How Far? The Extent of Chemical Change (DP IB Chemistry: HL): Exam Questions

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.

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

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]

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 K_c.

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 

K_c >> 1 at 200 C and 100 kPa

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

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

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

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

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.

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 K_c. Explain your answer. 

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 K_c.

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

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.

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

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

Is the student correct? Justify your answer.

Predict and explain the effect of increasing the pressure by compressing the plunger on the colour of the gas mixture.

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)   K_c = 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]

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.

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]

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.

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.

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

Using Sections 1 and 2 of the data booklet, calculate the standard Gibbs free energy change, ΔG^Ө, in kJ mol^-1, for this reaction at 230 ^OC.

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

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

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 K_c.

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

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

Distinguish between these two terms.

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.

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, K_c.

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 K_c.

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, ΔG^θ, is -8.40 kJ.

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

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

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

2Fe (s) + O₂ (g) ⇌ Fe₂O₃ (s)

• ΔH^θ = -824.2 kJ mol^-1

• ΔS^θ = -270.5 J K^-1 mol^-1

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

Using Section 1 of the data booklet, calculate the standard Gibbs free energy change, ΔG^θ, for the oxidation of iron to iron(III) oxide at 300 K.

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

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) + O₂ (g) ⇌ Fe₂O₃ (s)

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 K_c 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, K_c, and the position of equilibrium. Justify your answer.

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

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

2SO₃ (g) ⇌ 2SO₂ (g) + O₂ (g)

Calculate the equilibrium constant, K_c, for the reaction:

4SO₂ (g) + 2O₂ (g) ⇌ 4SO₃ (g)

Give your answer to 2 decimal places.

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 K_c 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 K_c.

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

3G + H ⇌ 4J + K

Write the expression for the equilibrium constant, K_c.

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 K_c at temperature, T, is 1.29.

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

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

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

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

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

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

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

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 K_c in this reaction, giving your answer to 2 significant figures. 

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

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

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

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

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, K_c, to 3 significant figures. 

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

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.

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

2Fe(s) + O₂ (g) → Fe₂O₃ (s) 
ΔH^θ = -824.2 kJ mol^{- 1} 
ΔS^θ = -270.5 J mol^-1

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

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 K_c at temperature, T, is 1.29.

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

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

Is the student correct? Justify your answer.

Using sections 1 and 2 of the data booklet, determine the value for the standard Gibbs free energy change, ΔG^θ , for the reverse reaction in part(a) given that temperature T= 30°C.

Give your answer, in kJ, to 2 significant figures.

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

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