Equilibrium Exam-style MCQs | A-Level Chemistry OCR

Question 1

This question is about the reversible reaction below.

2NO_2(g) ⇌ N₂O_4(g)

A chemist investigating this reaction started with 10 moles of NO₂ and allowed the system to reach equilibrium. If 3 moles of N₂O₄ are formed, the number of moles of NO₂ at equilibrium is

A

8.5

B

7

C

6

D

4

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Question 2

0.200 mol of sulfur dioxide and 0.200 mol of oxygen are placed in a 1.00 dm³ sealed container.

The gases are allowed to react until equilibrium is reached.

2SO₂ + O₂ ⇌ 2SO₃

At equilibrium there is 0.100 mol of SO₃ in the container.

What is the value of K_c?

A

0.150 mol dm^-3

B

0.800 mol dm^-3

C

1.25 mol dm^-3

D

6.67 mol dm^-3

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Question 3

The reaction between sulfur dioxide and oxygen is reversible.

2SO_2(g) + O_2(g) ⇌ 2SO_3(g) K_c = 280 mol^-1dm³ at 1,000 K

In an equilibrium mixture at 1,000 K the sulfur trioxide concentration is 6.00 mol dm^-3.

The sulfur dioxide concentration is twice the oxygen concentration.

What is the sulfur dioxide concentration?

A

0.175 mol dm^-3

B

0.254 mol dm^-3

C

0.318 mol dm^-3

D

0.636 mol dm^-3

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Question 4

The value of the equilibrium constant, K_c , for the reaction to form ethyl ethanoate from ethanol and ethanoic acid is 4.0 at 60°C.

C₂H₅OH + CH₃CO₂H ⇌ CH₃CO₂C₂H₅ + H₂O

1.0 mol of ethanol and 1.0 mol of ethanoic acid are allowed to reach equilibrium at 60°C.

What is the number of moles of ethyl ethanoate formed?

31 mol

32 mol

41 mol

43 mol

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Question 5

Nitrogen reacts with hydrogen to produce ammonia.

N₂(g) + 3H₂(g) 2NH₃(g)

A mixture of 2.00 mol of nitrogen, 6.00 mol of hydrogen, and 2.40 mol of ammonia is allowed to reach equilibrium in a sealed vessel of volume 1 dm³ under certain conditions. It was found that 2.32 mol of nitrogen were present in the equilibrium mixture.

What is the value of K_c under these conditions?

2.32×(6.96)3(1.76)2

2.32×(6.32)3(1.76)2

2.32×(6.32)3(2.08)2

2.32×(6.00)3(2.40)2

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Question 6

Sulfur trioxide is manufactured from sulfur dioxide and oxygen, using the Contact process.

Which condition affects the value of the equilibrium constant, K_c?

A

adjusting the temperature

B

increasing the pressure

C

removing SO₃ from the equilibrium mixture

D

using a catalyst

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Question 7

Two moles of compound P were placed in a vessel. The compound P was partly decomposed by heating. A dynamic equilibrium between chemicals P, Q and R was established.

At equilibrium, x mol of R were present and the total number of moles present was (2 + x).

What is the equation for this equilibrium?

A

P ⇌ 2Q + R

B

2P ⇌ 2Q + R

C

2P ⇌ Q + R

D

2P ⇌ Q + 2R

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Question 8

The equilibrium involving phosphorus(V) chloride, phosphorus(III) chloride and chlorine gas is

PCl_5(g) ⇌ PCl_3(g) + Cl_2(g)

The numerical value of K_p is 1.8 at 250°C.

The expression for the partial pressure of chlorine gas is

A

total pressure × (moles of chlorine gas ÷ total moles of gas)

B

total pressure × moles of chlorine gas

C

total pressure ÷ (moles of chlorine gas ÷ total moles of gas)

D

total pressure ÷ moles of chlorine gas

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Question 9

Sulfur dioxide and oxygen react to form sulfur trioxide. The reaction is reversible.

2SO_2(g) + O_2(g) ⇌ 2SO_3(g) K_p = 2.96 × 10^-9Pa^-1 at 700°C

The reaction is allowed to reach equilibrium at 700°C. The partial pressure of O_2(g) is 375 kPa and the partial pressure of SO_3(g) is 20.3 kPa.

What is the partial pressure of SO_2(g)?

