Syllabus Edition

First teaching 2023

First exams 2025

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How Far? The Extent of Chemical Change (HL IB Chemistry)

Exam Questions

4 hours73 questions
1a1 mark

Distinguish between the terms reaction quotient, Q, and equilibrium constant, Kc.

1b1 mark

Write an expression for the reaction quotient, Q, for this reaction.

2SO2 (g) + O2 (g) not stretchy rightwards harpoon over leftwards harpoon 2SO3 (g)

1c1 mark

The equilibrium constant, Kc, for the reaction is 0.282 at temperature T whilst the reaction quotient is calculated to be 0.5.

Deduce the direction of the initial reaction.

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2a1 mark

Urea can be made by the direct combination of ammonia and carbon dioxide gases.

2NH3 (g) + CO2 (g) not stretchy rightwards harpoon over leftwards harpoon CO(NH2)2 (g) + H2O (g) 

Write the equilibrium constant expression, Kc.

2b1 mark

ΔH < 0 for the forward reaction.

Predict the effect on the equilibrium constant, Kc, when the temperature is increased.

2c1 mark

Predict what will happen to the equilibrium position if there is a decrease in pressure.

2d
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1 mark

The Kc value for the reaction is determined to be 2 x 10-9 mol dm-3 at 298 K.

Determine the magnitude of Kc if the reaction is reversed.

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3a1 mark

The following reaction was allowed to reach equilibrium at 761 K.

H2 (g) + I2 (g) not stretchy rightwards harpoon over leftwards harpoon 2HI (g)               ΔHθ < 0

Determine the Kc expression for this reaction.

3b
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1 mark

The Kc value for the reaction in part a) is found to be 48.52.

Deduce the Kc value for the following reaction.

1 halfH2 (g) + begin mathsize 14px style 1 half end styleI2 (g) rightwards harpoon over leftwards harpoon HI (g)  

3c1 mark

The temperature of the reaction is increased to 703 K and the new Kvalue is found to be 54.30.

Explain why the value of Kc has changed.

3d1 mark

A catalyst is added in an attempt to speed up the rate of reaction.

State what will happen to the value of Kc.

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4a1 mark

State what is meant by the term dynamic equilibrium.

4b2 marks

Describe two characteristics of a reaction at equilibrium.

4c2 marks

State and explain the effect of a catalyst on the position of equilibrium.

4d1 mark

Methanoic acid reacts with methanol to form the ester methyl methanoate.

HCOOH (l) + CH3OH (l) rightwards harpoon over leftwards harpoon HCOOCH3 (l) + H2O (l)

The esterification reaction is exothermic. State the effect of increasing temperature on the value of the equilibrium constant (Kc) for this reaction.

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5a2 marks

State Le Chatelier's principle.

5b1 mark

Sulfur trioxide, SO3, decomposes to establish an equilibrium producing sulfur dioxide, SO2, and oxygen as shown in the reaction.

2SO3 (g) ⇌ 2SO2 (g) + O2 (g)        ΔH = +196 kJ mol-1

State the effect on the yield of sulfur dioxide if the concentration of sulfur trioxide is increased.

5c1 mark

Give the expression for Kc for the reaction outlined in part (b).

5d
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2 marks

For the reaction outline in part (a), at dynamic equilibrium, the concentrations of each compound are given in the table below when the temperature is 600°C. 

 

SO3

SO2

O2

Concentration at equilibrium (mol dm-3)

0.093

0.100

0.200


Calculate the value of Kc to 3 significant figures.

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6a1 mark

The reaction below shows the decomposition of dinitrogen tetroxide, N2O4, into two molecules of nitrogen dioxide, NO2

N2O4 (g) → 2NO2 (g)        ΔH = +58 kJ mol-1

A dynamic equilibrium is reached at a temperature of 298K. The concentrations of each of the compounds at equilibrium are shown in the table below.

 

N2O4

NO2

Concentration at equilibrium (mol dm-3)

0.0647

0.0206

Give the expression for Kc for this reaction.

6b
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2 marks

Calculate a value for Kc to three significant figures.

6c1 mark

State the units for Kc for the reaction outlined in part (a).

6d3 marks

At the start of the reaction outlined in part (a) dinitrogen tetroxide, N2O4, is the only compound present.

Sketch two lines on the graph shown below to show the change in concentration for both dinitrogen tetroxide, N2O4, and nitrogen dioxide, NO2 as the reaction reaches dynamic equilibrium. 

