OCR A-Level Chemistry: Enthalpy, Rates & Equilibrium

In a manufacturing process, ethene is hydrogenated to ethane in the gas phase. The reaction is slow at room temperature, so a finely divided nickel catalyst is added. The reaction is exothermic.

$\text{C}_2\text{H}_4(\text{g}) + \text{H}_2(\text{g}) \rightleftharpoons \text{C}_2\text{H}_6(\text{g})$C2​H4​(g)+H2​(g)⇌C2​H6​(g)

Describe and explain how adding the nickel catalyst increases the rate of this reaction. Your answer should refer to the activation energy, to the Boltzmann distribution of molecular energies, and should make clear what the catalyst does and does not do to the position of equilibrium.

(6 marks)

Methanol is burned completely in oxygen:

$\text{CH}_3\text{OH}(\text{l}) + 1\tfrac{1}{2}\,\text{O}_2(\text{g}) \rightarrow \text{CO}_2(\text{g}) + 2\text{H}_2\text{O}(\text{l})$CH3​OH(l)+121​O2​(g)→CO2​(g)+2H2​O(l)

The standard enthalpy changes of formation of the substances involved are given below.

Substance	$\Delta_f H^{\circ}$Δf​H∘ / kJ mol⁻¹
CH₃OH(l)	−239
O₂(g)	0
CO₂(g)	−394
H₂O(l)	−286

(a) Use the data, with the aid of an enthalpy cycle or Hess's law, to calculate the standard enthalpy change of combustion of methanol. (4 marks)

(b) Explain why the standard enthalpy of formation of oxygen, O₂(g), is zero. (2 marks)

The gaseous compound D is manufactured from gases X and Y by the reversible reaction below.

$2\text{X}(\text{g}) + \text{Y}(\text{g}) \rightleftharpoons 2\text{D}(\text{g})$2X(g)+Y(g)⇌2D(g)

The percentage of D present at equilibrium was measured at a range of temperatures and total pressures once equilibrium had been reached. The results are shown in the table.

Temperature / °C	Yield of D at 50 atm / %	Yield of D at 250 atm / %
200	71	88
350	48	70
500	26	45

(a) State and explain whether the forward reaction is exothermic or endothermic. (3 marks)

(b) Explain, using Le Chatelier's principle, why the yield of D is higher at 250 atm than at 50 atm for any given temperature. (2 marks)

Methanol is made industrially by passing carbon monoxide and hydrogen over a copper-based catalyst. The reaction is exothermic and reaches a dynamic equilibrium.

$\text{CO}(\text{g}) + 2\text{H}_2(\text{g}) \rightleftharpoons \text{CH}_3\text{OH}(\text{g}) \qquad \Delta H = -91 \text{ kJ mol}^{-1}$CO(g)+2H2​(g)⇌CH3​OH(g)ΔH=−91 kJ mol−1

A chemical engineer chooses to run the plant at a high pressure of about 50–100 atm but at a moderate temperature of about 250 °C, rather than at a much lower temperature even though that would give a higher equilibrium yield.

(a) Explain why a high pressure is used to maximise the equilibrium yield of methanol. (2 marks)

(b) Explain why the engineer uses a moderate temperature of 250 °C rather than a much lower temperature, even though the equilibrium yield would be greater at the lower temperature. (3 marks)

An effervescent indigestion tablet contains a carbonate that reacts with stomach acid, releasing carbon dioxide gas. A student investigates how quickly the gas is released. They find that a tablet that has been crushed to a fine powder fizzes much faster than a whole tablet, and that both react faster in warm water than in iced water.

(a) Use collision theory to explain why the crushed (powdered) tablet reacts faster than the whole tablet. (2 marks)

(b) Use collision theory to explain why both forms react faster in warm water than in iced water. (2 marks)

This question is about enthalpy and equilibrium.

(a) State what is meant by the standard enthalpy change of formation of a compound. (1 mark)

(b) The following homogeneous equilibrium is established in a sealed flask:

$2\text{SO}_2(\text{g}) + \text{O}_2(\text{g}) \rightleftharpoons 2\text{SO}_3(\text{g})$2SO2​(g)+O2​(g)⇌2SO3​(g)

Write the expression for the equilibrium constant, $K_c$Kc​, for this reaction. (1 mark)

(c) State Le Chatelier's principle. (1 mark)