Equilibrium constant (Kp) for Homogeneous Systems (A Level only) (AQA A Level Chemistry)

Exam Questions

4 hours30 questions
1a2 marks

Define dynamic equilibrium.

1b3 marks

A general equilibrium reaction proceeds as shown.

A (g) + B (g) ⇋ C (g) + D (g)

Describe the steps you would perform to calculate the mole fraction of C.

1c1 mark

After calculating the mole fraction of C, describe how you would calculate the partial pressure of C for the general reaction shown in part (b).

1d3 marks

State three reasons why the KP expression for the following general equation is incorrect.

A (s) + 2B (g) ⇋ C (g) + 2D (g)

KPfraction numerator left square bracket straight A right square bracket cross times 2 left square bracket straight B right square bracket over denominator left square bracket straight C right square bracket cross times left square bracket straight D right square bracket squared end fraction

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2a2 marks

At 550 K, ammonia and hydrogen chloride react in an equilibrium reaction to produce ammonium chloride gas.

Write a balanced chemical equation, including state symbols, for this reaction.

2b2 marks

10.0 g of ammonia and 10.0 g of hydrogen chloride were left to reach equilibrium in a flask at a temperature of 550 K.

Calculate the number of moles of ammonia and hydrogen chloride at the start of the reaction.

2c2 marks

At equilibrium, the reaction contained 0.132 moles of ammonium chloride.

Using your values from part (b), complete Table 1 to show the number of moles of each chemical at equilibrium.

(If you did not get an answer for part (b), you may assume that the initial moles of NH3 are 0.650 and HCl are 0.350. These are not the correct answers)

Table 1

 

NH3 (g)

+ HCl (g)

→ NH4Cl (g)

Initial moles

 

 

0

Change in moles

 

 

 

Equilibrium moles

 

 

0.132

2d2 marks

Use your answers to part (c), to calculate the mole fraction of each chemical at equilibrium.

(If you did not get an answer for part (c), you may assume that the equilibrium moles of NH3  and HCl are 0.480 and 0.175, respectively. These are not the correct answers.)

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3a2 marks

When 1.20 moles of hydrogen iodide is heated to 700 K in a closed container, the following equilibrium is achieved.

2HI (g) ⇋ H2 (g) + I2 (g)

When equilibrium was reached, there were 0.36 moles of hydrogen in the mixture.

Calculate the number of moles of each gas in the equilibrium mixture.

3b2 marks

Using your answers to part (a), calculate the mole fraction of each gas in the equilibrium mixture.

(If you did not get an answer for part (a), you may assume that the equilibrium moles of HI are 0.50, H2 are 0.40 and I2 are 0.40. These are not the correct answers.)

3c2 marks

At equilibrium, the total pressure in the flask was 120 kPa.

Calculate the partial pressure of each gas in the equilibrium mixture.

Partial pressure of hydrogen iodide ____________________ kPa

Partial pressure of hydrogen ____________________ kPa

Partial pressure of iodine ____________________ kPa

(If you did not get an answer for part (b), you may assume that the mole fraction of HI is 0.34, H2 is 0.33 and I2 is 0.33. These are not the correct answers.)

3d2 marks
i)
Give an expression for the equilibrium constant, KP, for the reaction of hydrogen iodide forming hydrogen and iodine, as described in part (a).

ii)

Calculate the value of the equilibrium constant, KP, for this reaction.

(If you did not get an answer for part (c), you may assume that the partial pressure of HI is 52.0 kPa, H2 is 34 kPa and I2 is 34 kPa. These are not the correct answers)

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4a5 marks

6.55 g of nitrosyl chloride was heated in a flask to a temperature of 650 K and left to reach equilibrium according to the following equation.

2NOCl (g) ⇋ 2NO (g) + Cl2 (g)

At equilibrium, there were 0.05 moles of nitric oxide in the mixture.

Calculate the mole fraction of each gas at equilibrium.

4b5 marks

At equilibrium, the total pressure was 250 kPa.

Calculate the value of the equilibrium constant, KP, for this reaction and give its units.

4c2 marks

Explain the effect, if any, to the equilibrium composition caused by an increase in pressure.

4d1 mark

State the effect, if any, to the value of the equilibrium constant, KP, caused by an increase in pressure.

