OCR A Level Chemistry

Topic Questions

A LevelChemistryOCRTopic Questions5. Physical Chemistry & Transition Elements (A Level Only)5.2 Equilibria

5.2 Equilibria

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1
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The reversible reaction of sulfur dioxide and oxygen to form sulfur trioxide is shown below.

SO2 (g)   +   O2 (g)rightwards harpoon over leftwards harpoon  2SO3 (g)

An equilibrium mixture contains 2.4 mol SO, 1.2 mol O2 and 0.4 mol SO3.

The total pressure is 250 atm.

What is the partial pressure of SO3?

  • 15 atm

  • 25 atm

  • 100 atm

  • 200 atm

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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

What is the correct Kp expression for the following reaction?

2SO2 (g) + O2 (g) ⇌ 2SO3 (g)

  • Kp = fraction numerator p to the power of 2 space end exponent SO subscript 3 over denominator p squared italic space SO subscript 2 space times space p straight O subscript 2 end fraction

  • Kp = fraction numerator p squared space 2 SO subscript 3 over denominator p squared space 2 SO subscript 2 times p straight O subscript 2 end fraction

  • Kp = fraction numerator p squared space SO subscript 2 times p straight O subscript 2 over denominator p squared space SO subscript 3 end fraction

  • Kp = fraction numerator p squared space 2 SO subscript 2 times p straight O subscript 2 over denominator p squared space 2 SO subscript 3 end fraction

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4
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3.00 mol of gas A reacts with 4.00 moles of gas B in a sealed vessel of volume 1.0 dm3 and total pressure of 1.50 kPa an equilibrium is established which contains 2.00 moles of gas C.  

A + B ⇌ 2C

What is the mole fraction of gas B at equilibrium?

  • 0.29

  • 0.42

  • 0.75

  • 0.40

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51 mark

The Kp expression for a reaction is Kp = fraction numerator p space CH subscript 3 OH over denominator p squared italic space straight H subscript 2 times p italic space CO end fraction

What is the correct equation for the reaction?

  • CH3OH (g)  ⇋ CO (g) + 2H2 (g)

  • 2CO (g) + 4H2 (g) ⇋ 2CH3OH (g)

  • CO (g) + 2H2 (g) ⇋ CH3OH (g)

  • 2CO (g) + H2 (g) ⇋ 2CH3OH (g)

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11 mark

Two students set up the equilibrium system below.

CH3COOC2H5 (l) + H2O (l) rightwards harpoon over leftwards harpoon C2H5OH (l) + CH3COOH (l)

The students titrated samples of the equilibrium mixture with sodium hydroxide, NaOH(aq), to determine the concentration of CH3COOH. The students used their results to calculate a value for Kc. The students’ values for Kc were different.

Which of the reason(s) below could explain why the calculated values for Kc were different?

1. Each student carried out their experiment at a different temperature.

2. Each student used a different concentration of NaOH (aq) in their titration.

3. Each student titrated a different volume of the equilibrium mixture.

  • 1, 2 and 3 

  • 1 and 2

  • 2 and 3

  • 1 only 

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2
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9.20 g of dinitrogen tetroxide, N2O4 (g), was heated to a temperature of 350 K at a pressure of 13.5 kPa and dissociated into nitrogen dioxide, NO2 (g). 

N2O4 (g)   ⇌   2NO2 (g)

At equilibrium, 70 % of N2O4 (g) had dissociated. 

What is the numerical value of Kp at 350 K?

  • 4.50

  • 22.05

  • 51.96

  • 0.02

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3
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Chloromethane can be made from the following reaction. 0.80 moles of methanol and hydrogen chloride are added to the reaction container. 

CH3OH (g) + HCl (g) ⇌ CH3Cl (g) + H2O (g) 

At 120 ℃, the value for Kp is 6.00 x 103 

Which statement about the number of moles of reactants and products at equilibrium is correct?

  • The number of moles of CH3Cl and H2O is 0.79

  • The number of moles of CH3OH and HCl is 0.79

  • The number of moles of CH3Cl and H2O is 1.26

  • The number of moles of CH3OH and H2O is 0.79

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41 mark

Methanol can be produced by a multi-step process using methane and water as the
starting materials. The first step is the production of synthesis gas, which is a mixture of carbon monoxide and hydrogen that is used to synthesise many chemicals including methanol. 

CH4 (g) + H2O (g) ⇋ CO (g) + 3H2 (g)    ∆H = +206 kJ mol-1

Which of the following changes would increase the value of Kp for this equilibrium?

  • Increasing temperature

  • Increasing pressure

  • Decreasing temperature

  • Decreasing pressure

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51 mark

What are possible Kp units for the decomposition of calcium carbonate?

  • No units

  • kPa-1

  • kPa-2

  • MPa

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1
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When 5.00 mol of A reacts with 5.00 mol of B in a sealed vessel of volume 1.0 dm3 and total pressure of 1.5 kPa an equilibrium is established which contains 3.00 moles of C.  

A + B ⇌ 2C

What is the equilibrium partial pressure of A?

  • 2.00 kPa

  • 0.525 kPa

  • 0.350 kPa

  • 0.450 kPa

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2
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When dinitrogen tetroxide gas is heated, it dissociates into nitrogen dioxide gas:

N2O4 (g)  ⇌  2NO2 (g) 

At a particular temperature, the equilibrium partial pressure of nitrogen dioxide was 0.67 kPa and dinitrogen tetroxide was 0.33 kPa. 

What is the numerical value of Kp at this temperature? 

  • 1.36

  • 0.49

  • 0.74

  • 2.03

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31 mark

The temperature of the gaseous equilibrium below is increased.

2SO2 (g) + O2 (g) ⇌ 2SO3 (g)   ΔH = -197 kJ mol-1

Which of the following statements is correct?

  • The position of equilibrium moves to the right

  • The partial pressure of SO3 increases

  • The value of Kp decreases

  • The mole fraction of SO2 decreases

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4
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NO (g), H2 (g), N2 (g) and H2O (g) exist in equilibrium.

2NO (g) + 2H2 (g) ⇌ N2 (g) + 2H2O (g)

At room temperature and pressure, the equilibrium lies well to the right-hand side.

Which of the following could be the equilibrium constant for this equilibrium?

  • 1.54 × 10−3 mol dm−3

  • 6.50 × 102 mol dm−3

  • 1.54 × 10−3 mol−1 dm3

  • 6.50 × 102 mol−1 dm3

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5
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A mixture of N2 and O2 gases has a total pressure of 1.42 atm. The mole fraction of N2 is 0.700.

What is the partial pressure, in atm, of O2 in the mixture?

  • 0.210

  • 0.426

  • 0.710

  • 0.994

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