1.8 Chemical Equilibria, Le Chatelier's Principle & Kc | AQA A Level Chemistry Topic Questions 2017 (2024)

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11 mark

Which of the following statements about dynamic equilibrium is not correct?

The concentration of reactants and products remains constant
The system must be open
The rate of the forward and backward reactions are equal
The equilibrium will shift to oppose any change in conditions

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21 mark

Ethyl ethanoate is formed in an esterification reaction.

ethanol + ethanoic acid $⇌$ ethyl ethanoate + water

The forward reaction is exothermic.

How can the value of the equilibrium constant Kc be increased?

Increasing the temperature
Decreasing the temperature
Increasing the pressure
Adding a catalyst

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31 mark

In the reaction where gaseous iodine reacts with hydrogen, an equilibrium is established at 450°C.

H2 (g) + I2 (g) $⇌$ 2HI (g)

colourless purple colourless

The forward reaction is exothermic

Which change in conditions will cause the purple colour of the equilibrium mixture to become darker?

Decrease in pressure
Increase in pressure
Decrease in temperature
Increase in temperature

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41 mark

An equilibrium is established in the reaction.

AB (g) + CD (g) $⇌$ E (g) + F (g) ΔH = + 89 kJ mol^-1

Which statement is not correct about this equilibrium?

The forward reaction is endothermic
$K_{c} = \frac{[E] [F]}{[AB] [CD]}$
A decrease in temperature will increase the yield of AB (g) and CD (g)
An increase in pressure will increase the yield of E (g) and F (g)

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51 mark

Which is the correct K_c expression for the following equilibrium?

2SO₂ (g) + O₂ (g) $⇌$ 2SO₃ (g)

$K_{c} = \frac{{[{SO}_{2}]}^{2} [O_{2}]}{{[{SO}_{3}]}^{2}}$
$K_{c} = \frac{[{SO}_{3}]}{[{SO}_{2}] [O_{2}]}$
$K_{c} = \frac{[{SO}_{2}] [O_{2}]}{[{SO}_{3}]}$
$K_{c} = \frac{{[{SO}_{3}]}^{2}}{{[{SO}_{2}]}^{2} [O_{2}]}$

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61 mark

Which is the correct K_c expression for the reaction between hydrogen and fluorine?

H₂ (g) + F₂ (g) $⇌$ 2HF (g)

$K_{c} = \frac{{[HF]}^{2}}{[H_{2}] [F_{2}]}$
$K_{c} = \frac{[HF]}{[H_{2}] [F_{2}]}$
$K_{c} = \frac{2 [HF]}{[H_{2}] [F_{2}]}$
$K_{c} = \frac{[H_{2}] [F_{2}]}{{[HF]}^{2}}$

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71 mark

Ammonia, NH₃, is manufactured in the Haber process via the following reaction

3H₂ (g) + N₂ (g) $⇌$ 2NH₃ (g) ΔH = -91.8 kJ mol^-1

Which statement about the Haber process is correct?

Nickel is used as a catalyst
Increasing the temperature gives a higher yield of N₂
Increasing the temperature gives a higher yield of NH₃
Decreasing the pressure gives a higher yield of NH₃

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61 mark

Ammonia is produced from hydrogen and nitrogen in the Haber process.

N₂(g) + 3H₂(g) $⇌$ 2NH₃(g)

A mixture of 2.50 moles of nitrogen, 3.50 moles of hydrogen and 1.68 moles of ammonia is allowed to reach equilibrium in a sealed container under certain conditions. It was found that 1.32 moles of ammonia were present in the equilibrium mixture.

The reaction container had a volume of 10 dm³.

What is the value of K_c under these conditions?

1.2
0.99
0.026
2.35

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71 mark

When a chemical reaction reaches equilibrium which of the following can be said about the concentrations of the substances present and the rates of reaction?

	Concentrations of the reactants and products	Rates of forward and backward reactions
A	Continue to change	Remain the same
B	Remain constant	Are different
C	Continue to change	Are different
D	Remain constant	Remain the same

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81 mark

Iodine monochloride, ICl, reacts according to the following equation.

2ICl (l) $⇌$ I₂ (g) + Cl₂ (g) ΔH^ϴ = -26 kJ

What will change if the temperature is lowered?

	The position of equilibrium	The value of K_c
A	will shift to the left	increases
B	will shift to the right	decreases
C	no change	decreases
D	will shift to the right	increases

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91 mark

What are the correct units for K_c for the following equation

2CH₃CH₂COOH + HOCH₂CH₂CH₂OH $⇌$ C₉H₁₆O₄ + 2H₂O

mol dm^-3
mol² dm^-6
no units
mol^-1 dm³

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101 mark

Sulfur trioxide, SO₃, decomposes to establish the following equilibrium producing sulfur dioxide, SO₂, and oxygen.

