R2.3 How far? The extent of chemical change

Subjects

Features

Practice exam-style IB Chemistry questions for How far? The extent of chemical change, aligned with the syllabus and grouped by topic.

Verified by Dennis M.

Paper

Difficulty

Status

Level

This content can't be printed from the browser. Use Export PDF with Pro for an offline copy.

Question 1

SL • Paper 1A

Easy

Calculator Permitted

SL • Paper 1A

Easy

Calculator Permitted

A sealed flask contains liquid propanone and propanone vapour at constant temperature. The system has reached dynamic equilibrium.

What is true for this system?

A.

Condensation has stopped and evaporation continues.

B.

The rates of evaporation and condensation are equal.

C.

Evaporation has stopped and condensation continues.

D.

The amounts of liquid and vapour must be equal.

Question 2

SL • Paper 1A

Easy

Calculator Permitted

SL • Paper 1A

Easy

Calculator Permitted

A catalyst is added to a reversible reaction mixture at constant temperature before equilibrium has been reached.

What is the effect of the catalyst?

A.

The time taken to reach equilibrium decreases and $K$ increases.

B.

The equilibrium composition changes and $K$ is unchanged.

C.

The equilibrium composition changes and $K$ increases.

D.

The time taken to reach equilibrium decreases and $K$ is unchanged.

Question 3

SL • Paper 1A

Easy

Calculator Permitted

SL • Paper 1A

Easy

Calculator Permitted

For the homogeneous equilibrium

$2NO_2(g) \rightleftharpoons N_2O_4(g)$

what is the equilibrium constant expression?

A.

$K=\dfrac{[N_2O_4]}{2[NO_2]}$

B.

$K=\dfrac{2[NO_2]}{[N_2O_4]}$

C.

$K=\dfrac{[NO_2]^2}{[N_2O_4]}$

D.

$K=\dfrac{[N_2O_4]}{[NO_2]^2}$

Question 4

SL • Paper 1A

Easy

Calculator Permitted

SL • Paper 1A

Easy

Calculator Permitted

At a fixed temperature, the equilibrium constant for

$A(g)+2B(g) \rightleftharpoons C(g)$

is $4.0\times10^3$ .

What is the equilibrium constant for the reverse reaction at the same temperature?

A.

$2.5\times10^{-4}$

B.

$1.6\times10^7$

C.

$4.0\times10^3$

D.

$4.0\times10^{-3}$

Build a Practice Exam — Chemistry HL

Test your knowledge with a custom paper containing questions from selected topics.

Question 5

SL • Paper 1A

Easy

Calculator Permitted

SL • Paper 1A

Easy

Calculator Permitted

Carbon dioxide is in equilibrium between air and water in a closed container.

$CO_2(g) \rightleftharpoons CO_2(aq)$

The pressure is increased at constant temperature.

What is the effect on the equilibrium position and on $K$ ?

A.

Equilibrium shifts to $CO_2(aq)$ and $K$ is unchanged.

B.

Equilibrium shifts to $CO_2(aq)$ and $K$ increases.

C.

Equilibrium shifts to $CO_2(g)$ and $K$ is unchanged.

D.

Equilibrium shifts to $CO_2(g)$ and $K$ decreases.

Question 6

SL • Paper 2

Easy

Calculator Permitted

SL • Paper 2

Easy

Calculator Permitted

A small amount of liquid bromine is sealed in a flask at constant temperature. After some time the intensity of the orange-brown vapour remains constant.

State why the flask must be sealed for equilibrium to be established.

[1]

Describe two characteristics of the system when dynamic equilibrium has been reached.

[2]

Question 7

SL • Paper 2

Easy

Calculator Permitted

SL • Paper 2

Easy

Calculator Permitted

Nitrosyl chloride decomposes according to the following homogeneous equilibrium.

$2\text{NOCl}(g) \rightleftharpoons 2\text{NO}(g) + Cl_2(g)$

Deduce the equilibrium constant expression for the reaction as written.

[1]

At a fixed temperature the value of $K$ for the reaction as written is $3.20 \times 10^{-3}$ . Calculate the value of $K$ for the reverse reaction at the same temperature and state which side is favoured for the reaction as written.

