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ASChemistryOCRExam Questions3. Periodic Table & Energy3.6 Chemical Equilibrium

Chemical Equilibrium (OCR AS Chemistry)

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3.6 Chemical Equilibrium

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3.6 Chemical Equilibrium

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11 mark

The reversible reaction below is allowed to reach equilibrium.


H2(g) + I2(g)  rightwards harpoon over leftwards harpoon   2HI(g)        ΔH = –9.4 kJmol–1


Which change in conditions would be expected to shift the equilibrium position towards the products?

  • Decrease the pressure

  • Decrease the temperature

  • Increase the pressure

  • Increase the temperature

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21 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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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

  • Kc = 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) ⇌ 2SO3 (g)

  • Kc = fraction numerator left square bracket SO subscript 2 right square bracket squared 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

  • Kc = 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

  • Kc = 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

  • Kc = 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) ⇌ 2HF (g)

  • Kc = 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 straight F subscript 2 right square bracket end fraction

  • Kc = 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 straight F subscript 2 right square bracket end fraction

  • Kc = fraction numerator 2 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

  • Kc = fraction numerator left square bracket straight H subscript 2 right square bracket space left square bracket straight F subscript 2 right square bracket over denominator left square bracket HF right square bracket squared end fraction

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7
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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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81 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?

  • Increasing the pressure gives a higher yield of H2

  • 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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9
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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)   ⇋  2ICl3 (l)

What is the value for Kc?

  • 2.7

  • 16

  • 1600

  • 4

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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)   ⇌   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

The reversible reaction below is at equilibrium.

2SO2(g) + O2(g)  → 2SO3(g)    ∆H = −197 kJ mol−1

Which changes in pressure and temperature would shift the equilibrium position towards the products?

  Pressure Temperature
A Decrease Decrease
B Decrease Increase
C Increase Decrease
D Increase Increase

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    21 mark

    The reversible reaction below is at equilibrium.

    N2 (g) + 3H2 (g) rightwards harpoon over leftwards harpoon 2NH3 (g)

    What is the expression for Kc?

    • fraction numerator left square bracket straight N subscript 2 space left parenthesis straight g right parenthesis right square bracket space left square bracket straight H subscript 2 space left parenthesis straight g right parenthesis right square bracket cubed over denominator left square bracket NH subscript 3 space left parenthesis straight g right parenthesis right square bracket squared end fraction

    • fraction numerator left square bracket NH subscript 3 space left parenthesis straight g right parenthesis right square bracket squared over denominator left square bracket straight N subscript 2 space left parenthesis straight g right parenthesis right square bracket space left square bracket straight H subscript 2 space left parenthesis straight g right parenthesis right square bracket cubed end fraction

    • fraction numerator left square bracket straight N subscript 2 space left parenthesis straight g right parenthesis right square bracket space plus 3 space left square bracket straight H subscript 2 space left parenthesis straight g right parenthesis right square bracket over denominator 2 left square bracket NH subscript 3 space left parenthesis straight g right parenthesis right square bracket end fraction

    • fraction numerator 2 left square bracket NH subscript 3 space left parenthesis straight g right parenthesis right square bracket over denominator left square bracket straight N subscript 2 space left parenthesis straight g right parenthesis right square bracket space plus 3 left square bracket straight H subscript 2 space left parenthesis straight g right parenthesis right square bracket end fraction

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    31 mark

    Carbon monoxide reacts with steam in the following reaction equation:

    CO (g) + H2O (g) → CO2 (g) + H2 (g)   ∆H = –40 kJ mol–1

    Which change will shift the position of equilibrium to the right hand side of the equation?

    • Decrease in pressure

    • Increase in pressure

    • Decrease in temperature

    • Increase in temperature

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    41 mark

    Chloroethene, CH2=CHCl, is prepared in the presence of a solid catalyst using the equilibrium reaction below.

    CH2ClCH2Cl (g) → CH2=CHCl (g) + HCl (g)   ∆H = +51 kJ mol–1

    Which change would result in an increased equilibrium yield of chloroethene?

    • Increasing the pressure

    • Increasing the surface area of the catalyst

    • Increasing the temperature

    • Use of a homogeneous catalyst

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    5
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    At 450 oC, the value for KC for the following reaction is 60

    H2 (g) + I2 (g) ⇌ 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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    6
    Sme Calculator    1 mark

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

    2NO2 (g) ⇌ 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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    71 mark

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

    X2Y4 (g)  ⇌  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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    81 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 the forwards and backwards reactions
    A Continue to change Remains the same
    B Remain constant Are different
    C Continue to change Are different
    D Remain constant Remains the same

     

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      91 mark

      The reversible reaction below is at equilibrium.

      2SO2(g) + O2(g)  → 2SO3(g)    ∆H = −197 kJ mol−1

      Which changes in pressure and temperature would shift the equilibrium position towards the products?

        Pressure Temperature
      A Decrease Decrease
      B Decrease Increase
      C Increase Decrease
      D Increase Increase

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        11 mark

        The dynamic equilibrium that forms between chromate(VI) and dichromate(VI) ions is shown below. Solutions of chromate ions are yellow and solutions of dichromate ions are orange.

        2CrO42- (aq) + 2H+ (aq) ⇌ Cr2O72- (aq) + 2H2O (l)

        Which of the following statements is/are correct?

        1. Addition of aqueous sodium hydroxide shifts the position of the equilibrium to the left.
        2. This reaction is a reversible and redox reaction.
        3. A homogeneous catalyst can be used to shift the position of the equilibrium to the right.

        • 1, 2 and 3

        • Only 1 and 2

        • Only 2 and 3

        • Only 1

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        21 mark

        The following results are from an experiment involving an unknown equilibrium.

        Condition

        Chemical shift

        Increased temperature

        Right

        Decreased pressure

        Left

        Addition of a catalyst

        No shift

         

        Which of the following statements is correct?

        • The forward reaction is exothermic

        • There are fewer moles of gas on the right-hand side

        • The enthalpy change for the backwards reaction is positive

        • The reaction must be carried out in an open system

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        31 mark

        Haemoglobin carries oxygen in the blood around the body. It forms the following equilibrium where Hb is haemoglobin.

        O2 (g) + HbH+ (aq) ⇌ HbO2 (aq) + H+ (aq)

        Which of the following statements is/are correct?

        1. If the pH of the blood is too low there will be more HbO2 increasing the capacity of haemoglobin to carry oxygen.
        2. The central Fe(II) ion has an electron configuration of 1s22s22p63s23p63d6 and forms dative covalent bonds with oxygen.
        3. On inhalation, the equilibrium in the capillaries surrounding the lungs shifts to the right due to an increased concentration of oxygen.

        • 1, 2 and 3

        • Only 1 and 2

        • Only 2 and 3

        • Only 1

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        41 mark

        Ethyl ethanoate was added to water. 600 cm3 of the reaction mixture at equilibrium contained 0.27 mol of ethyl ethanoate, 0.45 mol of water and the same number of moles of each product.

        Kc = 5.26.

        What is the concentration of each product?

        • 0.799 mol dm-3

        • 0.639 mol dm-3

        • 1.33 mol dm-3

        • 1.78 mol dm-3

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        51 mark

        For the reaction below which of the following statements is/are correct?

        3A (l) + B (l) ⇌ 2C (l) + D (l)

        1. If Kc increases when temperature is increased, the forward reaction has a positive enthalpy change.
        2. The unit for Kc in this reaction is mol-1 dm3.
        3. If the concentration of B increases, the value for Kc will increase

        • 1, 2 and 3

        • Only 1 and 2

        • Only 2 and 3

        • Only 1

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