A-Level CIE Chemistry 1.8 Chemical Equilibria, Le Chatelier's Principle & Kc: 2 (a) (i) Explain the meaning of

Question

2 (a) (i) Explain the meaning of the term enthalpy change of formation.

The enthalpy change of formation refers to the heat energy change that occurs when one mole of a compound is formed from its elements in their standard states. This value indicates the stability of the compound; if the enthalpy change is negative, the formation of the compound is exothermic, while a positive change indicates an endothermic process.

(ii) Give the equation for the reaction for which the enthalpy change corresponds to the standard enthalpy change of formation of liquid sulfur trioxide, SO₃. Include state symbols.

S(s) + 3/2O₂(g) → SO₃(l)

(b) Ammonia is manufactured by the Haber process.

N₂(g) + 3H₂(g) ⇌ 2NH₃(g)

(i) Use bond energies from the Data Booklet to calculate the enthalpy change of reaction for the Haber process. Include a sign in your answer.

Enthalpy change = Bond energies of reactants - Bond energies of products = [944 + 3(436)] - [2(NH₃)] = 2252 - 2340 = -88 kJ mol⁻¹

(ii) State the essential operating conditions for the Haber process.

- Iron catalyst
- 400-500°C
- High pressure

(iii) Explain the choices of temperature and pressure for the Haber process.

High temperature increases the reaction rate, while low temperature favors the yield of ammonia. Hence, an optimal balance must be achieved.

(c) One of the major uses of ammonia is in the manufacture of fertilizers such as diammonium hydrogen phosphate, (NH₄)₂HPO₄.

(i) Write an equation for the formation of diammonium hydrogen phosphate by the reaction between ammonia and phosphoric acid, H₃PO₄.

2NH₃ + H₃PO₄ → (NH₄)₂HPO₄

(ii) Explain this reaction in terms of the Brønsted-Lowry theory.

Ammonia acts as a Brønsted base, accepting a proton (H⁺) from phosphoric acid, which acts as a Brønsted acid, donating a proton.

(d) The use of nitrate fertilizers can give rise to environmental consequences in terms of effects on both rivers and the atmosphere.

(i) Explain how the uncontrolled use of nitrate fertilizers can result in a severe reduction in water quality in rivers.

Excessive nitrates lead to algal blooms, which deplete oxygen in the water and result in 'dead zones', harming aquatic life.

(ii) Oxides of nitrogen are produced by the action of bacteria on nitrate fertilizers. Explain the problems associated with the release of oxides of nitrogen into the atmosphere. Include an equation in your answer.

Nitrogen oxides can contribute to air pollution, acid rain, and the formation of ground-level ozone. For example:

2NO + O₂ → 2NO₂

2 (a) (i) Explain the meaning of the term enthalpy change of formation - CIE - A-Level Chemistry - Question 2 - 2015 - Paper 1

Worked Solution & Example Answer:2 (a) (i) Explain the meaning of the term enthalpy change of formation - CIE - A-Level Chemistry - Question 2 - 2015 - Paper 1

Explain the meaning of the term enthalpy change of formation.

Give the equation for the reaction for which the enthalpy change corresponds to the standard enthalpy change of formation of liquid sulfur trioxide, SO₃. Include state symbols.

Use bond energies from the Data Booklet to calculate the enthalpy change of reaction for the Haber process. Include a sign in your answer.

State the essential operating conditions for the Haber process.

Explain the choices of temperature and pressure for the Haber process.

Write an equation for the formation of diammonium hydrogen phosphate by the reaction between ammonia and phosphoric acid, H₃PO₄.

Explain this reaction in terms of the Brønsted-Lowry theory.

Explain how the uncontrolled use of nitrate fertilizers can result in a severe reduction in water quality in rivers.

Explain the problems associated with the release of oxides of nitrogen into the atmosphere. Include an equation in your answer.

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