Equilibrium I (Edexcel A Level Chemistry): Exam Questions

This question is about the oxidation of ammonia.

One equation for the oxidation of ammonia is

4NH₃ (g) + 3O₂ (g)  2N₂ (g) + 6H₂O (g)

Write the expression, including units, for the equilibrium constant K_c for this reaction.

Expression ..................................................

 Units ..................................................

Nitric acid is made from ammonia. One of the stages in nitric acid production involves the oxidation of ammonia to produce nitrogen(II) oxide, NO. In this process, a mixture of ammonia and oxygen is passed over a platinum-rhodium catalyst. One manufacturer uses a pressure of 5 atm and a temperature of 850 °C.

The equation for this reaction is different from that in 3(a). 

4NH₃ (g) + 5O₂ (g) → 4NO (g) + 6H₂O (g)   Δ_rH = −904.8 kJ mol⁻¹

i) Use this equation, and the enthalpy changes of formation of nitrogen(II) oxide and water, to calculate the enthalpy change of formation of ammonia in kJ mol⁻¹.

You may find it helpful to draw a Hess cycle first. You must show your working.

 Δ_fH (NO (g)) = +90.4 kJ mol⁻¹

 Δ_fH (H₂O (g)) = −241.8 kJ mol⁻¹ 

(3)

ii) Calculate the atom economy by mass for the formation of NO in this reaction.

Give your answer to an appropriate number of significant figures.

(2)

In fact, this oxidation to form nitrogen(II) oxide is an equilibrium reaction. 

i) Explain the effect, if any, of increasing pressure on the equilibrium yield of NO in this reaction.

 4NH₃ (g) + 5O₂ (g) 4NO (g) + 6H₂O (g)

 (2)

 ii) Explain the effect, if any, of an increase in pressure on the rate of this reaction. 

(2)

 iii) The platinum-rhodium catalyst used in this reaction is a heterogeneous catalyst.

State what is meant by the term ‘heterogeneous’ and why a catalyst has no effect on the yield of the products in the reaction.

 (2)

The diagram shows a Maxwell-Boltzmann distribution of particle energies, including the activation energy, E_a, for a reaction.

An increase in temperature will