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5a2 marks

Dinitrogen tetroxide, N2O4, exists in an equilibrium with nitrogen dioxide, NO2, according to the following equation.

N2O4 (g) ⇌ 2NO2 (g)   ΔH = +58 kJ mol–1

i)
Write an expression for the equilibrium constant, Kp, for this reaction.

ii)
Rearrange the KP expression to calculate the partial pressure of nitrogen dioxide.
5b2 marks

The equilibrium constant, KP, for the reaction in part (a) has a value of 48 kPa at 400 K.

i)

At equilibrium, the partial pressure of the dinitrogen tetroxide was 75.8 kPa.

Calculate the partial pressure of the nitrogen dioxide.

ii)
Calculate the total pressure of the reaction vessel.
5c1 mark

Describe the effect, if any, on the value of the equilibrium constant, KP, of adding a catalyst.

5d4 marks

Explain the effect on the equilibrium constant, KP, of increasing the temperature of this reaction.

N2O4 (g) ⇌ 2NO2 (g)               ΔH = +58 kJ mol–1 

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1a3 marks

This question is about the equilibrium that occurs in the Haber Process.

The equation for the reaction between nitrogen and hydrogen is shown below:

N2 (g) + 3H2 (g) ⇌ 2NH3 (g)              T = 600 K

The total pressure in the flask was 250 kPa at equilibrium, and the mole fraction of ammonia in the mixture was 0.80.

Calculate the partial pressure of each gas in the container.

1b5 marks

Nitrogen monoxide and oxygen were mixed in a reaction vessel and left to react until they reached equilibrium, at a fixed temperature and a total pressure of 1.75 × 104 kPa.

  2NO (g) +  O2 (g)  ⇌ 2NO2 (g)

The equilibrium mixture contained 0.0950 mol of nitrogen dioxide and 0.540 mol of nitrogen monoxide.

i)
Calculate the number of moles of oxygen present in the equilibrium mixture.

ii)

Calculate the mole fraction of oxygen in the equilibrium mixture.

iii)
Calculate the partial pressure of oxygen in the equilibrium mixture.
1c7 marks

In a heated vessel, the below reaction reached equilibrium. At a constant temperature, a mixture of 0.456 mol of PCl3 and 0.278 mol of Cl2 of fixed volume reacted together.

Cl2 (g) +  PCl3 (g) ⇌  PCl5 (g)

At equilibrium, the total pressure was 235 kPa and 0.156 mol of PCl5 had been formed.

i)
Calculate the number of moles of each reactant in the equilibrium mixture and the total number of moles of gas present.

ii)
Calculate the mole fraction and the partial pressure of PCl3 in the mixture.

iii)
Write an expression for the equilibrium constant, Kp, for this equilibrium.
1d5 marks

Two gases, A and B, were added to a flask which was then sealed. The flask was heated to 500 K and the following equilibrium was established.

A (g) + 3B (g)  2C (g)

The partial pressure of C in the equilibrium mixture was 5.0 kPa when the total pressure was 21.0 kPa.

i)
Calculate the partial pressures of A and B at equilibrium. 

ii)

Write the Kp expression for this reaction and state the units.

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2a5 marks

In a reaction vessel a mixture of three different gases are at equilibrium. The partial pressure of each gas is as follows:

      H2                     = 12400 kPa

      CO                    = 7350 kPa

      CH3OH             = 2620 kPa

The reaction taking place in the reaction vessel is shown below:

2H2 (g) + CO (g)  ⇌ CH3OH (g)

i)
Write an expression for the equilibrium constant, Kp, for the reaction above.

ii)
Calculate the value of the equilibrium constant, Kp, for the reaction under these conditions. You must include units in your answer.

iii)
State the effect, if any, of an increase in pressure on the yield of CH3OH and also the effect on the value of Kp.
2b2 marks

Table 1 below shows the partial pressures of an equilibrium mixture containing hydrogen, nitrogen and ammonia.

N2 (g) + 3H2 (g) 2NH3 (g)

Calculate the value of the equilibrium constant, Kp, in this reaction and state its units.

Table 1

Gas

Partial Pressure / kPa

Hydrogen

1.30 x 102

Nitrogen

1.60 x 102

Ammonia

1.20 x 103

2c2 marks

In a reaction vessel a mixture of gases were heated until they reached equilibrium.

