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IBChemistry: SLDPExam Questions7. Equilibrium7.1 Equilibrium

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First teaching 2014

Last exams 2024

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Equilibrium (DP IB Chemistry: SL)

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7.1 Equilibrium

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7.1 Equilibrium

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11 mark

Dinitrogen tetraoxide, N2O4, and nitrogen dioxide exist in equilibrium. This is represented by the reaction below:

2NO2 (g)  ⇌  N2O4 (g)            increment straight H = -57 kJ mol-1

Which conditions give the greatest percentage of NO2 at equilibrium?

  • high pressure and high temperature 

  • low pressure and high temperature 

  • high pressure and low temperature 

  • low pressure and low temperature 

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21 mark

Which statement below best describes a dynamic equilibrium?

  • the rate of the forward reaction is equal to the backwards reaction, and the concentrations of reactants and products is constant

  • the rate of the forward reaction is equal to the backwards reaction in a closed system, and the concentrations of the reactants and products are equal

  • the rate of the forward reaction is equal to the backwards reaction in a closed system, and the concentrations of the reactants and products is constant

  • the rate of reaction changes in either direction to counteract a change in conditions

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31 mark

Ethanol is manufactured by reacting steam with ethene.

C2H4 (g) + H2O (g) ⇌ C2H5OH (g)                        increment straight H = -45 kJ mol-1

What would increase the equilibrium yield of ethanol in this process?

1

adding a catalyst
2

increasing the pressure
3

decreasing the temperature

  • 1 and 2 only

  • 1 and 3 only

  • 2 and 3 only

  • 1, 2 and 3

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41 mark

Propyl ethanoate is formed in an esterification reaction. How can the value of the equilibrium constant Kc be increased?

Propan-1-ol + ethanoic acid  ⇌  propyl ethanoate + water          begin mathsize 16px style increment H end style = -10 kJ mol-1

  • increasing the temperature

  • adding a catalyst 

  • increasing the pressure 

  • lowering the temperature 

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51 mark

In the reaction where gaseous iodine reacts with hydrogen,an equilibrium is established at 450 degreeC. The reaction is exothermic.

H2 (g) +   I2 (g) ⇌ 2HI (g)
 colourless   purple   colourless


Which change in conditions will cause the purple colour of the equilibrium mixture to become paler?

  • decrease in pressure

  • decrease in temperature

  • increase in pressure

  • increase in temperature

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61 mark

Nitrosyl chloride decomposes into nitrogen monoxide and chlorine according to the following equation.

2NOCl (g)   ⇌   2NO +  Cl2 (g)

What is the correct expression for Kc?

  • fraction numerator left square bracket NOCl right square bracket squared over denominator left square bracket NO right square bracket squared space left square bracket Cl subscript 2 right square bracket end fraction

  • fraction numerator left square bracket NO right square bracket space left square bracket Cl subscript 2 right square bracket over denominator left square bracket NOCl right square bracket end fraction

  • fraction numerator 2 left square bracket NO right square bracket space left square bracket Cl subscript 2 right square bracket over denominator 2 left square bracket NOCl right square bracket end fraction

  • fraction numerator left square bracket NO right square bracket squared space left square bracket Cl subscript 2 right square bracket over denominator left square bracket NOCl right square bracket squared end fraction

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71 mark

An equilibrium is established in the reaction.

AB (aq) + CD (aq)  ⇌  AC (aq) + BD (aq)             increment straight H = +180 kJ mol-1

Which factors would affect the value of Kc  in this equilibrium?

  • change in temperature in the absence of a catalyst

  • change in pressure in the presence of a catalyst

  • increasing the concentration of AB

  • increasing the concentration of AC

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81 mark

What is the equilibrium constant expression for the following equation?

