O-LevelChemistryRates of Reaction5070/21 · May/June 2019 · Q810 Marks

A scientist heats a sample of phosphorus(V) chloride in a closed container. A dynamic equilibrium is established. PCl₅(g) ⇌ PCl₃(g) + Cl₂(g)

(a)Describe what is meant by the term dynamic equilibrium.[2]
(b)The pressure of the equilibrium mixture is increased. The temperature of the equilibrium mixture is kept constant. Predict and explain what will happen, if anything, to the composition of the equilibrium mixture. prediction explanation[2]
(c)(i)The temperature of the equilibrium mixture is increased. The pressure of the equilibrium mixture is kept constant. Suggest why the position of equilibrium moves to the right.[1]
(c)(ii)Explain why the rate of the reaction increases.[2]
(d)Draw the 'dot-and-cross' diagram for a molecule of PCl₃. Only include the outer shell electrons.[2]
(e)PCl₅ reacts with water to form hydrogen chloride and phosphoric acid, H₃PO₄· Construct an equation for this reaction.[1]

Mark scheme

Marking points from the official mark scheme (May/June 2019). The mark or mark type is shown in brackets.

[2] when a reversible reaction (in a closed system) (1)
[2] (reaches a point that the) rate of forward reaction equals the rate of the backward reaction (1)
[2] more PCl₅/concentration of PCl₅ increases / less PCl₃/less Cl₂ / concentration of Cl₂ decreases / concentration of PCl₃ decrease (1)
[2] there are fewer moles of gas on the left hand side of the reaction (1)
[1] the reaction absorbs heat / the (forward) reaction is endothermic (1)
[2] particles have more kinetic energy / particles moving faster (1)
[2] more successful collisions / more energetic collisions / more effective collisions / more particles with equal or above activation energy (1)
[2] outer shell of phosphorus is correct (3 bond pairs with chlorine and 2 non-bonding electrons) (1)
[2] rest of outer shells of all three chlorine atoms correct (1)
[1] PCl₅ + 4H₂O → H₃PO₄ + 5HCl

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