Cl2 (g) +  PCl3 (g)  ⇌ PCl5 (g)         ΔH = +124 kJ mol-1

State how you would alter the temperature and pressure to increase the yield of the products in the equilibrium reaction above.

2d6 marks
1.0 moles of hydrogen and 2.0 moles of fluorine were sealed into a container. After the mixture had reached equilibrium, at a pressure of 150 kPa, 0.3 moles of hydrogen fluoride were formed.
 
i)
Write a balanced equation to show the reaction between hydrogen and fluorine.
 
ii)
Calculate Kp and give any appropriate units.

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3a3 marks

The equation below shows hydrogen and nitrogen reacting together to form ammonia in equilibrium, as part of the Haber process. The enthalpy value for this reaction is -91 kJ mol-1.

N2(g) + 3H2(g) 2NH3(g)

Explain the effect, if any, of an increase in temperature on the value of Kp for this reaction if the volume remains constant and justify your answer.

3b3 marks

Hydrogen is required for the synthesis of ammonia in the Haber process. The below equation shows how the hydrogen is produced by reacting methane and water vapour to form carbon monoxide and hydrogen.

CH4(g) + H2O(g) CO(g) + 3H2(g)          ΔH = +206 kJ mol-1

State the effect, if any, of an increase in temperature on the equilibrium constant, Kp and on the yield of hydrogen produced.

Explain your answer.

3c6 marks

In the Contact Process for the production of sulfuric acid, purified sulfur dioxide and oxygen react together to form sulfur trioxide. The partial pressure of SO2 was 24 kPa and the total pressure in the vessel was 118 kPa.

2SO2(g) + O2(g)  2SO3 (g)      ΔH = -190 kJ mol-1

Calculate the value of Kp, including units.

3d3 marks

An equilibrium mixture of the three gases below is prepared in a container of fixed volume.

CO(g)   +   Cl2(g)      COCl2(g)          ΔH = −108 kJ mol-1

The reaction was repeated at a different temperature and the value of Kp increased.

State whether the temperature had been increased or decreased for the second reaction. Explain your answer.

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4a4 marks

An equilibrium mixture of the three gases below is prepared in a container of fixed volume.

CO(g)   +   Cl2(g)      COCl2(g)          ΔH = −108 kJ mol-1

State and explain the effect, if any, on the value of Kp in each of the following situations:

i)
Changing the conditions of an equilibrium mixture from 300 K and 100 atm to 500 K and 100 atm. 

ii)
Changing the conditions of an equilibrium mixture from 300 K and 100 atm to 300 K and 300 atm.

iii)
Introducing a catalyst to an equilibrium mixture at 300 K and 100 atm.
4b7 marks

Methanol can be synthesised along with water when carbon dioxide and hydrogen gas are reacted together in a sealed container. In an experiment below, 1.0 mol of carbon dioxide and 3.0 mol of hydrogen were reacted together. After the mixture had reached equilibrium, at a pressure of 550 kPa, the yield of methanol was 0.91 mol.

CO2(g) + 3H2(g)  CH3OH(g) + H2O(g)

Calculate a value for Kp. Give your answer to the appropriate number of significant figures. Give units with your answer.

4c5 marks

The equilibrium below shows carbon dioxide and hydrogen being produced from water vapour and carbon monoxide reacting together in a reaction vessel.

H2O(g) + CO(g)  H2(g) + CO2(g)         ΔH = - 41.2 kJ mol-1

i)
State an expression for Kp for this reaction.

ii)
When 1.00 mol of steam and 1.00 mol of carbon monoxide were allowed to reach equilibrium, 0.60 mol of hydrogen was formed at a pressure of 400 kPa. Calculate a value for Kp.
4d1 mark

Methanol can be synthesised by reacting carbon monoxide and hydrogen at a temperature of 69℃ and a pressure of 1.75 MPa, as shown in the equation below.

CO(g) + 2H2(g) CH3OH(g)

State what the units for Kp would be in kPa for this reaction.

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5a6 marks

Producing methanol by an industrial process involves several stages and optimum conditions in order to convert the starting reactants, methane and steam, into gaseous products. Figure 1 below, shows the percentage of methane conversion into gaseous products at equilibrium under different conditions.

