram must match the scale shown for the production of $\mathrm{NH}_{3}$ .

Figure for Question 9 — Edexcel A-Level Chemistry AS

An answer that makes reference to the following
points:

- $\Delta H$ labelled

and

arrow pointing downwards

(1)

- labelled reaction profiles for uncatalysed and catalysed reactions

(1)

- correct scale for activation energies

(1)

Example of labelled reaction profile:
$\mathrm{H} / \mathrm{kJ} \mathrm{mol}^{-1}$
reaction progress
Allow arrows to start/end within one small square of correct
placement and penalise incorrect placement once only
Allow -92 / 'enthalpy change' for $\Delta H$
Do not award double headed arrow
Allow any form of unambiguous labelling, eg values
Allow double headed arrows
Do not award downward arrows
Do not award $E_{\text {cat }}>E_{\text {a }}$
Accept accuracy of ± one small square
Ignore scale shown on y-axis

Figure for Answer to Question 9 — Edexcel A-Level Chemistry AS

[Maximum number: 1]

Ammonium nitrate, $\mathrm{NH}_{4} \mathrm{NO}_{3}$ , is used in the manufacture of fertilisers and explosives. It is produced on a large scale using only methane, water and air.
The process has four stages.

The reaction in Stage 2 has an activation energy, $E_{\text {cat }}=+70 \mathrm{~kJ} \mathrm{~mol}^{-1}$ .

The uncatalysed reaction between $\mathrm{N}_{2}$ and $\mathrm{H}_{2}$ has an activation energy, $E_{\mathrm{a}}=+290 \mathrm{~kJ} \mathrm{~mol}^{-1}$ .

Suggest why the use of the catalyst makes Stage 2 more sustainable.

[Maximum number: 3]

Some diesel cars contain an extra catalytic converter for the reduction of nitrogen oxides $\left(\mathrm{NO}_{\mathrm{x}}\right)$ in exhaust gases.
A solution of urea is used for this process.

Figure for Question 11 — Edexcel A-Level Chemistry AS

urea

The catalytic converter contains metal oxides. When the exhaust gases pass through the catalytic converter, ammonia reacts with $\mathrm{NO}_{\mathrm{x}}$ gases to form nitrogen and water.

Explain how a catalyst increases the rate of a chemical reaction.

Use the Maxwell-Boltzmann distribution shown and refer to the collision theory.

Number of particles with energy, E

[Maximum number: 1]

Which statement is not true for a system at dynamic equilibrium?

A

the position of equilibrium is affected by temperature, by pressure and by catalysts

B

the concentration of reactants is constant

C

the forward and backward reactions occur at equal rates

D

the equilibrium can be reached from either direction

[Maximum number: 2]

Fuels

Fuels burn in oxygen to release a lot of energy.
Many hydrocarbons and alcohols are used as fuels. During complete combustion, they produce carbon dioxide and water.

Petrol contains 2,2,4-trimethylpentane, an isomer of octane, that promotes smooth combustion.

Figure for Question 13 — Edexcel A-Level Chemistry AS

2,2,4-trimethylpentane

Alcohols, such as methanol and ethanol, can be used as fuels either on their own or as additives in petrol.

Ethanol can be manufactured by reacting ethene with steam.

\mathrm{C}_{2} \mathrm{H}_{4}(\mathrm{~g})+\mathrm{H}_{2} \mathrm{O}(\mathrm{~g}) \rightleftharpoons \mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}(\mathrm{~g}) \quad \Delta_{\mathrm{r}} H=-45 \mathrm{~kJ} \mathrm{~mol}^{-1}

This reaction is usually carried out in industry at $300^{\circ} \mathrm{C}$ and 70 atm pressure using a catalyst.

Explain the effect on the equilibrium position and the equilibrium yield of ethanol if the reaction is carried out at $300^{\circ} \mathrm{C}$ and 200 atm pressure.

[Maximum number: 5]

The Sabatier process uses carbon dioxide and hydrogen to produce methane and water.

\mathrm{CO}_{2}(\mathrm{~g})+4 \mathrm{H}_{2}(\mathrm{~g}) \rightleftharpoons \mathrm{CH}_{4}(\mathrm{~g})+2 \mathrm{H}_{2} \mathrm{O}(\mathrm{~g}) \quad \Delta_{\mathrm{r}} H=-165 \mathrm{~kJ} \mathrm{~mol}^{-1}

The process is carried out at a temperature of $400^{\circ} \mathrm{C}$ and a pressure of 30 atm , in the presence of a nickel catalyst.

Discuss the advantages and disadvantages of using such high temperatures and pressures in the process.

Edexcel A-Level Chemistry AS 9 Introduction to Kinetics and Equilibria Questions

Exam points

Covered syllabus points

Covered syllabus points

Question 1

Question (a)

Question (i)

Question (ii)

Question (b)

Question (c)

Question (i)

Question (ii)

Question (d)

Question (i)

Question (e)

Question 2

Question (a)

Question (b)

Question 3

Question (a)

Question (i)

Question (ii)

Question (b)

Question (i)

Question (ii)

Question 4

Question 5

Question 6

Question 7

Question 8

Question 9

Question 10

Question 11

Question 12

Question 13

Question 14