Edexcel IAL Chemistry A2 Topic 13 Chemical equilibria

Pearson Edexcel IAL Chemistry syllabusChemistry YCH11First assessment 2019

Start from the balanced equation, construct the correct concentration or partial-pressure expression and build an equilibrium table before explaining what can change K.

Exam points

Write Kc or Kp expressions with stoichiometric powers and omitted pure solids or liquids.
Calculate equilibrium concentrations or partial pressures and constants with appropriate units.
Distinguish changes to composition from the temperature-dependent value of the equilibrium constant.

[Maximum number: 2]

The reaction shown is at equilibrium. The forward reaction is endothermic.

\mathrm{C}(\mathrm{~s})+\mathrm{CO}_{2}(\mathrm{~g}) \rightleftharpoons 2 \mathrm{CO}(\mathrm{~g})

(a)

Which will increase when the temperature is lowered?

the mole fraction of carbon dioxide

the partial pressure of carbon monoxide

the rate of the backward reaction

the value of $K_{\mathrm{p}}$ for the forward reaction

[ 1 ]

(b)

At $680^{\circ} \mathrm{C}$ and $1 \mathrm{~atm}, 52.6 \%$ of the molecules in the gas mixture are carbon monoxide. What is the partial pressure of carbon dioxide, in atmospheres?

0.237

0.263

0.474

0.526

[ 1 ]

[Maximum number: 1]

The total entropy change, $\Delta S_{\text {total }}$ , of a reaction at 298 K is $-85.0 \mathrm{JK}^{-1} \mathrm{~mol}^{-1}$ .
What is the value of the equilibrium constant for this reaction at 298 K ?

\left[R=8.31 \mathrm{~J} \mathrm{~mol}^{-1} \mathrm{~K}^{-1}\right]

$3.61 \times 10^{-5}$

$9.07 \times 10^{-1}$

$9.66 \times 10^{-1}$

$2.77 \times 10^{4}$

[Maximum number: 8]

This question is about sulfuric acid and its salts.

(a)

The manufacture of sulfuric acid involves the equilibrium

2 \mathrm{SO}_{2}(\mathrm{~g})+\mathrm{O}_{2}(\mathrm{~g}) \rightleftharpoons 2 \mathrm{SO}_{3}(\mathrm{~g}) \quad \Delta_{\mathrm{r}} H=-197 \mathrm{~kJ} \mathrm{~mol}^{-1}

[ 8 ]

(i)

A catalyst of vanadium(V) oxide is used in this reaction.

State the effect, if any, of the catalyst on the value of the equilibrium constant, $K_{p}$ .

[ 1 ]

(ii)

The temperature used for this reaction in industry is 700 K .

Explain, in terms of the equilibrium constant and the equilibrium position, the effect of an increase in temperature on the equilibrium yield of sulfur trioxide.
(2)

[ 2 ]

(iii)

Write the expression for the equilibrium constant, $K_{p}$ , for this equilibrium. State symbols are not required.

[ 1 ]

(iv)

A mixture of 2.00 mol of sulfur dioxide and 1.00 mol of oxygen is allowed to reach equilibrium at 5.00 atm pressure. 1.60 mol of sulfur trioxide is formed.

Calculate the value of $K_{p}$ .
Include units and give your answer to an appropriate number of significant figures.
(4)

[ 4 ]

[Maximum number: 4]

Esters are used in flavourings and perfumes.
They can be made by reactions involving alcohols.

A flask containing a mixture of 0.200 mol of ethanoic acid and 0.150 mol of ethanol was left at $25^{\circ} \mathrm{C}$ , in the presence of a catalyst, until equilibrium had been established.

\mathrm{CH}_{3} \mathrm{COOH}(\mathrm{l})+\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}(\mathrm{l}) \rightleftharpoons \mathrm{CH}_{3} \mathrm{COOC}_{2} \mathrm{H}_{5}(\mathrm{l})+\mathrm{H}_{2} \mathrm{O}(\mathrm{l})

The ethanoic acid present in the equilibrium mixture required $34.8 \mathrm{~cm}^{3}$ of a $2.50 \mathrm{~mol} \mathrm{dm}^{-3}$ solution of sodium hydroxide for complete neutralisation.

Calculate the value of the equilibrium constant, $K_{c}$ , for this reaction at $25^{\circ} \mathrm{C}$ .

[Maximum number: 6]

Ammonia is manufactured in the Haber Process.

\mathrm{N}_{2}(\mathrm{~g})+3 \mathrm{H}_{2}(\mathrm{~g}) \rightleftharpoons 2 \mathrm{NH}_{3}(\mathrm{~g})

In this process, pressures between 100 atm and 300 atm and temperatures between 675 K and 725 K are usually used.

