O-LevelChemistryRates of Reaction5070/22 · Oct/Nov 2015 · Q1010 Marks

At 200°C and 200 atmospheres pressure, phosphorus(V) chloride forms an equilibrium mixture with phosphorus(III) chloride and chlorine. PCl5(g) ⇌ PCl3(g) + Cl2(g) phosphorus(V) chloride phosphorus(III) chloride

(a)Predict and explain the effect of decreasing the pressure on the position of this equilibrium. The temperature remains constant.[2]
(b)Predict and explain the effect of increasing the concentration of chlorine on the position of this equilibrium.[2]
(c)(i)The table shows the percentage of phosphorus(III) chloride in the equilibrium mixture at different temperatures. The pressure is the same in each case. temperature/°C | % PCl3 in the mixture ---|--- 200 | 48 300 | 95 400 | 99 Describe how the composition of this equilibrium mixture changes with temperature.[1]
(c)(ii)Explain what this tells you about the energy change in this reaction.[1]
(d)How is the position of equilibrium affected by the presence of a catalyst?[1]
(e)The rate of this reaction increases with increase in temperature. Explain why.[2]
(f)Phosphorus(V) chloride reacts with water. Phosphoric acid, H3PO4, and hydrogen chloride are formed. Construct the equation for this reaction.[1]

Mark scheme

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

[2] (a) position of equilibrium moves to the right/moves in forward direction/moves to the product side/moves to make more PCl3/moves to make more Cl2 (1) (if pressure decreases) reaction goes in direction of increasing number of moles (in stoichiometric) equation)/goes in direction of more moles of products/ products have larger (gas) volume than reactants (1)
[2] (b) position of equilibrium moves to left moves in backward direction (1) to reduce the concentration of added substance/to reduce the concentration of chlorine (1)
[1] (c) (i) increasing temperature increases the % of PCl3 (or reverse argument) (1)
[1] (c) (ii) reaction is endothermic (because as temperature increases the amount of product increases) (1)
[1] (d) no effect/position of equilibrium remains the same (1)
[2] (e) molecules move faster/molecules have more energy (1) more molecules have energy greater than activation energy (1)
[1] (f) PCl5 + 4H2O → H3PO4 + 5HCl

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