[2]

Question 8

SL • Paper 2

Easy

Calculator Permitted

SL • Paper 2

Easy

Calculator Permitted

Two weak monoprotic acids have the following acid dissociation constants at the same temperature.

$K_a(HA) = 1.8 \times 10^{-5}$

$K_a(HB) = 6.3 \times 10^{-10}$

State which acid is stronger and explain your answer in terms of extent of ionization.

[2]

Calculate the equilibrium constant for the reverse of the dissociation of $HA$ at the same temperature.

[1]

Build a Practice Exam — Chemistry HL

Test your knowledge with a custom paper containing questions from selected topics.

Question 9

SL • Paper 1A

Medium

Calculator Permitted

SL • Paper 1A

Medium

Calculator Permitted

For the equilibrium

$N_2(g)+3H_2(g) \rightleftharpoons 2NH_3(g)\qquad \Delta H<0$

what happens when the temperature is increased?

A.

The equilibrium shifts right and $K$ decreases.

B.

The equilibrium shifts left and $K$ decreases.

C.

The equilibrium shifts left and $K$ increases.

D.

The equilibrium shifts right and $K$ increases.

Question 10

HL • Paper 1A

Medium

Calculator Permitted

HL • Paper 1A

Medium

Calculator Permitted

For the equilibrium

$H_2(g)+I_2(g) \rightleftharpoons 2HI(g)$

$K=50$ at a particular temperature. At one instant, $[H_2]=0.20\ \text{mol dm}^{-3}$ , $[I_2]=0.30\ \text{mol dm}^{-3}$ and $[HI]=0.60\ \text{mol dm}^{-3}$ .

What is the value of $Q$ and the direction favoured?

A.

$Q=6.0$ ; reverse reaction favoured

B.

$Q=50$ ; the mixture is at equilibrium

C.

$Q=6.0$ ; forward reaction favoured

D.

$Q=0.17$ ; forward reaction favoured

Question 11

HL • Paper 1A

Medium

Calculator Permitted

HL • Paper 1A

Medium

Calculator Permitted

For the homogeneous equilibrium

$A(g) \rightleftharpoons 2B(g)$

$1.00\ \text{mol dm}^{-3}$ of $A$ is placed in a sealed container. At equilibrium, $[A]=0.64\ \text{mol dm}^{-3}$ .

What is the value of $K$ ?

A.

$0.20$

B.

$0.81$

C.

$0.41$

D.

$1.13$

Question 12

HL • Paper 1A

Medium

Calculator Permitted

HL • Paper 1A

Medium

Calculator Permitted

For the equilibrium

$A(g)+B(g) \rightleftharpoons C(g)$

$K=10$ at a fixed temperature. At equilibrium, $[A]=0.20\ \text{mol dm}^{-3}$ and $[B]=0.10\ \text{mol dm}^{-3}$ .

What is $[C]$ at equilibrium?

A.

$0.020\ \text{mol dm}^{-3}$

B.

$500\ \text{mol dm}^{-3}$

C.

$2.0\ \text{mol dm}^{-3}$

D.

$0.20\ \text{mol dm}^{-3}$

Build a Practice Exam — Chemistry HL

Test your knowledge with a custom paper containing questions from selected topics.

Question 13

HL • Paper 1A

Medium

Calculator Permitted

HL • Paper 1A

Medium

Calculator Permitted

For a reaction at $298\ \text{K}$ , $K=1.0\times10^{-2}$ .

Using $\Delta G^\circ=-RT\ln K$ , what is $\Delta G^\circ$ ?

A.

$-11.4\ \text{kJ mol}^{-1}$

B.

$+11.4\ \text{kJ mol}^{-1}$

C.

$-5.70\ \text{kJ mol}^{-1}$

D.

$+5.70\ \text{kJ mol}^{-1}$

Question 14

SL • Paper 2

Medium

Calculator Permitted

SL • Paper 2

Medium

Calculator Permitted

The chromate-dichromate equilibrium is shown.

$2CrO_4^{2-}(aq) + 2H^+(aq) \rightleftharpoons Cr_2O_7^{2-}(aq) + H_2O(l)$

yellow $CrO_4^{2-}$ , orange $Cr_2O_7^{2-}$

A few drops of aqueous sodium hydroxide are added to an equilibrium mixture at constant temperature.

