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

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What is a reversible reaction?

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What is a reversible reaction?
One in which the products can react to re-form the reactants, shown by ⇌.
What is dynamic equilibrium?
The state in a closed system where the forward and reverse reactions continue at equal rates, so concentrations stay constant.
Why must the system be closed for equilibrium to be reached?
Otherwise reactants or products could escape and the concentrations could not stay constant.
State Le Chatelier's principle.
If a system at equilibrium is disturbed, the position of equilibrium shifts to oppose the change.
Effect of increasing the concentration of a reactant on the position of equilibrium?
The equilibrium shifts to the right, using up the extra reactant and forming more product.
Effect of increasing pressure on a gaseous equilibrium?
The equilibrium shifts towards the side with fewer moles of gas.
When does pressure have no effect on the position of equilibrium?
When there are equal moles of gas on both sides of the equation.
Effect of raising the temperature on an equilibrium?
The position shifts in the endothermic direction.
Effect of a catalyst on the position of equilibrium and on the time to reach it?
No change to the position or yield; equilibrium is reached sooner because both rates increase equally.
Why is about 450 °C used in the Haber process?
A compromise: lower temperatures give a higher yield (exothermic) but too slow a rate.
Why is 200 atm used in the Haber process instead of a higher pressure?
Higher pressure would raise the yield but needs stronger equipment and more energy, so the cost is not justified.
Why are unreacted gases recycled in the Haber process?
The conversion per pass is low, so recycling increases the overall yield and wastes less.

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
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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).