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Chemical equilibria, Le Chatelier’s principle and Kc​

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Question 15

Tetrafluoroethene is made from chlorodifluoromethane in this reversible reaction:

2CHClF2(g)⇌C2F4(g)+2HCl(g)ΔH=+128 kJ mol−1 2\text{CHClF}_2\text{(g)} \rightleftharpoons \text{C}_2\text{F}_4\text{(g)} + 2\text{HCl(g)} \quad \Delta H = +128\text{ kJ mol}^{-1} 2CHClF2​(g)⇌C2​F4​(g)+2HCl(g)ΔH=+128 kJ mol−1

A 3.50 mol3.50\text{ mol}3.50 mol sample of CHClF2\text{CHClF}_2CHClF2​ is placed in a container of volume 18.5 dm318.5\text{ dm}^318.5 dm3 and heated. When equilibrium is reached, the mixture contains 0.380 mol0.380\text{ mol}0.380 mol of CHClF2\text{CHClF}_2CHClF2​.

1.

Calculate the amount, in moles, of C2F4\text{C}_2\text{F}_4C2​F4​ and of HCl\text{HCl}HCl in the equilibrium mixture.

[3]
2.

Give an expression for KcK_cKc​ for this equilibrium.

[1]
3.

Calculate a value for KcK_cKc​. Give its units.

[3]
4.

State and explain the effect of using a higher temperature on the equilibrium yield of tetrafluoroethene.

[2]
5.

Chemists provided evidence that was used to support a ban on the use of chlorodifluoromethane as a refrigerant. Many refrigerators now use butane as a refrigerant. State the environmental problem that chlorodifluoromethane can cause, and give one reason why butane does not cause this problem.

[2]
Markscheme

Chemical equilibria, Le Chatelier’s principle and Kc Questions

  1. A Level
  2. /Chemistry
  3. /Chemical equilibria, Le Chatelier’s principle and Kc

24 exam-style questions on AQA A Level Chemistry Chemical equilibria, Le Chatelier’s principle and $K_c$, covering Chemical equilibria and Le Chatelier's principle and Equilibrium constant $K_c$ for homogeneous systems. Each one has a worked solution and a mark scheme showing where the marks go.

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