Deduce the equilibrium constant expression for this reaction, omitting water.

[1]

Predict and explain the colour change after adding sodium hydroxide.

[2]

State the effect of adding sodium hydroxide on the value of $K$ .

[1]

Question 15

SL • Paper 2

Medium

Calculator Permitted

SL • Paper 2

Medium

Calculator Permitted

In a sealed bottle of carbonated water, carbon dioxide is involved in the heterogeneous equilibrium:

$CO_2(g) \rightleftharpoons CO_2(aq)$

The bottle is opened at constant temperature.

State the change in pressure of $CO_2(g)$ when the bottle is opened.

[1]

Explain the effect on the equilibrium position and the value of $K$ .

[2]

Question 16

SL • Paper 2

Medium

Calculator Permitted

SL • Paper 2

Medium

Calculator Permitted

Dinitrogen tetroxide and nitrogen dioxide form the following equilibrium in a sealed syringe.

$N_2O_4(g) \rightleftharpoons 2NO_2(g)$

The forward reaction is endothermic. $N_2O_4$ is colourless and $NO_2$ is brown.

Predict and explain the effect of increasing the temperature on the colour intensity and on $K$ .

[3]

State the effect of decreasing the volume of the syringe at constant temperature on the equilibrium position.

[1]

Build a Practice Exam — Chemistry HL

Test your knowledge with a custom paper containing questions from selected topics.

Question 17

HL • Paper 2

Medium

Calculator Permitted

HL • Paper 2

Medium

Calculator Permitted

At a particular temperature, $K = 49.0$ for the equilibrium:

$H_2(g) + I_2(g) \rightleftharpoons 2HI(g)$

At one instant, the concentrations are:

$[H_2] = 0.0200\ \text{mol dm}^{-3}$ , $[I_2] = 0.0100\ \text{mol dm}^{-3}$ and $[HI] = 0.0600\ \text{mol dm}^{-3}$ .

Calculate the reaction quotient, $Q$ .

[2]

Determine the direction in which the reaction proceeds to reach equilibrium.

[1]

Question 18

HL • Paper 2

Medium

Calculator Permitted

HL • Paper 2

Medium

Calculator Permitted

A homogeneous equilibrium is established at constant temperature.

$A(g) + B(g) \rightleftharpoons C(g)$

Initially, $[A] = 0.500\ \text{mol dm}^{-3}$ and $[B] = 0.500\ \text{mol dm}^{-3}$ , with no $C$ present. At equilibrium, $[C] = 0.300\ \text{mol dm}^{-3}$ .

Determine the equilibrium concentrations of $A$ and $B$ .

[1]

Calculate the value of $K$ at this temperature.

[2]

Question 19

SL • Paper 1B

Medium

Calculator Permitted

SL • Paper 1B

Medium

Calculator Permitted

A small amount of liquid bromine is placed in a sealed glass vessel at constant temperature. The graph shows the rates of evaporation and condensation of bromine over time.

Identify the time at which dynamic equilibrium is first reached.

[1]

Describe two characteristics of the system after this time.

[2]

Explain why the same equilibrium would not be established if the vessel were open.

[1]

Question 20

HL • Paper 1A

Medium

Calculator Permitted

HL • Paper 1A

Medium

Calculator Permitted

A weak acid dissociates according to

$HA(aq) \rightleftharpoons H^+(aq)+A^-(aq)$

For $0.100\ \text{mol dm}^{-3}$ $HA$ , $K_a=1.6\times10^{-5}$ . Using the small- $K$ approximation, what is the equilibrium concentration of $H^+(aq)$ ?

A.

$1.3\times10^{-3}\ \text{mol dm}^{-3}$

B.

$4.0\times10^{-2}\ \text{mol dm}^{-3}$

C.

$1.6\times10^{-6}\ \text{mol dm}^{-3}$

D.

$1.6\times10^{-4}\ \text{mol dm}^{-3}$

Build a Practice Exam — Chemistry HL

Test your knowledge with a custom paper containing questions from selected topics.

Question 21

HL • Paper 1A

Medium

Calculator Permitted

HL • Paper 1A

Medium

Calculator Permitted

For a reaction at $298\ \text{K}$ , $\Delta G^\circ=-17.1\ \text{kJ mol}^{-1}$ .