2SO₃ (g) $⇌$ 2SO₂ (g) + O₂ (g) ΔH = +196 kJ mol^-1

What will change if the pressure is decreased?

	Yield of SO₂ (g)	The value of K_c
A	increase	increase
B	increase	no change
C	no change	increase
D	decrease	no change

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11 mark

1.5 moles of hydrogen and 1.5 moles of iodine react to form hydrogen iodide in a 7 dm³ container. At equilibrium 30% of hydrogen had reacted.

H₂ (g) + I₂ (g) $⇌$ 2HI (g) ΔH = -11 kJ mol^-1

What is the concentration of hydrogen iodide at equilibrium at a given temperature?

0.45 mol dm^-3
0.064 mol dm^-3
0.13 mol dm^-3
0.90 mol dm^-3

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21 mark

The graph shows the equilibrium yield of E and F present during the following reaction

A (g) + B (g) $⇌$ 3E (g) + 2F (g) ΔH = - 98 kJ mol^-1

What are the correct labels for the graph?

	x-axis	y-axis	Curve X	Curve Y
A	Pressure	Reactant yield	High temperature	Low temperature
B	Pressure	Product yield	Low temperature	High temperature
C	Temperature	Product yield	High pressure	Low pressure
D	Temperature	Reactant yield	Low pressure	High pressure

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31 mark

A mixture of 1.20 moles of CO, 2.50 moles of H₂O and 1.00 mole of CO₂ were allowed to reach equilibrium at 500 K in a reaction vessel with a volume of 6 dm³.

CO (g) + H₂O (g) $⇌$ CO₂ (g) + H₂ (g)

0.64 moles of hydrogen were formed in the equilibrium

What is the correct value of the concentration of CO (g) at equilibrium?

0.56 mol dm^-3
1.84 mol dm^-3
3.36 mol dm^-3
0.093 mol dm^-3

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41 mark

The value of K_c for the equilibrium between carbon monoxide, oxygen and carbon dioxide is 4.

Reaction 1 2CO (g) + O₂ (g) $⇌$ 2CO₂ (g)

Under the same conditions, what is the value for K_c for the following equilibrium?

Reaction 2 2CO₂ (g) $⇌$ 2CO (g) + O₂ (g)

0.5
0.25
4
1

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51 mark

A sample of bromine gas was heated in a sealed container and an equilibrium was established.

Br₂ (g) $⇌$ 2Br (g)

Which of the following statements is correct?

A decrease in temperature will shift the equilibrium to the left hand side
If more bromine gas is added the concentration of the bromine atoms will remain the same
When a catalyst is added the concentration of bromine atoms increases
Increasing pressure will shift the equilibrium to the right hand side

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61 mark

Which statement does not correctly describe a state of equilibrium for a reaction involving gases?

Equilibrium can be reached from either direction
Equilibrium is dynamic
Concentrations of reactants and products remain constant
Equilibrium is achieved in an open system

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71 mark

The K_c value for the following reaction is 4.7 x 10^-3

N₂ + O₂ $⇌$ 2NO

The equilibrium concentrations of oxygen and nitrogen was 0.35 mol dm^-3 and 0.42 mol dm^-3. What is the equilibrium concentration of nitrogen monoxide?

0.180 mol dm^-3
6.91 x 10^-4 mol dm^-3
0.026 mol dm^-3
4.77 x 10^-7 mol dm^-3

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81 mark

Which of the following statements is correct about the equilibrium shown

2W ⇋ Z K_c = 1.3 x 10^-4

[W] ≫ [Z]
[W] ＞[Z]
[W] = [Z]
[W]＜[Z]

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91 mark

A reacts with B to form C according to the following equation:

A (g) + B (g) $⇌$ 2C (g)

The value of K_c for this reaction has been measured at different temperatures

K_c = 42 at 355 $°$ C

K_c = 65 at 450 $°$ C

Which of the following statements is correct?

The forward reaction is exothermic
The forward reaction is endothermic
The reaction does not proceed at 355 $°$ C
The reaction has gone to completion at 450 $°$ C

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101 mark

Which of the following conditions and reasons will increase the amount of chlorine produced?

2NOCl (g) $⇌$ 2NO (g) + Cl₂ (g) ΔH = - 92.4 kJ mol^-1

	Condition	Reason	Condition	Reason
A	Increase in temperature	Forward reaction is exothermic	Decrease in pressure	Fewer moles of gaseous product than reactant
B	Increase in temperature	Forward reaction is endothermic	No change in pressure	Equal numbers of moles of gases
C	Decrease in temperature	Forward reaction is exothermic	Decrease in pressure	More moles of gaseous product than reactant
D	Decrease in temperature	Forward reaction is exothermic	Increase in pressure	Equal numbers of moles of gases

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