3Cl2 (g)   +   I2 (g)  ⇋  2ICl3 (l)

  • fraction numerator 2 left square bracket ICl subscript 3 right square bracket over denominator 3 left square bracket Cl subscript 2 right square bracket plus left square bracket straight I subscript 2 right square bracket end fraction

  • fraction numerator 2 left square bracket ICl subscript 3 right square bracket over denominator 3 left square bracket Cl subscript 2 right square bracket space left square bracket straight I subscript 2 right square bracket end fraction

  • fraction numerator left square bracket Cl subscript 2 right square bracket cubed space left square bracket straight I subscript 2 right square bracket over denominator left square bracket ICl subscript 3 right square bracket squared end fraction

  • fraction numerator left square bracket ICl subscript 3 right square bracket squared over denominator left square bracket Cl subscript 2 right square bracket cubed space left square bracket straight I subscript 2 right square bracket end fraction

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91 mark

Hydrogen iodide can be made by the direct combination of hydrogen and iodine in the following equilibrium reaction. What is the expression for the equilibrium constant?

H2 (g)   +    I2 (g)   ⇋   2HI (g)

  • fraction numerator left square bracket 2 HI right square bracket 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

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

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

  • 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

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101 mark

What is the significance of the value of Kc ≫1 in a reversible reaction?

  • The reaction is almost complete

  • Very little reaction occurs

  • Equilibrium is established very quickly

  • The rate of the forward reaction is larger than the rate of the backward reaction

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11 mark

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

2CH4 (g)  ⇌   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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21 mark

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

X2Y4 (g)  ⇌  2XY2 (g)         increment straight H to the power of empty set = +60 kJ mol-1 

At constant pressure, if the temperature of the equilibrium mixture of the gases is increased, will the volume of the mixture increase or decrease and why?

  • the volume will increase, but only because of a shift of equilibrium towards the right 

  • the volume will increase, both because of a shift of equilibrium towards the right and also because of thermal expansion 

  • the volume will stay the same because any thermal expansion could be exactly counteracted by a shift of equilibrium towards the left

  • the volume will decrease because a shift of equilibrium towards the left would more than counteract any thermal expansion 

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31 mark

The reaction between nitrogen and hydrogen produces ammonia gas:

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

If the equilibrium constant for the reaction is x, what is the equilibrium constant for the following reaction?

2N2 (g) + 6H2 (g)  rightwards harpoon over leftwards harpoon  4NH3 (g)

  • x

  • 2x

  • x2

  • 2x2

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41 mark

For the following reaction at equilibrium

H2S (aq)  +  Mg2+ (aq)  ⇋   MgS (s)   +  2H+ (aq)              ΔH < 0

Which of the following changes will result in more MgS being produced?

  • Adding sodium hydroxide

  • Decreasing the pressure

  • Adding a catalyst

  • Increasing the temperature

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51 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

Remain the same

C

Continue to change

Are different

D

Remain constant

Are different

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    61 mark

    The Haber process is used to produce ammonia industrially by the following reaction

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

    Which statements explain why a catalyst is used?

    1

    A catalyst lowers the activation energy
    2

    A catalyst moves the position of equilibrium towards the product
    3

    A catalyst allows the same rate to be achieved at a lower temperature

    • 1 and 2 only

    • 1 and 3 only

    • 2 and 3 only

    • 1, 2 and 3

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    71 mark

    Study the following equilibrium reaction and determine which of the changes listed below will shift the equilibrium to the right hand side.

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

    1

    using a catalyst
    2

    reducing the oxygen concentration
    3

    increasing the volume of the container

    • 1 and 2 only

    • 1 and 3 only

    • 2 and 3 only

    • 1, 2 and 3

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    81 mark

    Which of the following conclusions can be made about this reaction?

    H2O (g)   ⇋   H2 (g)   + 1 halfO2 (g)            Kc = 8 x 10-41 at 25 degreeC

    • The reaction does not proceed

    • The reaction goes almost to completion

    • The products have a higher concentration than the reactants

    • The concentrations of reactants and products are the same.

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    91 mark

    The value of Kc at 298K for the following reaction is 4.0. The reaction takes several days to react in equilibrium at room temperature.

    CH3COOH (l) +   CH3CH2OH (l)  ⇋  CH3COOCH2CH3 (l)   +  H2O (l)

    A student carried out the reaction shown and a day later measured the concentrations and calculated a value for the reaction quotient, Q. She found that the value of Q was 8.7.

    This value tells you that:

    • The reaction has not reached equilibrium and is moving left to right.

    • The reaction has not reached equilibrium and is moving right to left.

    • The reaction has reached equilibrium, but there are more products than reactants.