Using $\Delta G^\circ=-RT\ln K$ , what is the approximate value of $K$ ?

A.

$1.0\times10^{-3}$

B.

$1.0\times10^{1}$

C.

$1.0\times10^{-1}$

D.

$1.0\times10^{3}$

Question 22

HL • Paper 2

Medium

Calculator Permitted

HL • Paper 2

Medium

Calculator Permitted

A $0.200\ \text{mol dm}^{-3}$ solution of a weak acid, $HA$ , is prepared at $298\ \text{K}$ .

$HA(aq) \rightleftharpoons H^+(aq) + A^-(aq)$

$K_a = 1.6 \times 10^{-5}$

Use the approximation $[HA]_{\text{initial}} \approx [HA]_{\text{eqm}}$ .

Calculate $[H^+]$ in the solution.

[2]

Calculate the pH of the solution.

[1]

State why the approximation is reasonable in this calculation.

[1]

Question 23

HL • Paper 2

Medium

Calculator Permitted

HL • Paper 2

Medium

Calculator Permitted

For a reaction at $298\ \text{K}$ , the standard Gibbs energy change is $+12.4\ \text{kJ mol}^{-1}$ .

Use $\Delta G^\circ = -RT\ln K$ and $R = 8.31\ \text{J K}^{-1}\ \text{mol}^{-1}$ .

Calculate the equilibrium constant, $K$ , at $298\ \text{K}$ .

[2]

State whether reactants or products are favoured under standard conditions.

[1]

Question 24

HL • Paper 2

Medium

Calculator Permitted

HL • Paper 2

Medium

Calculator Permitted

At $350\ \text{K}$ , a homogeneous equilibrium has $K = 2.50 \times 10^4$ .

Use $\Delta G^\circ = -RT\ln K$ and $R = 8.31\ \text{J K}^{-1}\ \text{mol}^{-1}$ .

Calculate $\Delta G^\circ$ in $\text{kJ mol}^{-1}$ .

[2]

Explain what the sign of $\Delta G^\circ$ indicates about the equilibrium position.

[1]

Build a Practice Exam — Chemistry HL

Test your knowledge with a custom paper containing questions from selected topics.

Question 25

SL • Paper 1B

Medium

Calculator Permitted

SL • Paper 1B

Medium

Calculator Permitted

Nitrosyl chloride decomposes in a sealed vessel according to the homogeneous equilibrium:

$2\text{NOCl}(g) \rightleftharpoons 2\text{NO}(g) + \text{Cl}_2(g)$

The table gives equilibrium concentrations at one temperature.

Species	Equilibrium concentration / mol dm $^{-3}$
NOCl	0.100
NO	0.300
Cl2	0.150

Deduce the equilibrium constant expression, $K$ , for the reaction as written.

[2]

Calculate $K$ using the equilibrium concentrations in the table.

[1]

State what the magnitude of $K$ suggests about the extent of the reaction as written.

[1]

Question 26

SL • Paper 1B

Medium

Calculator Permitted

SL • Paper 1B

Medium

Calculator Permitted

The equilibrium constant for the formation of methanol is shown at different temperatures.

$\text{CO}(g)+2\text{H}_2(g) \rightleftharpoons \text{CH}_3\text{OH}(g)$

Identify the temperature at which methanol is most favoured at equilibrium.

[1]

At $500\ \text{K}$ , $K=5.0$ for the reaction as written. Calculate $K$ for the reverse reaction at this temperature.

[1]

Use the data to explain whether the forward reaction is exothermic or endothermic.

[2]

Question 27

SL • Paper 1B

Medium

Calculator Permitted

SL • Paper 1B

Medium

Calculator Permitted

The equilibrium below is established in aqueous solution. The product ion is red.

$\text{Fe}^{3+}(aq)+\text{SCN}^-(aq) \rightleftharpoons \text{FeSCN}^{2+}(aq)$

Absorbance is used to monitor the concentration of $\text{FeSCN}^{2+}$ after different additions.

Describe and explain the effect of adding $\text{Fe}^{3+}(aq)$ on the equilibrium mixture.

