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A LevelChemistryAQAExam QuestionsPhysical ChemistryChemical Equilibria, Le Chatelier’s Principle & Kc

Chemical Equilibria, Le Chatelier’s Principle & Kc (AQA A Level Chemistry)

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Chemical Equilibria, Le Chatelier’s Principle & Kc

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Chemical Equilibria, Le Chatelier’s Principle & Kc

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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 rightwards harpoon over leftwards harpoon 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)        rightwards harpoon over leftwards harpoon          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) rightwards harpoon over leftwards harpoon E (g) + F (g)     ΔH = + 89 kJ mol-1

Which statement is not correct about this equilibrium?

  • The forward reaction is endothermic

  • straight K subscript straight c space equals space fraction numerator left square bracket straight E right square bracket space left square bracket straight F right square bracket over denominator left square bracket AB right square bracket space left square bracket CD right square bracket end fraction

  • 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 Kc expression for the following equilibrium?

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

  • straight K subscript straight c space equals space fraction numerator left square bracket SO subscript 2 right square bracket squared space space left square bracket straight O subscript 2 right square bracket over denominator left square bracket SO subscript 3 right square bracket squared end fraction

  • straight K subscript straight c space equals space fraction numerator left square bracket SO subscript 3 right square bracket over denominator left square bracket SO subscript 2 right square bracket space left square bracket straight O subscript 2 right square bracket end fraction

  • straight K subscript straight c space equals space fraction numerator left square bracket SO subscript 2 right square bracket space left square bracket straight O subscript 2 right square bracket over denominator left square bracket SO subscript 3 right square bracket end fraction

  • straight K subscript straight c space equals space fraction numerator left square bracket SO subscript 3 right square bracket squared over denominator left square bracket SO subscript 2 right square bracket squared space left square bracket straight O subscript 2 right square bracket end fraction

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61 mark

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

H2 (g)  +  F2 (g)  rightwards harpoon over leftwards harpoon  2HF (g)

  • straight K subscript straight c space equals space fraction numerator left square bracket HF right square bracket squared over denominator left square bracket straight H subscript 2 right square bracket space left square bracket space straight F subscript 2 right square bracket end fraction

  • straight K subscript straight c space equals space fraction numerator left square bracket HF right square bracket over denominator left square bracket straight H subscript 2 right square bracket space left square bracket space straight F subscript 2 right square bracket end fraction

  • straight K subscript straight c space equals space fraction numerator 2 space left square bracket HF right square bracket over denominator left square bracket straight H subscript 2 right square bracket space left square bracket straight F subscript 2 right square bracket end fraction

  • straight K subscript straight c space equals space fraction numerator left square bracket straight H subscript 2 right square bracket space left square bracket space straight F subscript 2 right square bracket over denominator left square bracket HF right square bracket squared end fraction

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71 mark

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

3H2 (g) + N2 (g)  rightwards harpoon over leftwards harpoon  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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81 mark

The reaction between Cl2 and I2 is carried out in a 1 dm3 reaction vessel and allowed to reach equilibrium.

At equilibrium, there were 0.5 moles of Cl2 and I2 and 1 mole of ICl3.

3Cl2 (g) + I2 (g)  rightwards harpoon over leftwards harpoon  2ICl3 (l)

What is the correct calculation for Kc?

  • fraction numerator 2 space cross times space 1 over denominator left parenthesis 3 space cross times space 0.5 right parenthesis space cross times space 0.5 end fraction

  • fraction numerator 1 squared over denominator 0.5 cubed space cross times space 0.5 end fraction

  • fraction numerator 0.1 squared over denominator 0.05 cubed space cross times space 0.05 end fraction

  • fraction numerator 1 over denominator 0.5 cross times space 0.5 end fraction

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91 mark

The expression for Kc for the reaction between H2 and I2 is 

fraction numerator left square bracket HI right square bracket squared over denominator left square bracket straight H subscript 2 right square bracket space left square bracket straight I subscript 2 right square bracket end fraction

This reaction occurred in a 10 dm3 reaction vessel and was allowed to reach equilibrium. 

At equilibrium there were 2.34 moles of HI, 1.5 moles of H2 and 0.25 moles of I2.

What is the correct value for Kc?

  • 14.6

  • 62.4

  • 6.2

  • 3.1

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101 mark

Nitrosyl chloride decomposes into nitrogen monoxide and chlorine according to the following equation. The forward reaction is endothermic.