You should make reference to the information given in the table in part (a).

kc-graph

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7a1 mark

The following reaction was allowed to reach equilibrium at 761 K.

H2 (g) + I2 (g) not stretchy rightwards harpoon over leftwards harpoon 2HI (g)               ΔHθ < 0

State the equilibrium constant expression, Kc , for this reaction.

7b
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1 mark

The following equilibrium concentrations, in mol dm–3, were obtained at 761 K.

[H2 (g)] [I2 (g)] [HI (g)]
8.72 x 10-4 2.72 x 10-3 1.04 x 10-2

Calculate the value of the equilibrium constant at 761 K.

7c
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1 mark

Determine the value of ΔGθ, in kJ, for the above reaction at 761 K using section 1 of the data booklet.

7d1 mark

Comment on whether this reaction is feasible.

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8a
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2 marks

Methanoic acid and methanol react to form the ester methyl methanoate and water as follows:

H2COOH (I) + CH3OH (I) ⇌ HCOOCH3 (I) + H2O (I)

At 35 oC, the free energy change, ΔG, for the reaction is -3.79 kJ mol-1.

Using sections 1 and 2 of the data booklet, calculate the value of Kc for this reaction to 2 decimal places.

8b2 marks

Using your answer to part (a), predict and explain the position of the equilibrium.

8c1 mark

The value for ΔG = -4.21 kJ mol-1 as the temperature is increased to 50°C.

State what happens to the value of the equilibrium constant.

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1a2 marks

Ammonia gas can be synthesized by the direct combination of nitrogen gas and hydrogen gas. When the two gases are reacted together in a sealed container the following equilibrium reaction takes place:

N2 (g)  + 3H2 (g) 2NH3 (g)            H = -92.6 kJ

Describe two characteristics of a reaction in a state of dynamic equilibrium.

1b1 mark

Write the equilibrium constant expression, Kc, for the reaction in part (a).

1c4 marks

Explain, with a reason, how each of the following changes can affect the position of equilibrium in part (a).

i)

The volume of the container is increased.

[2]

ii)
Ammonia is removed from the container.
[2]
1d1 mark

Ammonia is manufactured industrially by the Haber process in which iron is used as a catalyst. Explain the effect of a catalyst on the position of equilibrium and the value of Kc.

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2a1 mark

Sulfuric acid is produced on an industrial scale in the Contact Process. The middle step of the process involves the following equilibrium reaction:               

2SO2 (g) + O2 (g) 2SO3 (g)               ∆H = -198 kJ 

Kc >> 1 at 200 degreeC and 100 kPa

Outline what the information given about Kc tells you about the extent of the reaction at the conditions specified.

2b4 marks

The actual operating conditions of the Contact Process are 450 degreeC and 200 kPa. Explain the choice of using these operating conditions in terms of temperature and pressure.

2c1 mark

Suggest, with a reason, whether using pure oxygen instead of air would be an improvement to the Contact Process.

2d1 mark

Write the equilibrium constant expression for the reverse reaction of the Contact Process.

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3a2 marks

A sample of chlorine gas is reacted with sulfur dioxide at 375 oC in a 1dm3 container. The equilibrium reaction produces colourless sulfuryl chloride, SO2Cl2, and the enthalpy change for the reaction is -84 kJ mol-1.

Write the equation for the reaction and deduce the equilibrium constant expression.

3b3 marks

If the reaction in part (a) is carried out at 300 oC, predict what will happen to the equilibrium concentration of SO2Cl2 and the value of Kc. Explain your answer. 

3c3 marks

If the reaction in (a) is now carried out in a 2.00 dm3 container, predict, with a reason what will happen to the equilibrium concentration of SO2Cl2 and the value of Kc.

3d2 marks

If the same reaction is carried out in part (a) with a catalyst, explain how this will affect the equilibrium concentration of SO2Cl2.

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4a2 marks

A reaction mixture was set up in a syringe containing dinitrogen tetraoxide gas and nitrogen dioxide gas as shown in the equation below:

N2O4 (g) ⇋ 2NO2 (g)                ΔH = +58 kJ mol-1

The appearance of the gases is quite different; dinitrogen tetraoxide is a pale-yellow gas, whereas nitrogen dioxide is dark brown in colour.

State why this equilibrium reaction is considered homogeneous and deduce the equilibrium constant expression for the reaction.

4b3 marks

Explain why the reaction mixture turns darker in colour when it is heated. 

4c3 marks

The reaction which takes place in part (a) has a Kc value of 3.21. A student claims that increasing the temperature of this reaction will increase the value of Kc.