[2]

Silver ions react with $\text{SCN}^-(aq)$ . Explain the change after adding silver ions.

[1]

State the effect of these concentration changes on the value of $K$ at constant temperature.

[1]

Question 28

HL • Paper 1B

Medium

Calculator Permitted

HL • Paper 1B

Medium

Calculator Permitted

At a fixed temperature, $K=50.0$ for the equilibrium:

$\text{H}_2(g)+\text{I}_2(g) \rightleftharpoons 2\text{HI}(g)$

The table gives concentrations in two reaction mixtures.

Mixture	$[H_2]$ / mol dm $^{-3}$	$[I_2]$ / mol dm $^{-3}$	$[HI]$ / mol dm $^{-3}$
A	0.200	0.200	0.400
B	0.0500	0.100	0.500

Calculate the reaction quotient, $Q$ , for mixture A.

[1]

Determine the direction in which mixture A will proceed to reach equilibrium.

[2]

Mixture B has $Q=50.0$ . State what this indicates about mixture B.

[1]

Build a Practice Exam — Chemistry HL

Test your knowledge with a custom paper containing questions from selected topics.

Question 29

HL • Paper 2

Medium

Calculator Permitted

HL • Paper 2

Medium

Calculator Permitted

The equilibrium constant for the reaction below is $K = 0.250$ at a particular temperature.

$2A(g) \rightleftharpoons B(g)$

At one instant, $[A] = 0.400\ \text{mol dm}^{-3}$ and $[B] = 0.0200\ \text{mol dm}^{-3}$ .

Calculate $Q$ for the mixture at this instant.

[2]

Determine the direction in which the reaction is favoured and state the sign of $\Delta G$ under these conditions.

[2]

Question 30

SL • Paper 1B

Hard

Calculator Permitted

SL • Paper 1B

Hard

Calculator Permitted

Carbon dioxide is allowed to reach equilibrium between the gas phase and water in a sealed syringe.

$\text{CO}_2(g) \rightleftharpoons \text{CO}_2(aq)$

The plunger is then pushed in at constant temperature.

State the change in the concentration of dissolved $\text{CO}_2$ after the plunger is pushed in.

[1]

Explain the change using Le Châtelier's principle.

[2]

State the effect of the pressure change on $K$ for this equilibrium.

[1]

Question 31

SL • Paper 1B

Hard

Calculator Permitted

SL • Paper 1B

Hard

Calculator Permitted

Dinitrogen tetroxide and nitrogen dioxide form the equilibrium below. Nitrogen dioxide is brown and dinitrogen tetroxide is colourless.

$\text{N}_2\text{O}_4(g) \rightleftharpoons 2\text{NO}_2(g)$

For the reaction as written, $\Delta H_r^\circ=+57\ \text{kJ mol}^{-1}$ . The diagrams show the colour intensity of sealed tubes under different conditions.

Predict and explain the effect of increasing temperature on the colour of the mixture.

[2]

Compare the effect of adding a catalyst with the effect of increasing temperature.

[2]

State the effect of cooling on the value of $K$ for the reaction as written.

[1]

Question 32

HL • Paper 1B

Hard

Calculator Permitted

HL • Paper 1B

Hard

Calculator Permitted

A homogeneous equilibrium is established at constant temperature.

$A(g)+B(g) \rightleftharpoons C(g)$

The table shows initial concentrations and one measured equilibrium concentration.

Species	Initial concentration / mol dm $^{-3}$	Equilibrium concentration / mol dm $^{-3}$
A(g)	0.500
B(g)	0.500
C(g)	0.000	0.250

Complete the equilibrium concentrations of A and B.

[2]

Calculate $K$ for the equilibrium.

[1]

State whether products or reactants are favoured by this equilibrium constant.

[1]

Build a Practice Exam — Chemistry HL

Test your knowledge with a custom paper containing questions from selected topics.

Question 33

HL • Paper 1B

Hard

Calculator Permitted

HL • Paper 1B

Hard

Calculator Permitted

A weak monoprotic acid, HA, is dissolved in water.

$\text{HA}(aq) \rightleftharpoons \text{H}^+(aq)+\text{A}^-(aq)$

At $298\ \text{K}$ , $K_a=1.8\times10^{-5}$ for HA. The initial concentration of HA is $0.100\ \text{mol dm}^{-3}$ .

