O-LevelChemistryRates of Reaction5070/21 · May/June 2023 · Q68 Marks
This question is about the energy changes that take place during chemical reactions.
- (a)Methane reacts with chlorine to make chloromethane. The reaction is exothermic. CH4(g) + Cl2(g) → CH3Cl(g) + HCl(g) Draw, on the axes provided in Fig. 6.1, the reaction pathway diagram for this reaction. Include labels for the: • axes • reactants • products • enthalpy change of reaction, ΔΗ • activation energy, Ea [Figure 6.1][5]
- (b)Hydrogen iodide decomposes to make hydrogen and iodine. 2H–I → H–H + I–I Calculate the enthalpy change of this reaction. Use the bond energies in Table 6.1. Table 6.1 bond bond energy in kJ/mol H–H 436 I–I 151 H–I 298 enthalpy change of reaction ______ kJ/mol[3]
This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.
Mark scheme
Marking points from the official mark scheme (May/June 2023). The mark or mark type is shown in brackets.
- 6(a) correct labels for axes (1) reactants on a line (1) product on a line to the right and below reactant line (1) vertical line with arrow and correct label for enthalpy change (1) vertical line with arrow and correct label for activation energy (1)
- 6(b) energy associated with bond breaking = 2 × 298 OR 596 (1) energy associated with bond forming = 436 + 151 OR 587 (1) (enthalpy change = 596–587) = +9 (kJ/mol) (1)
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