Is the student correct? Justify your answer.

4d3 marks

Using Le Chatelier’s principle, explain what would be seen if the plunger of the syringe was pressed and the gases within the syringe were compressed.

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5a3 marks

During an esterification reaction, methanol and ethanoic acid react together to form the ester, methyl ethanoate, and water as shown below:

CH3OH (l) + CH3COOH (l) ⇋ CH3COOCH3 (l) + H2O (l)   Kc = 7.21 at 298K

A chemist sets up the reaction and allows it to reach dynamic equilibrium at a constant temperature.

i)
State the meaning of the term dynamic equilibrium.
[2]
ii)
Give one key condition which must be satisfied for a reversible reaction to reach dynamic equilibrium. 
[1]
5b2 marks

Once the reaction in part (a) is set up, the chemist leaves it for 24 hours to make sure that it has reached equilibrium.

State how the chemist could check to make sure that the reaction mixture had reached equilibrium.

5c3 marks

When the chemist sampled the concentrations of the substances in the reaction mixture and calculated a value for the reaction quotient, she determined the value of Q to be 5.34. 

i)
State the meaning of the term reaction quotient.
[1]
ii)
Deduce, with a reason, whether the reaction had reached equilibrium and what conclusion can be drawn from the value of Q.
[2]
5d3 marks

Adding more ethanoic acid to the reaction mixture will increase the yield of the ester produced.

Use Le Chatelier’s principle to explain the above statement.

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6a1 mark

Nitrogen(II) oxide is an atmospheric pollutant linked to acid rain. It can be formed by the combustion of fossil fuels or from the following dissociation of nitrosyl chloride. 

2NOCl (g) ⇌ 2NO (g) + Cl2 (g)    

Predict, giving your reason, the sign of the standard entropy change for the forward reaction.

6b1 mark

At 230 OC, the value of KC for the dissociation of nitrosyl chloride is 4.5 x 10-3. Describe the significance of the value of KC

6c2 marks

Using Sections 1 and 2 of the Data Booklet, calculate the standard Gibbs free energy change, ΔGӨ, in kJ mol-1, for this reaction at 230 OC.

6d1 mark

At 465 OC, the value of KC for the dissociation of nitrosyl chloride is 9.2 x 10-2. 

In terms of the equilibrium position, suggest how this KC value supports the fact that the forward reaction is endothermic.

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7a5 marks

The following reaction is used to manufacture sulfuric acid. 

2SO2 (g) + O2 (g) ⇌ 2SO3 (g) 

A mixture of 2.00 mol SO2 (g) and 1.40 mol O2 (g) is placed inside a 1.00 dm3 flask and allowed to reach equilibrium at a temperature, T1. At equilibrium, 0.30 mol of SO3 (g) was present.

Determine the equilibrium concentration of SO2 (g) and O2 (g), and hence calculate the value of KC, including units, at this temperature.

7b2 marks

Using Sections 1 and 2 of the Data Booklet and your answer to (a), calculate the standard Gibbs free energy change, ΔGӨ, in kJ mol-1, for this reaction at a temperature of 700K.

7c1 mark

Experimental data can be used to calculate the reaction quotient, Q, and the equilibrium constant, KC.

Distinguish between these two terms.

7d2 marks

1.20 mol SO2 (g), 1.60 mol O2 (g) and 0.85 mol SO3 (g) were mixed in a 1.00 dm3 container at temperature, T2. 

2SO2 (g) + O2 (g) ⇌ 2SO3 (g) 

Use your answer to (a) to deduce the direction of this reaction, showing your working.

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8a2 marks

Carbon monoxide and chlorine react to form phosgene, COCl2, according to the following equation. 

CO (g) + Cl2 (g) ⇌ COCl2 (g)

Deduce the equilibrium constant expression, KC, including units for this reaction.

8b4 marks

0.50 mol CO (g) and 0.30 mol Cl2 (g) were mixed in a 10.0 dm3 container. At equilibrium, 0.10 mol of COCl2 (g) was present.

Determine the equilibrium concentration of CO (g) and Cl2 (g), and hence calculate the value of KC.

8c3 marks

Use Sections 1 and 2 of the Data Booklet with your answer to (b) to deduce, showing your working, the temperature of the reaction at which the standard Gibbs free energy change, ΔGӨ, is -8.40 kJ.

8d2 marks

At 873 K, the standard Gibbs free energy change, ΔGӨ, was found to be +11.7 kJ.

Deduce, giving your reasons, whether the forward reaction is endothermic or exothermic. Use your answer to (c).