State	HA / mol dm $^{-3}$	H $^+$ / mol dm $^{-3}$	A $^-$ / mol dm $^{-3}$
Initial	0.100	0	0
Change	$-x$	$+x$	$+x$
Equilibrium	$0.100 - x$	$x$	$x$

Using the small- $K_a$ approximation, calculate $[\text{H}^+]$ at equilibrium.

[2]

Calculate the pH of the solution.

[1]

Justify the use of the approximation for this solution.

[1]

Question 34

HL • Paper 1B

Hard

Calculator Permitted

HL • Paper 1B

Hard

Calculator Permitted

The equilibrium constant for a reaction at $298\ \text{K}$ is measured as $K=3.2\times10^4$ .

The relationship between standard Gibbs energy change and $K$ is:

$\Delta G^\circ=-RT\ln K$

Quantity	Value	Units
Equilibrium constant, $K$	$3.2 \times 10^{4}$	dimensionless
Temperature, $T$	298	$K$
Gas constant, $R$	8.31	$\text{J mol}^{-1}\ \text{K}^{-1}$

Calculate $\Delta G^\circ$ in $\text{kJ mol}^{-1}$ at $298\ \text{K}$ .

[2]

State which side of the equilibrium is favoured under standard conditions.

[1]

State the value of $\Delta G$ when the reaction mixture has reached equilibrium.

[1]

Question 35

SL • Paper 2

Hard

Calculator Permitted

SL • Paper 2

Hard

Calculator Permitted

A small amount of liquid bromine is placed in a sealed flask. After some time the intensity of the brown vapour becomes constant.

The bromine system reaches a physical equilibrium.

I.

Explain why the flask must be sealed for equilibrium to be established.

[2]

II.

State one macroscopic observation that indicates equilibrium has been reached.

[1]

Compare and contrast this physical equilibrium with a chemical equilibrium.

[3]

Question 36

SL • Paper 2

Hard

Calculator Permitted

SL • Paper 2

Hard

Calculator Permitted

Methanol can be manufactured from carbon monoxide and hydrogen in a reversible reaction.

$CO(g)+2H_2(g)\rightleftharpoons CH_3OH(g)$

The equilibrium law can be applied to this homogeneous equilibrium.

I.

Deduce the equilibrium constant expression for the reaction as written.

[1]

II.

Explain how the stoichiometry of the equation is represented in the equilibrium expression.

[2]

At a certain temperature $K=12$ for the reaction as written. Discuss what this value of $K$ indicates about the equilibrium, determine $K$ for the reverse reaction, and state what happens to $K$ if more $H_2$ is added at constant temperature.

[3]

Build a Practice Exam — Chemistry HL

Test your knowledge with a custom paper containing questions from selected topics.

Question 37

SL • Paper 2

Hard

Calculator Permitted

SL • Paper 2

Hard

Calculator Permitted

Ammonia is produced in the Haber process.

$N_2(g)+3H_2(g)\rightleftharpoons 2NH_3(g)\qquad \Delta H<0$

Two changes are considered for an equilibrium mixture at constant temperature.

I.

Explain the effect of increasing pressure on the equilibrium yield of ammonia.

[2]

II.

Explain the effect of adding an iron catalyst on the equilibrium composition and on $K$ .

[2]

Discuss the effect of increasing temperature on the Haber equilibrium.

[2]

Question 38

HL • Paper 1B

Hard

Calculator Permitted

HL • Paper 1B

Hard

Calculator Permitted

For a reversible reaction at constant temperature, the reaction quotient $Q$ was calculated at different times after mixing the reactants. The equilibrium constant is $K=12$ at this temperature.

For the curve that starts below $K$ , identify the direction favoured at the first labelled time.

[1]

For the curve that starts above $K$ , explain why the reverse reaction is favoured initially.

[2]

Explain the relationship between $Q$ , $K$ and Gibbs energy when equilibrium is reached.

[2]

Question 39

HL • Paper 1B

Hard

Calculator Permitted

HL • Paper 1B

Hard

Calculator Permitted

For the equilibrium below at $298\ \text{K}$ , the standard Gibbs energy change is $+4.80\ \text{kJ mol}^{-1}$ .

