O-LevelChemistryRates of Reaction5070/22 · May/June 2015 · Q212 Marks

Hydrogen reacts with halogens to form hydrogen halides.

(a)Predict which halogen reacts most violently with hydrogen.[1]
(b)(i)The reaction between hydrogen and chlorine is exothermic. H2(g) + Cl2(g) → 2HCl(g) ΔH = –185 kJ/mol Explain, in terms of bond breaking and bond forming, why this reaction is exothermic.[2]
(b)(ii)When one mole of chlorine molecules reacts, 185kJ of energy is released. Calculate the amount of energy released when 106.5 g of chlorine reacts.[2]
(c)(i)Hydrogen reacts with iodine in a reversible reaction. This reaction reaches an equilibrium if carried out in a closed system. H2(g) + I2(g) ⇌ 2HI(g) ΔH = +53 kJ/mol The reaction is studied at a temperature of 400 °C. Describe and explain what happens to the position of equilibrium if the pressure is increased.[2]
(c)(ii)The reaction is studied at 25 atmospheres pressure. Describe and explain what happens to the position of equilibrium if the temperature is decreased.[2]
(d)(i)Hydrogen iodide dissolves in water to form hydroiodic acid, HI(aq). Hydroiodic acid is a strong acid. Write an equation to show the dissociation of hydroiodic acid.[1]
(d)(ii)Hydroiodic acid reacts with calcium. Write the equation for this reaction.[1]
(d)(iii)Hydroiodic acid reacts with sodium carbonate. Write the ionic equation for this reaction.[1]

Mark scheme

Marking points from the official mark scheme (May/June 2015). The mark or mark type is shown in brackets.

[1] (a) Fluorine (1)
[2] (b) (i) Bond breaking absorbs energy AND bond making releases energy/bond breaking is endothermic AND bond making is exothermic (1)
Less energy absorbed than released/more energy released than absorbed/ endothermic energy change is less than exothermic energy change/ exothermic energy change is more than endothermic energy change (1)
[2] (ii) Moles of chlorine = 1.5 (1)
Energy released = 277.5 (kJ) (1)
[2] (c) (i) Unchanged/does not move (1)
Same number of moles (of gas) on both sides/equal volumes (of gases) on both sides/equal number of molecules on both sides (of the equation) (1)
[2] (ii) Moves to the left/backward reaction favoured/moves to reactants/moves to H2 or I2 (1)
(Forward) reaction is endothermic/reverse reaction is exothermic (1)
[1] (d) (i) HI → H⁺ + I¯ (1)
OR
H2O + HI → H3O+ + I¯ (1)
[1] (ii) Ca + 2HI → CaI2 + H2
[1] (iii) CO32- + 2H* → H2O + CO2 (1)
OR
CO32- + 2H+ → H2CO3 (1)
OR
CO32- + H+ → HCO3¯ (1)

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