A

19.3 kPa

B

609 kPa

C

18,300 kPa

D

609,000 kPa

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Question 10

In this question you should assume that all gases behave ideally.

Hydrogen and iodine react reversibly in the following reaction. The system reaches dynamic equilibrium.

H_2(g) + I_2(g) ⇌ 2HI_(g) ∆H = -9.5 kJ mol^-1

Which statement must be true for the K_p of this equilibrium to be constant?

A

the partial pressures of H₂, I₂ and HI are equal

B

the external pressure is constant

C

the forward and reverse reactions have stopped

D

the temperature is constant

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Question 11

Hydrogen iodide dissociates into hydrogen and iodine.

2HI_(g) ⇌ H_2(g) + I_2(g)

In an experiment, b mol of hydrogen iodide were put into a sealed vessel at pressure p. At equilibrium, x mol of the hydrogen iodide had dissociated.

Which expression for K_p is correct?

(b−x)2x2

(b−x)2x2p2

4b(b−x)x2p2

4(b−x)2x2

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Question 12

Methanol can be produced from hydrogen and carbon monoxide.

2H_2(g) + CO_(g) ⇌ CH₃OH_(g)

What is the expression for K_p for this reaction?

Kp=p(CH3OH)2p(H2)2×p(CO)

Kp=p(CH3OH)p(H2)2×p(CO)

Kp=p(H2)2×p(CO)p(CH3OH)

Kp=p(CO)×2p(H2)2p(CH3OH)

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Question 13

The equation for the reaction between carbon monoxide and hydrogen is shown.

CO_(g) + 3H_2(g) ⇌ CH_4(g) + H₂O_(g)

What are the units of K_pfor this reaction?

A

kPa

B

kPa^-1

C

kPa²

D

kPa^-2

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Question 14

The reversible reaction of nitrogen and hydrogen to form ammonia is shown below.

N_2(g) + 3H_2(g) ⇌ 2NH_3(g)

In the equilibrium mixture, the partial pressure of N₂is 18.75 MPa and the partial pressure of H₂is 2.50 MPa.

The total pressure is 25 MPa.

What is the value of K_p, in MPa^-2?

A

1.2 × 10^-4

B

0.048

C

0.075

D

21

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Question 15

The reversible reaction of sulfur dioxide and oxygen to form sulfur trioxide is shown below.

2SO_2(g) + O_2(g) ⇌ 2SO_3(g)

An equilibrium mixture contains 2.4 mol SO₂, 1.2 mol O₂and 0.4 mol SO₃.

The total pressure is 250 atm.

What is the partial pressure of SO₃?

A

15 atm

B

25 atm

C

100 atm

D

200 atm

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Question 16

An equilibrium mixture is prepared in a container of fixed volume.

CO_(g) + Cl_2(g) ⇌ COCl_2(g) ΔH = -108 kJ mol^-1

The temperature of this mixture is decreased and the mixture is allowed to reach a new equilibrium.

Which is greater for the new equilibrium than for the original equilibrium?

A

the mole fraction of carbon monoxide

B

the partial pressure of chlorine

C

the total pressure of the mixture

D

the value of the equilibrium constant, K_p

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Question 17

When one mole of ammonia is heated to a given temperature, 50% of the compound dissociates and the following equilibrium is established.

NH_3(g) ⇌ $\frac{1}{2}$ N_2(g) + $\frac{3}{2}$ H_2(g)

What is the total number of moles of gas present in this equilibrium mixture?

A

1.5

B

2.0

C

2.5

D

3.0

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Question 18

The equation for the reaction between sulfur dioxide and oxygen is shown.

2SO_2(g) + O_2(g) ⇌ 2SO_3(g)

In an experiment, 2.00 mol of sulfur dioxide are mixed with 2.00 mol of oxygen.

The total amount of the three gases at equilibrium is 3.40 mol

What is the mole fraction of sulfur trioxide in the equilibrium mixture?

A

0.176

B

0.353

C

0.600

D

1.200

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Question 19

Nitrogen reacts with hydrogen in this exothermic reaction

N_2(g) + 3H_2(g) ⇌ 2NH_3(g)

Which change increases the equilibrium yield of ammonia but has no effect on the value of the equilibrium constant K_p?

A

add a catalyst

B

increase the partial pressure of nitrogen

C

decrease the temperature

D

decrease the total pressure

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