(a)

Write the expression for the equilibrium constant, $K_{\mathrm{p}}$ , for the reaction in the Haber Process.

\mathrm{N}_{2}(\mathrm{~g})+3 \mathrm{H}_{2}(\mathrm{~g}) \rightleftharpoons 2 \mathrm{NH}_{3}(\mathrm{~g})

(b)

A mixture of nitrogen, hydrogen and ammonia is at equilibrium at 255 atm and 700 K . The partial pressure of nitrogen is 25 atm and that of hydrogen is 150 atm .

Calculate the equilibrium constant, $K_{p}$ , for the formation of ammonia, at 700 K . Include units in your answer.

[ 3 ]

(c)

The pressure of the system is doubled at constant temperature.

Explain the effect of this on the yield of ammonia by considering the change in the $K_{\mathrm{p}}$ expression and hence the position of equilibrium.
(3)

[ 3 ]

[Maximum number: 3]

The equation for the formation of ammonia in the Haber Process is shown

1 / 2 \mathrm{~N}_{2}(\mathrm{~g})+1 \frac{1}{2} \mathrm{H}_{2}(\mathrm{~g}) \rightleftharpoons \mathrm{NH}_{3}(\mathrm{~g})

(a)

The industrial synthesis of ammonia

1 / 2 \mathrm{~N}_{2}(\mathrm{~g})+11 / 2 \mathrm{H}_{2}(\mathrm{~g}) \rightleftharpoons \mathrm{NH}_{3}(\mathrm{~g})

is carried out at pressures of about 20000 kPa and temperatures between 700 K and 750 K . These temperatures are higher than the answer to (c)(iii).

[ 3 ]

(i)

State the relationship between the total entropy, $\Delta S_{\text {total }}$ , and the equilibrium constant, K.

[ 1 ]

(ii)

Calculate the value of the equilibrium constant K at 750 K.

[ 2 ]

[Maximum number: 16]

The reversible reaction between hydrogen chloride and oxygen produces water vapour and chlorine.

4 \mathrm{HCl}(\mathrm{~g})+\mathrm{O}_{2}(\mathrm{~g}) \rightleftharpoons 2 \mathrm{H}_{2} \mathrm{O}(\mathrm{~g})+2 \mathrm{Cl}_{2}(\mathrm{~g}) \quad \Delta H=-114 \mathrm{~kJ} \mathrm{~mol}^{-1}

(a)

Explain what effect, if any, each of the following changes has on the yield of chlorine at equilibrium and on the equilibrium constant, $K_{\mathrm{p}}$ .

[ 7 ]

(i)

An increase in the total pressure

[ 3 ]

(ii)

An increase in the temperature

[ 2 ]

(iii)

The use of a catalyst

[ 2 ]

(b)

0.850 mol of hydrogen chloride was mixed with 0.600 mol of oxygen and allowed to reach equilibrium in a closed flask.
At equilibrium the total pressure was 1.50 atm and there was 0.250 mol of chlorine in the flask.

4 \mathrm{HCl}(\mathrm{~g})+\mathrm{O}_{2}(\mathrm{~g}) \rightleftharpoons 2 \mathrm{H}_{2} \mathrm{O}(\mathrm{~g})+2 \mathrm{Cl}_{2}(\mathrm{~g})

[ 9 ]

(i)

Complete the table.

[ 3 ]

(ii)

Write the expression for the equilibrium constant, $K_{\mathrm{p}}$ .

[ 1 ]

(iii)

Use your answers to (b)(i) and (b)(ii) to calculate the value for $K_{\mathrm{p}}$ . Give your answer to an appropriate number of significant figures, and include units.

[ 3 ]

(iv)

Use your answer to (b)(iii) to calculate a value for the total entropy change of the reaction, $\Delta S_{\text {total }}$ .

[ 2 ]

Exam points

Question 3

Question 3(a)

Question 3(b)

Question 6

Question 18

Question 18(a)

Question 18(a)(i)

Question 18(a)(ii)

Question 18(a)(iii)

Question 18(a)(iv)

Question 21(a)(i)

Question 19

Question 19(b)(i)

Question 19(b)(ii)

Question 19(b)(iii)

Question 22

Question 22(d)

Question 22(d)(i)

Question 22(d)(ii)

Question 20

Question 20(a)

Question 20(a)(i)

Question 20(a)(ii)

Question 20(a)(iii)

Question 20(b)

Question 20(b)(i)

Question 20(b)(ii)

Question 20(b)(iii)

Question 20(b)(iv)