Using the two graphs in Figure 1, deduce the optimum conditions needed for the conversion of methane and steam into gaseous products. Explain your ideas.

Figure 1

oLkM0-o__1

5b7 marks

A lab technician added two gases to a flask and then sealed it, allowing equilibrium to be reached. The reaction which took place inside the flask is shown below:

3A(g)  +  2B(g)   2C(g)  +  D(g)        ΔH = –180 kJ mol-1

i)
Write an expression for the equilibrium constant, Kp, for this reaction.

ii)
State one change in the conditions which would increase the rate of reaction and simultaneously decrease the value of Kp. Justify your suggestion.
5c2 marks

In the gas phase equilibrium below, state the change, if any, that would occur to the value of Kp, if the pressure was increased inside the reaction vessel. Explain your answer.

A + B2 + C ⇌ ABC + B        ΔH = −60 kJ mol-1

5d4 marks

During the synthesis of methanol, the final stage involves the reaction between carbon monoxide and hydrogen gas as shown below. During this stage, an equilibrium was established at 590 K.

CO (g) + 2H2 (g) CH3OH (g)

The mixture contained a partial pressure of 370kPa of H2 and 27.4 kPa of CH3OH. The total pressure was 620 kPa.

i)
State the Kp expression for this equilibrium.

ii)

The value for Kp for this reaction is 8.76 x 10-7 kPa-2.

Using the value for Kp, calculate the partial pressure of carbon monoxide. Show your working out.

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1a3 marks

In the Contact Process, sulfur dioxide and oxygen were mixed in a 2:1 mole ratio and left to reach equilibrium in a flask at a temperature of 750 K. The equation for the reaction between sulfur dioxide and oxygen is shown.

2SO2 (g) + O2 (g) ⇌ 2SO3 (g)       ∆H = -196 kJ mol-1

At equilibrium, the mole fraction of SO3 (g) in the mixture was 0.60 and the total pressure in the flask was 120 kPa.

Calculate the partial pressure of each gas in this equilibrium mixture.

1b3 marks

Calculate the value of the equilibrium constant (KP) for the reaction shown in part (a).

Give your answer, to the appropriate number of significant figures, in standard form and state the units.

1c2 marks

Explain the effect, if any, of increasing the temperature on the yield and rate of reaction for the Contact Process.

2SO2 (g) + O2 (g) ⇌ 2SO3 (g)       ∆H = -196 kJ mol-1

1d3 marks

Explain the effect, if any, of increasing the temperature on the value of KP for the Contact Process.

2SO2 (g) + O2 (g) ⇌ 2SO3 (g)          ∆H = -196 kJ mol-1

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2a6 marks

Methanol can be produced by a multi-step process using methane and water as the starting materials.

Step 1 is the production of synthesis gas, which is a mixture of carbon monoxide and hydrogen that is used to synthesise many chemicals including methanol.

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

The general  industrial conditions for synthesis gas production are pressures of between 1000 - 2000 kPa, at temperatures of 1080 K and a nickel catalyst.

Figures 1 and 2 show the percentage conversion of methane into carbon monoxide and hydrogen under different conditions.

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Use the information, to discuss the industrial conditions used to form synthesis gas as an intermediate step in the production of methanol.

2b4 marks

Explain the effects of the general industrial conditions used for temperature and pressure, as stated in part (a), on the value of KP.

2c8 marks

Step 2 is the production of methanol from the synthesis gas mixture.

Step 2: CO (g) + 2H2 (g) → CH3OH (g)         ∆H = -90 kJ mol-1

A 57.4 mol sample of carbon monoxide was reacted with 98.6 mol of hydrogen. When equilibrium was reached at a given temperature, the total pressure of the mixture was 7.80 MPa and contained 21.1 mol of methanol.

Calculate a value for Kp. Give your answer to the appropriate number of significant figures. Give units with your answer.

2d4 marks
i)

Suggest temperature and pressure conditions, for use in industry, to increase the yield for step 2. 

Step 2: CO (g) + 2H2 (g) → CH3OH (g)            ∆H = -90 kJ mol-1

ii)
Explain the effect that the suggested conditions will have on the value of KP.

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3a7 marks

The total pressure of the following equilibrium mixture is 7.4 MPa.

