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Equilibria
2018 Paper One
Question Two
This question is worth 9-marks.
Issues
Total pressure as the sum of partial pressures missed
Incorrect units for the equilibrium constant
Failure to link Le Chatelier’s principle and that equilibrium shifts/moves to oppose the increase in temperature.
The total pressure = 150 kPa and the mole fraction of ammonia was 0.80:
partial pressure of the ammonia is 150 x 0.80 = 120 kPa
The other partial pressures are 150 – 120 = 30 kPa
Since there is a 1:3 mole ratio for nitrogen: hydrogen, the mole fraction for nitrogen is 0.25 and for hydrogen it's 0.75 (3 x 0.25)
pp nitrogen = 0.25 x 30 = 7.5 kPa
pp hydrogen = 0.75 x 30 = 22.5 or 23 kPa
pp of ammonia = 0.8 x 150 = 120 kPa
Straightforward, but the mark will be lost if square brackets are used
Enthalpy change is negative so reaction is exothermic in the forward direction.
Increasing temperature will cause a decrease in the value of the equilibrium constant
Equilibrium will move left in the endothermic direction
to reduce the temperature
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