Tetrafluoroethene is made from chloro difluoromethane in this reversible reaction. 2CHClF2(g) ⇌ C2F4(g) + 2HCl(g) ΔH° = +128 kJ mol⁻¹ A 2.00 mol sample of CHClF2 is placed in a container of volume 23.2 dm³ and heated. When equilibrium is reached, the mixture contains 0.270 mol of CHClF2. Calculate the amount, in moles, of C2F4 and of HCl in the equilibrium mixture. Give an expression for Kc for this equilibrium. Calculate a value for Kc. Give its units. State and explain the effect of using a higher temperature on the equilibrium yield of tetrafluoroethene. Chemists provided evidence that was used to support a ban on the use of chloro difluoromethane as a refrigerant. Many refrigerators now use pentane as a refrigerant. State the environmental problem that chloro difluoromethane can cause. Give one reason why pentane does not cause this problem.

A-Level AQA Chemistry Chemical Equilibria, Le Chateliers Principle & Kc: Tetrafluoroethene is made from c

Tetrafluoroethene is made from chloro difluoromethane in this reversible reaction - AQA - A-Level Chemistry - Question 2 - 2022 - Paper 2

Question 2

Tetrafluoroethene is made from chloro difluoromethane in this reversible reaction. 2CHClF2(g) ⇌ C2F4(g) + 2HCl(g) ΔH° = +128 kJ mol⁻¹ A 2.00 mol sample of CHClF2 i... show full transcript

Worked Solution & Example Answer:Tetrafluoroethene is made from chloro difluoromethane in this reversible reaction - AQA - A-Level Chemistry - Question 2 - 2022 - Paper 2

Step 1

Calculate the amount, in moles, of C2F4 and of HCl in the equilibrium mixture.

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Answer

Given the initial moles of CHClF2 as 2.00 mol, and that 0.270 mol remains at equilibrium:

Initial moles of CHClF2 = 2.00 mol Moles of CHClF2 at equilibrium = 0.270 mol

Moles of CHClF2 that reacted = 2.00 - 0.270 = 1.730 mol

From the balanced equation: 2 moles CHClF2 produce 1 mole C2F4 and 2 moles HCl. Therefore:

Moles of C2F4 formed = 1.730 mol / 2 = 0.865 mol Moles of HCl formed = 1.730 mol × 2 = 3.460 mol

Step 2

Give an expression for Kc for this equilibrium.

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Answer

The equilibrium constant expression for the reaction is given by:

$K_c = \frac{[C_2F_4][HCl]^2}{[CHClF_2]^2}$

Step 3

Calculate a value for Kc.

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Answer

At equilibrium:

Moles of C2F4 = 0.865 mol
Moles of HCl = 3.460 mol
Moles of CHClF2 = 0.270 mol

Volume of the container = 23.2 dm³

Calculating concentrations:

$[C_2F_4] = \frac{0.865}{23.2} = 0.0373 \, mol \, dm^{-3}$

$[HCl] = \frac{3.460}{23.2} = 0.149 \, mol \, dm^{-3}$

$[CHClF_2] = \frac{0.270}{23.2} = 0.0116 \, mol \, dm^{-3}$

Substituting into the expression for Kc:

$K_c = \frac{(0.0373)(0.149)^2}{(0.0116)^2}$

Calculating:

$K_c = 85.56321$

Step 4

Give its units.

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Answer

The units of Kc for this equilibrium is:

$Units = \frac{mol^2 \, dm^{-6}}{mol^2 \, dm^{-6}} = \text{dimensionless}$

Step 5

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

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Answer

Effect on yield:

The yield of tetrafluoroethene will increase with a higher temperature.

Explanation:

Since the reaction is endothermic (ΔH° is positive), increasing the temperature shifts the equilibrium position to the right, favoring the production of the end products (C2F4 and HCl) according to Le Chatelier’s principle.

Step 6

State the environmental problem that chloro difluoromethane can cause.

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Answer

Chloro difluoromethane can cause ozone depletion in the stratosphere.

Step 7

Give one reason why pentane does not cause this problem.

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Answer

Pentane does not contain chlorine atoms, which are responsible for ozone depletion.

Tetrafluoroethene is made from chloro difluoromethane in this reversible reaction - AQA - A-Level Chemistry - Question 2 - 2022 - Paper 2

Worked Solution & Example Answer:Tetrafluoroethene is made from chloro difluoromethane in this reversible reaction - AQA - A-Level Chemistry - Question 2 - 2022 - Paper 2

Calculate the amount, in moles, of C2F4 and of HCl in the equilibrium mixture.

Give an expression for Kc for this equilibrium.

Calculate a value for Kc.

Give its units.

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

State the environmental problem that chloro difluoromethane can cause.

Give one reason why pentane does not cause this problem.

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