$\text{N}_2\text{O}_4(g) \rightleftharpoons 2\text{NO}_2(g)$

A mixture is then prepared with non-equilibrium concentrations of the two gases.

Item	Value	Units
Reaction	$\text{N}_2\text{O}_4(g) \rightleftharpoons 2\text{NO}_2(g)$
$T$	298	K
ΔG $^\circ$	+4.80	$\text{kJ mol}^{-1}$
$Q$ (prepared mixture)	0.050	dimensionless

Calculate $K$ for the reaction as written at $298\ \text{K}$ .

[2]

Determine $K$ for the reverse reaction at the same temperature.

[1]

For the prepared mixture, $Q=0.050$ . Predict the direction in which the reaction will proceed.

[1]

Question 40

SL • Paper 2

Hard

Calculator Permitted

SL • Paper 2

Hard

Calculator Permitted

An aqueous equilibrium involving cobalt ions is shown.

$[\text{Co(H}_2\text{O)}_6]^{2+}(aq)+4Cl^-(aq)\rightleftharpoons [\text{CoCl}_4]^{2-}(aq)+6H_2O(l)$

The forward reaction is endothermic. The octahedral complex is pink and the tetrahedral complex is blue.

Changes are made to a solution at equilibrium.

I.

Predict and explain the colour change when concentrated hydrochloric acid is added.

[2]

II.

Predict and explain the effect of heating the equilibrium mixture on the value of $K$ .

[2]

Discuss whether adding water as solvent should be included in the equilibrium expression for this reaction.

[2]

Build a Practice Exam — Chemistry HL

Test your knowledge with a custom paper containing questions from selected topics.

Question 41

SL • Paper 2

Hard

Calculator Permitted

SL • Paper 2

Hard

Calculator Permitted

Nitrogen dioxide and dinitrogen tetroxide form an equilibrium in a sealed gas syringe.

$N_2O_4(g)\rightleftharpoons 2NO_2(g)$

$N_2O_4$ is colourless and $NO_2$ is brown.

The plunger is pushed in at constant temperature.

I.

Predict the direction of the equilibrium shift after compression and explain your answer.

[2]

II.

State the effect of compression on the value of $K$ .

[1]

Evaluate the statement: "The mixture becomes less brown after compression because compression removes $NO_2$ molecules from the syringe."

[3]

Question 42

SL • Paper 2

Hard

Calculator Permitted

SL • Paper 2

Hard

Calculator Permitted

A gas $X$ is used in an industrial absorption process represented by the heterogeneous equilibrium:

$X(g)\rightleftharpoons X(aq)$

The dissolution of $X$ is exothermic.

Pressure and temperature are changed independently.

I.

Explain the effect of increasing pressure on the amount of $X(aq)$ at equilibrium.

[2]

II.

Explain the effect of decreasing temperature on the value of $K$ for the equilibrium as written.

[2]

Evaluate the proposal that the absorption process should use high pressure, low temperature and a catalyst to maximize the equilibrium amount of dissolved $X$ .

[3]

Question 43

HL • Paper 2

Hard

Calculator Permitted

HL • Paper 2

Hard

Calculator Permitted

At $700\ \text{K}$ , the equilibrium constant for the reaction below is $K=4.0$ .

$CO(g)+H_2O(g)\rightleftharpoons CO_2(g)+H_2(g)$

At one instant, $[CO]=0.50\ \text{mol dm}^{-3}$ , $[H_2O]=0.40\ \text{mol dm}^{-3}$ , $[CO_2]=0.60\ \text{mol dm}^{-3}$ and $[H_2]=0.20\ \text{mol dm}^{-3}$ .

The reaction quotient is used before equilibrium is reached.

I.

Calculate $Q$ for the mixture at this instant.

[2]

II.

Deduce the direction in which the reaction proceeds to reach equilibrium.

[2]

Explain why comparing $Q$ with a value of $K$ measured at a different temperature would be invalid.

