Chemical Equilibria (Edexcel International A Level Chemistry): Exam Questions

This question is about sulfuric acid and its salts.

The manufacture of sulfuric acid involves the equilibrium

2SO₂(g) + O₂(g) ⇌ 2SO₃(g)   Δ_rH = −197 kJ mol⁻¹

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)

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)

Sulfur trioxide is used to produce sulfuric acid.

i) Commercial concentrated sulfuric acid contains 98.5 % H₂SO₄ and 1.5 % water by mass.

The density of concentrated sulfuric acid is 1800 g dm⁻³.

Calculate the concentration of this sulfuric acid in mol dm⁻³.

(2)

ii) The pH of a 0.10 mol dm⁻³ solution of sulfuric acid at 25 °C is 0.97.

Calculate the concentration of hydrogen ions, in mol dm⁻³, in this solution.

(1)

iii) In an aqueous solution of sulfuric acid, the following equilibria exist.

Explain, in terms of these equilibria, why the concentration of hydrogen ions in a 0.10 mol dm⁻³ solution of sulfuric acid is not 0.20 mol dm⁻³.

No calculation is required.

(2)

A buffer solution is made from HSO₄^– and SO₄^2– ions.

i) Write two ionic equations involving HSO₄^– and SO₄^2– ions to show how this solution acts as a buffer.

State symbols are not required.

(2)

ii) A buffer solution is formed by mixing 25.0 cm³ of a solution that is 0.150 mol dm^-3 with respect to SO₄^2– ions with 75.0 cm³ of a solution that is 0.100 mol dm^-3 with respect to HSO₄^– ions.

 Calculate the pH of this buffer solution.

 [K_a for HSO₄^– ions = 0.012 mol dm^-3]

(5)

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

4HCl (g) + O₂ (g) ⇌ 2H₂O (g) + 2Cl₂ (g)   ∆H = –114 kJ mol^–1

Explain what effect, if any, each of the following changes has on the yield of chlorine at equilibrium and on the equilibrium constant, K_p.

i) An increase in the total pressure

(3)

ii) An increase in the temperature

(2)

iii) The use of a catalyst

(2)

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.

4HCl(g) + O₂(g) ⇌ 2H₂O(g) + 2Cl₂(g)

i) Complete the table.

(3)

ii) Write the expression for the equilibrium constant, K_p .

(1)

iii) Use your answers to (b)(i) and (b)(ii) to calculate the value for K_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, ΔS_total .

(2)

Draw a sketch of entropy against temperature for water to illustrate the entropy changes as temperature increases, including when water changes state.

A scale is not required for the vertical axis.