2NOCl (g)  rightwards harpoon over leftwards harpoon  2NO (g) +  Cl2 (g)

Which change in condition would change the value for Kc?

  • Decreasing the pressure

  • Adding a catalyst

  • Increasing the temperature

  • Increasing the pressure

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11 mark

At 450 oC, the value for KC for the following reaction is 60

H2 (g) + I2 (g)  rightwards harpoon over leftwards harpoon  2HI (g)

The equilibrium moles of H2 and I2 are 2 mol and 0.3 mol, respectively.

How many moles of HI (g) are present at equilibrium?

 

  • 36

  • 0.001

  • 6

  • 0.1

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21 mark

Ethyne and hydrogen are formed from methane, and a dynamic equilibrium is established. 

2CH4 (g) rightwards harpoon over leftwards harpoon 3H2 (g) + C2H2 (g)

What are the units for Kc? 

  • mol dm-3

  • mol2 dm-6

  • mol3 dm-9

  • mol4 dm-12

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31 mark

Nitrogen monoxide and oxygen can be formed from the thermal decomposition of nitrogen dioxide.

2NO2 (g) rightwards harpoon over leftwards harpoon 2NO (g) + O2 (g)

In an experiment, 4 moles of nitrogen dioxide were put into a 1.0 dm3 container and heated to a constant temperature. The equilibrium mixture contains 0.8 moles of oxygen.

What is the value of the equilibrium constant, Kc?

  • 0.032

  • 0.222

  • 0.853

  • 0.356

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41 mark

The dissociation of gas X2Y4 into XY2 is represented in the equation below. 

X2Y4 (g)  rightwards harpoon over leftwards harpoon  2XY2 (g)     ΔHϴ = + 60 kJ mol-1

Which of the following statements is correct?

  • the yield of XY2 will increase if the pressure is decreased

  • a decrease in temperature will increase the yield of XY2

  • the yield of XY2 will increase if a catalyst is added

  • an increase in temperature will shift the equilibrium to the left

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51 mark

A compound, WX, will dissociate into an X– ion and a W+ ion in solution. This is represented by the equation below.  

WX (aq) rightwards harpoon over leftwards harpoon X– (aq) + W+ (aq)

2.0 dm3 of a 0.4 mol dm-3 solution of WX was heated to a constant temperature until equilibrium was established. The equilibrium mixture is found to contain 0.1 mol of the W+ ion.  

What is the correct value of Kc?

  • 0.0143

  • 140

  • 0.00714

  • 0.143

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61 mark

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

N2 (g) + 3H2 (g) rightwards harpoon over leftwards harpoon 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 Kc 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)   rightwards harpoon over leftwards harpoon    I2 (g)   +   Cl2 (g)     ΔHϴ = -26 kJ

    What will change if the temperature is lowered?

     

    The position of equilibrium

    The value of Kc

    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 Kc for the following equation 

      2CH3CH2COOH + HOCH2CH2CH2OH rightwards harpoon over leftwards harpoon 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)  rightwards harpoon over leftwards harpoon  2SO2 (g) + O2 (g)    ΔH = +196 kJ mol-1

      What will change if the pressure is decreased? 

       

      Yield of SO2 (g)

      The value of Kc

      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)   rightwards harpoon over leftwards harpoon  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) rightwards harpoon over leftwards harpoon 3E (g) + 2F (g)     ΔH = - 98 kJ mol-1

        What are the correct labels for the graph?

        -aV5r5ur_20

         

        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) rightwards harpoon over leftwards harpoon 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 Kc for the equilibrium between carbon monoxide, oxygen and carbon dioxide is 4. 

          Reaction 1     2CO (g) + O2 (g) rightwards harpoon over leftwards harpoon 2CO2 (g) 

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

          Reaction 2    2CO2 (g) rightwards harpoon over leftwards harpoon 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) rightwards harpoon over leftwards harpoon 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 Kc value for the following reaction is 4.7 x 10-3

          N2 + O2 rightwards harpoon over leftwards harpoon 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               Kc = 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)   rightwards harpoon over leftwards harpoon   2C (g)

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

          Kc  =  42  at 355 degreeC

          Kc  =  65  at 450 degreeC

          Which of the following statements is correct?

          • The forward reaction is exothermic

          • The forward reaction is endothermic

          • The reaction does not proceed at 355 degreeC

          • The reaction has gone to completion at 450 degreeC

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          101 mark

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

          2NOCl (g)  rightwards harpoon over leftwards harpoon  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

            Choose your answer

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