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9a1 mark

The following thermochemical data is for the oxidation of iron to produce iron(III) oxide at 300 K.

2Fe (s) + 3 over 2O2 (g) ⇌ Fe2O3 (s)

  • ΔHӨ = -824.2 kJ mol-1
  • ΔSӨ = -270.5 J K-1 mol-1

Explain why the enthalpy value given is the enthalpy of formation, ΔHӨf, of iron(III) oxide.

9b2 marks

Using Section 1 of the Data Booklet, calculate the standard Gibbs free energy change, ΔGӨ, for the oxidation of iron to iron(III) oxide at 300 K.

9c3 marks

Use you answer to (b) and Sections 1 and 2 of the Data Booklet to calculate a value, in terms of e, for KC for this reaction at 300 K.

9d1 mark

Use your answer to (c) to explain why the following oxidation of iron to iron(III) oxide at 300 K can be considered to be irreversible. 

2Fe (s) + 3 over 2O2 (g) ⇌ Fe2O3 (s)

 

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10a2 marks

Ethanol and ethanoic acid react to form ethyl ethanoate according to the following equation. 

C2H5OH + CH3COOH ⇌ CH3COOC2H5 + H2O 

0.47 mol of ethanol and 0.25 mol of ethanoic acid were mixed in a 5.0 dm3 container and left to reach equilibrium. At equilibrium, there was found to be 0.28 mol of ethanol. Calculate the number of moles of the remaining chemicals at equilibrium.

10b2 marks

The reaction is performed in a 5.0 dm3 container. 

Deduce the equilibrium constant expression, KC, for the reaction of ethanol and ethanoic acid and explain why the number of moles can be used directly in your expression.

 

10c1 mark

Using your answer to part (b), calculate, showing your working, a value for the equilibrium constant expression, KC, for the reaction of ethanol and ethanoic acid.

10d2 marks

A second experiment reacting ethanol and ethanoic acid was performed. Analysis showed the equilibrium mixture to contain 0.16 mol ethanoic acid, 0.11 mol ethyl ethanoate and 0.12 mol water. Calculate the number of moles of ethanol in the equilibrium mixture.

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1a3 marks

The following dynamic equilibrium was reached at temperature, T, in a closed container.

2X (g) + Y (g) ⇌ 2Z (g)             ΔH = -65 kJ mol-1

The value of Kc for the reaction was 75.0 mol-1 dm3 when the equilibrium mixture contained 2.97 mol of Y and 5.38 mol of Z.

i)
Define dynamic equilibrium.
[2]
ii)
Write an expression for Kc for the reaction.
[1]
1b2 marks

If the conditions for a closed container are changed, it can affect the concentrations of the reactants, products and Kc

State the effect, if any, on the concentration of Y at equilibrium if temperature, T, is decreased and give a reason for your answer.

1c
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1 mark

Calculate the equilibrium constant for the following reaction at temperature, T

2Z (g) ⇌ 2X (g) + Y (g)

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2a4 marks

A 0.680 mol sample of SO3 is introduced into a reaction container and allowed to reach equilibrium at temperature T

2SO3 (g) ⇌ 2SO2 (g) + O2 (g)                ΔH = +196 kJ mol-1

The value of Kc for the reaction was 7.9 x 10-3 mol dm-3.

The size of the container for the reaction is increased. State the effect if any on the equilibrium constant, Kc, and the position of equilibrium. Justify your answer.

2b3 marks

The temperature of the reaction in part (a) is increased. State the effect, if any, on the equilibrium constant, Kc, and the position of equilibrium. Justify your answer.

2c
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1 mark

If the value of the equilibrium constant, Kc, is 2.7 x 10-2 at temperature T1 for the reaction:

2SO3 (g) ⇌ 2SO2 (g) + O2 (g)

Calculate the equilibrium constant, Kc, for the reaction:

4SO2 (g) + 2O2 (g) ⇌ 4SO3 (g)

Give your answer to 2 decimal places.

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3a2 marks

A mixture in a container at temperature, T, is allowed to reach equilibrium.

2E (g) ⇌ 2F (g) + G (g)     ΔH = -143 kJ mol-1


The value of Kc for the reaction at T is 2.98 mol dm-3. Comment on the relationship between the concentration of the reactant E and products F and G with regards to Kc.

3b1 mark

Reactants G and H react together to form products J and K according to the equation

3G + H ⇌ 4JK

Write the expression for the equilibrium constant, Kc.

