February/March 2026 Paper 42

2026 · 6 questions · 51 parts · 80 marks

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Context for question 4

When hydrogen reacts with carbon dioxide in a sealed vessel at 130

^{\circ}

C using a suitable catalyst, methanol is produced.

The equation is shown.

3 H_{2} (g) + C O_{2} (g) ⇌ C H_{3} OH (g) + H_{2} O (g) Δ H = - 40 kJ / mol

The activation energy,

E_{a}

, of the forward reaction is +220 kJ / mol.

4(a)Enthalpy Changes Energy Profiles CalorimetryEasy2 marks

State how the information shows that this reaction is reversible and exothermic.

Reversible

Exothermic

4(b)Dynamic Equilibria Equilibrium ConstantsMedium4 marks

Complete Table 4.1 to show the effect, if any, for each change of condition.

temperature is decreased: effect on the concentration of methanol at equilibrium

temperature is decreased: effect on the rate of the forward reaction

pressure in the reaction vessel is decreased: effect on the concentration of methanol at equilibrium

the catalyst is removed: effect on the rate of the forward reaction

4(c)

Figure 4.1 shows an incomplete reaction pathway diagram for the forward reaction.

Figure 4.1: an incomplete reaction pathway diagram of energy against progress of reaction, a curve rising over an activation-energy hump then settling below the reactant level, with no labels or arrows on it

4(c)(i)Enthalpy Changes Energy Profiles CalorimetryMedium3 marks

Complete the reaction pathway diagram in Figure 4.1.

In your diagram, include:

the formulas of the reactants and the products
an arrow, labelled $E_{a}$ , to show the activation energy of the forward reaction
an arrow, labelled $Δ H$ , to show the enthalpy change of the forward reaction.

Draw with the mouse or a finger.

4(c)(ii)Enthalpy Changes Energy Profiles CalorimetryEasy2 marks

Use Figure 4.1 and the information given to determine:

the enthalpy change, in kJ / mol, of the reverse reaction. Include a sign with your answer.

the activation energy, in kJ / mol, of the reverse reaction. Include a sign with your answer.

4(d)Enthalpy Cycles Bond EnthalpiesMedium4 marks

The equation for the reaction of hydrogen with carbon dioxide can be represented as shown in Figure 4.2.

Figure 4.2: the reaction drawn as displayed formulae - three H-H molecules plus O=C=O in equilibrium with methanol and H-O-H, with delta H = -40 kJ/mol

Table 4.2 shows some bond energies.

Table 4.2

bond	H–H	$C = O$	O–H	C–O
bond energy in kJ / mol	440	805	460	360

Use the bond energies in Table 4.2 and the value of

Δ H

for the forward reaction to calculate the C–H bond energy in kJ / mol.

Use the following steps.

Calculate the energy needed to break the bonds in the reactants.

Calculate the energy released when the bonds in water form.

Calculate the bond energy of the C–H bond.

0 words

4(e)Covalent BondingMedium3 marks

In Figure 4.3, complete the dot-and-cross diagram of a molecule of methanol.

Draw with the mouse or a finger.

4(f)AlcoholsEasy1 mark

Ethanol is a common fuel. Methanol can also be used as a fuel.

Complete and balance the symbol equation for the incomplete combustion of methanol, CH

_{3}

OH.

C H_{3} OH + \dots\dots\dots\dots \to CO + \dots H_{2} O

Missing formula and coefficient

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