Dynamic equilibrium and Le Chatelier's principleEdexcel A-Level Chemistry: Mind map

Exam questions on Dynamic equilibrium and Le Chatelier's principle

1. A sealed glass tube contains a mixture of colourless dinitrogen tetroxide and brown nitrogen dioxide at a constant temperature of 298 K: N₂O₄(g) ⇌ 2NO₂(g), ΔH = +57 kJ mol⁻¹. At the start the tube contained only N₂O₄. After some minutes the brown colour stops getting darker and then stays the same.
   Explain what is meant by dynamic equilibrium in this tube.2 marks
2. Ethanoic acid reacts with ethanol in a sealed flask at 60 °C in the presence of a few drops of concentrated sulfuric acid: CH₃COOH(l) + C₂H₅OH(l) ⇌ CH₃COOC₂H₅(l) + H₂O(l). The enthalpy change for the reaction is close to zero. Equal amounts of the two reactants are mixed and the mixture is left until its composition stops changing.
   Water is removed from the mixture as it forms, using a dehydrating agent. Use Le Chatelier's principle to explain the effect on the yield of ethyl ethanoate.2 marks
3. In the Haber process nitrogen and hydrogen react in a 1 : 3 mole ratio over an iron catalyst: N₂(g) + 3H₂(g) ⇌ 2NH₃(g), ΔH = −92 kJ mol⁻¹. In a pilot study at 200 atm the equilibrium mixture contained about 62% ammonia at 300 °C and about 18% ammonia at 500 °C. The iron catalyst works too slowly below about 400 °C. The industrial plant operates at about 450 °C and 200 atm, and unreacted gases are recycled.
   Use the data to explain why the plant operates at about 450 °C rather than 300 °C.3 marks