[2]

Question 44

HL • Paper 2

Hard

Calculator Permitted

HL • Paper 2

Hard

Calculator Permitted

Methanoic acid is a weak acid. At $298\ \text{K}$ , $K_a=1.8\times10^{-4}$ for:

$HCOOH(aq)\rightleftharpoons H^+(aq)+HCOO^-(aq)$

A solution initially contains $0.100\ \text{mol dm}^{-3}$ methanoic acid.

The small- $K$ approximation may be used.

I.

Explain why $[HCOOH]_{initial}\approx[HCOOH]_{eqm}$ is reasonable.

[2]

II.

Calculate $[H^+]$ and the pH of the solution using this approximation.

[3]

Discuss how the magnitude of $K_a$ relates to acid strength.

[2]

Build a Practice Exam — Chemistry HL

Test your knowledge with a custom paper containing questions from selected topics.

Question 45

HL • Paper 2

Hard

Calculator Permitted

HL • Paper 2

Hard

Calculator Permitted

At $298\ \text{K}$ , the equilibrium constant for a reaction is $K=3.2\times10^{-5}$ . The standard Gibbs energy change is related to $K$ by

$\Delta G^\circ=-RT\ln K$

The thermodynamic position of equilibrium is considered.

I.

Calculate $\Delta G^\circ$ in $\text{kJ mol}^{-1}$ using $R=8.31\ \text{J K}^{-1}\text{mol}^{-1}$ .

[3]

II.

Deduce which side of the equilibrium is favoured under standard conditions.

[1]

Evaluate a student's claim that a positive $\Delta G^\circ$ means the reaction can never form products.

[2]

Question 46

HL • Paper 2

Hard

Calculator Permitted

HL • Paper 2

Hard

Calculator Permitted

At a particular temperature, $K=1.80$ for the equilibrium:

$A(g)+B(g)\rightleftharpoons 2C(g)$

A mixture initially contains $0.600\ \text{mol dm}^{-3}$ of $A$ , $0.600\ \text{mol dm}^{-3}$ of $B$ and no $C$ .

The mixture is allowed to reach equilibrium.

I.

Set up the equilibrium concentrations in terms of $x$ , where $x$ is the decrease in $[A]$ .

[2]

II.

Calculate the equilibrium concentration of $C$ .

[3]

Evaluate whether the equilibrium mixture is product-favoured based on the value of $K$ and the calculated composition.

[2]

Question 47

HL • Paper 2

Hard

Calculator Permitted

HL • Paper 2

Hard

Calculator Permitted

At $350\ \text{K}$ , a reaction has $\Delta G^\circ=-18.0\ \text{kJ mol}^{-1}$ for the equation:

$D(g)\rightleftharpoons E(g)$

The equilibrium constant is calculated from the Gibbs energy change.

I.

Calculate $K$ for the reaction at $350\ \text{K}$ .

[3]

II.

Calculate $K$ for $E(g)\rightleftharpoons D(g)$ at the same temperature.

[1]

Explain how the signs and magnitudes of $\Delta G^\circ$ and $K$ describe the position of equilibrium.

[3]

Question 48

HL • Paper 2

Hard

Calculator Permitted

HL • Paper 2

Hard

Calculator Permitted

At a fixed temperature, $K=0.250$ for the homogeneous equilibrium:

$2X(g)\rightleftharpoons Y(g)+Z(g)$

At equilibrium, $[X]=0.300\ \text{mol dm}^{-3}$ , $[Y]=0.150\ \text{mol dm}^{-3}$ and $[Z]=0.150\ \text{mol dm}^{-3}$ . The mixture was prepared initially with only $X$ and $Y$ present.

The equilibrium data are checked and then used to infer the initial mixture.

I.

Verify that the stated equilibrium concentrations are consistent with $K=0.250$ .

[2]

II.

Determine the initial concentrations of $X$ and $Y$ .

[4]

Evaluate the conclusion that because $K<1$ , very little $Y$ and $Z$ can be present at equilibrium.

[2]

Build a Practice Exam — Chemistry HL

Test your knowledge with a custom paper containing questions from selected topics.

R2.2 How fast? The rate of chemical change

R3.1 Proton transfer reactions

The Ultimate All-in-One
IB Platform

All content on this website has been developed independently from and is not endorsed by the International Baccalaureate Organization. International Baccalaureate and IB are registered trademarks owned by the International Baccalaureate Organization.