3c3 marks

Diesters are compounds often used as synthetic lubricants for machinery such as compressors. The reaction below shows the formation of a diester from propanoic acid and propane-1,3-diol. 

2CH3CH2COOH + HOCH2CH2CH2OH ⇌ C9H16O4 + 2H2O

The value for Kc at temperature, T, is 1.29.

The forward reaction is slightly exothermic. At a different temperature, T1, the value for Kc increases to 22.78. 

State whether the new temperature, T1, is higher or lower than the original temperature. Justify your answer.

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4a3 marks

The graph below shows the effect of pressure and temperature on the equilibrium yield of gaseous molecules.

JJZI1Mjr_1

 

Using the graph, explain whether the forward reaction is exothermic or endothermic.

4b3 marks

Use the graph to explain whether the forward reaction will involve either an increase or decrease in the number of moles of a gas.

4c3 marks

The graph to show the relationship between temperature and Kc for a different dynamic equilibrium to produce a gaseous product is shown below.

GkgJwoUC_2
 

Use the information shown in the graph to establish whether the forward reaction is exothermic or endothermic. Justify your answer.

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5a
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6 marks

A 0.680 mol sample of SO3 is introduced into a 3.04 dm3 reaction container and allowed to reach equilibrium at temperature T.
32% of the SO3 had decomposed. 

Calculate the value for Kc in this reaction, giving your answer to 2 significant figures. 

2 SO3 (g) ⇌ 2 SO2 (g) + O2 (g)      ΔH = +196 kJ mol-1

5b4 marks

The size of the container for the reaction in part (a) is decreased.
State the effect if any on the equilibrium constant, Kc, and the position of equilibrium. Justify your answer.

5c3 marks

The temperature of the reaction in part (a) is increased.
State the effect, if any, on the equilibrium constant, Kc, and the position of equilibrium. Justify your answer.

5d2 marks

Comment on whether the reaction in part (a) is likely to take place spontaneously at temperature T.

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6a
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5 marks

A mixture of 1.32 moles of E, 1.49 moles of F and 0.752 moles of G were placed into a 5.0 dm3 container at temperature, T, and allowed to reach equilibrium. At equilibrium, the number of moles of E was 1.86.

Calculate the value of the equilibrium constant, Kc , to 3 significant figures. 

2 E (g) ⇌ 2 F (g) + G (g)        ΔH = -143 kJ mol-1

6b
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5 marks

Reactants G and H react together to form products J and K according to the equation

3G + H ⇌ 4J + K

A beaker contained 35 cm3 of 0.18 mol dm-3 of an aqueous solution of G

8.41 x 10-3 moles of H and 3.1 x 10-3 moles of J were also added to the beaker. The equilibrium mixture contained 4.1 x 10-3 moles of G

Calculate the number of moles of H, J and K at equilibrium.

6c
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3 marks

Using sections 1 and 2 of the data booklet, calculate the equilibrium constant at 300 K for the oxidation of iron: 

2Fe(s) + begin mathsize 14px style 3 over 2 end styleO2 (g) → Fe2O3 (s) 
ΔHΘ = -824.2 kJ mol- 1 
ΔSΘ = -270.5 J mol-1
6d1 mark

Suggest what the value for Kc calculated in part (c) suggests about the equilibrium position for the oxidation of iron.

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7a
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3 marks

Diesters are compounds often used as synthetic lubricants for machinery such as compressors. The reaction below shows the formation of a diester from propanoic acid and propane-1,3-diol. 

2CH3CH2COOH + HOCH2CH2CH2OH  ⇌  C9H16O4 + 2H2O

At equilibrium, the reaction mixture contained 3.25 moles of CH3CH2COOH, 1.15 moles of HOCH2CH2CH2OH, and 1.18 moles of C9H16O4.

The value for Kc at temperature, T, is 1.29.

Calculate the concentration of water in the reaction mixture at equilibrium. Give your answer to 3 significant figures. 

7b2 marks

A student deduced that in order to calculate the value of Kc for the reaction in part (a) you must work out the concentrations using the overall volume. 

Is the student correct? Justify your answer.

7c
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2 marks

Using sections 1 and 2 of the data booklet, determine the value for ΔG for the reverse reaction in part(a) given that temperature T= 30°C. Give your answer, in kJ, to 2 significant figures.

7d4 marks

The reverse reaction in part (a) is slightly endothermic. At a different temperature, T2, the value for ΔG decreases to -0.52 kJ mol-1. 

State whether the new temperature, T2, is higher or lower than the original temperature.
Justify your answer.

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