2SO3 (g) ⇋ 2SO2 (g) + O2 (g)

Calculate the value of KP when 65% of sulfur trioxide dissociates at a given temperature.

3b5 marks

In the reverse reaction, as shown, 3.5 moles of sulfur dioxide are mixed with 2.5 moles of oxygen.

2SO2 (g) + O2 (g) ⇋ 2SO3 (g)

The total amount of the three gases at equilibrium is 4.8 moles.

Calculate the mole fraction of each gas in the equilibrium mixture.

3c4 marks

The total pressure of the equilibrium mixture, shown in part (b), is 12.0 kPa.

Use your answers, from part (b), to calculate KP for this reaction.

(If you did not get answers for (b), you may assume the following mole fractions: SO2 (g) = 0.4, O2 (g) = 0.3, SO3 (g) = 0.3. These are not the correct answers)

3d1 mark

Sulfur trioxide used to be made using the Chamber process involving a lead catalyst. This process had issues producing higher acid concentrations. The Contact Process using platinum or vanadium catalysts was developed to remove this issue.

State the effect of a platinum catalyst on the value of KP.

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4a2 marks

Ammonia can react in different ways with oxygen.

Reaction 1: In the presence of a platinum catalyst, ammonia is oxidised to nitric oxide and oxygen is reduced.

Reaction 2: Ammonia is heated with oxygen in the absence of a catalyst producing nitrogen and one other gaseous product.

Write balanced chemical equations, with whole number coefficients, for these reactions.

4b1 mark

Give an expression for the equilibrium constant, KP, when Reaction 1, from part (a) occurs at 320 K.

4c8 marks

Reaction 1 used 2.0 moles of ammonia and 6.0 moles of oxygen. They were reacted at 750 K and 500 kPa. At equilibrium, 1.0 moles of nitric oxide were formed.

Calculate a value for the equilibrium constant, KP, including units, for this equilibrium.

Give your answer to an appropriate number of significant figures.

4d5 marks

Reaction 2 used the same conditions of 2.00 moles of ammonia and 6.00 moles of oxygen at 750 K and 500 kPa. At equilibrium, 0.99 mole of nitrogen gas was formed.

i)

Calculate the number of moles of each chemical in the equilibrium mixture. Give your answers to an appropriate number of significant figures.

ii)

Suggest whether the KP for this reaction will be higher or lower than the value in Reaction 1 in part (c). Justify your answer.

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5a4 marks

At high temperatures,  gaseous phosphorus (V) chloride dissociates according to the following equation.

PCl5 (g) ⇋ PCl3 (g) + Cl2 (g)

At 500K, the equilibrium amount of each gas is shown in Table 1.

Table 1

Chemical

 PCl5 (g)

PCl3 (g)

Cl2 (g)

Equilibrium moles

0.184

0.816

0.816


Calculate the total pressure of the reaction, if KP = 597 kPa.

Give your answers to an appropriate number of significant figures.

5b4 marks

Nitrogen dioxide decomposes according to the following equation.

2NO2 (g) ⇋ 2NO (g) + O2 (g)

At 700 K, the equilibrium amount of each gas is shown in Table 2.

Table 2

Chemical

NO2 (g)

NO (g)

O2 (g)

Equilibrium moles

0.96

0.04

0.02


Calculate the total pressure of the reaction, if KP = 6.8 x 10-3 kPa.

5c4 marks

Hydrogen iodide can undergo thermal decomposition according to the following equilibrium.

2HI (g) ⇋ H2 (g) + I2 (g)

The value of KP for this reaction at 700K is 0.0185.

Calculate the total pressure when the partial pressure of hydrogen iodide, at equilibrium, is 133 kPa. Give your answer to the appropriate number of significant figures.

5d6 marks

Dinitrogen tetroxide decomposes according to the following equilibrium.

N2O4 (g) ⇋ 2NO2 (g)

The value of KP for this reaction at 350 K is 395 kPa.

The equilibrium mixture contained 1.20 moles of nitrogen dioxide when an initial 2.00 moles of dinitrogen tetroxide were heated at 350 K.

Calculate the partial pressures of dinitrogen tetroxide and nitrogen dioxide.

Give your answer to the appropriate number of significant figures.

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