    • The reaction has reached equilibrium, but there are more reactants than products.

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    101 mark

    In the equilibrium reaction shown

    2ICl (l)   ⇋    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 right

    decreases

    B

    will shift to the right

    increases

    C

    will shift to the left

    decreases

    D

    will shift to the left

    increases

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      11 mark

      Study the following equilibrium reaction and determine which of the statements must be true.

      2X    ⇋   Y               Kc = 1.1

      • [X] ≫ [Y]

      • [X] ＞[Y]

      • [X] = [Y]

      • [X] ＜ [Y]

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      21 mark

      Hydrogen reacts with iodine according to the following equation

      H2 (g)  + I2 (g)   ⇋   2HI (g)

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

      Kc  =  60  at 355 degreeC

      Kc  =  47  at 450 degreeC

      From the information given which of the following must be true?

      • The reaction is exothermic

      • The reaction is endothermic

      • The reaction barely proceeds at 355 degreeC

      • The reaction almost goes to completion at 450 degreeC

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      31 mark

      The following Kc values were obtained for a reaction carried out at different temperatures, T1 to T4.

      Temperature

      Kc value

      T1

      1 x 10-2

      T2

      1 x 101

      T3

      1

      T4

      1 x 102


      Which of the following gives the correct amount of products in the mixtures from least to most?

      • T1 ＜T2 ＜T3＜T4

      • T4 ＜T3 ＜T2＜T1

      • T4 ＜T2 ＜T3＜T1

      • T1 ＜T3 ＜T2＜T4

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      41 mark

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

      H2 (g)  + I2 (g)   ⇋   2HI (g)              ΔHᶿ = -126 kJ

       

      Condition

      Reason

      Condition

      Reason

      A

      increase T

      exothermic reaction

      increase P

      two gaseous reactants but only one gaseous product

      B

      increase T

      endothermic reaction

      no change in P

      equal numbers of moles of gases

      C

      decrease T

      exothermic reaction

      decrease P

      two moles of gaseous product but only one mole of each gaseous reactant

      D

      decrease T

      exothermic reaction

      no change in P

      equal numbers of moles of gases

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        51 mark

        The blood-red complex iron (III) thiocynanate, [FeSCN]2+ is formed when iron (III) ions react with thiocyanate ions in the following equilibrium reaction:

         Fe3+ (aq) + SCN- (aq) ⇋ [FeSCN]2+    ΔHᶿ = +12 kJ
        yellow    colourless    red    


        Which of the following changes would make the solution go darker?

        1

        raising the temperature of the solution
        2

        adding iron(III) chloride solution
        3

        adding a catalyst

        • 1 and 2 only

        • 1 and 3 only

        • 2 and 3 only

        • 1, 2 and 3

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        61 mark

        Which of the following features is not a characteristic of a state of equilibrium?

        • Equilibrium is dynamic

        • Equilibrium is achieved in a closed system

        • Concentrations of reactants and products are equal

        • Equilibrium can be reached from either direction

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        71 mark

        What is the relationship between K subscript c 1 end subscript and  K subscript c 2 end subscript in the following reactions?

        2NOBr (g)  ⇋   2NO (g)   +  Br2 (g)        K subscript c 1 end subscript

        NO (g)   +  begin mathsize 16px style 1 half end styleBr2 (g) ⇋  NOBr (g)         K subscript c 2 end subscript 

        • begin mathsize 16px style 2 straight K subscript straight c 2 end subscript equals straight K subscript straight c 1 end subscript end style

        • begin mathsize 16px style straight K subscript straight c 2 end subscript equals fraction numerator 1 over denominator square root of straight K subscript straight c 1 end subscript end root end fraction end style

        • begin mathsize 16px style straight K subscript straight c 2 end subscript equals fraction numerator 1 over denominator 2 straight K subscript straight c 1 end subscript end fraction end style

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        81 mark

        Nitrogen dioxide can react with itself to produce a dimer molecule called dinitrogen tetroxide in the following equilibrium reaction.

        2NO2 (g)    ⇋  N2O4 (g)      Kc  = 0.01 at 25 degreeC

        In an experiment, 100 cm3 of nitrogen dioxide is placed in a gas syringe and the barrel is pushed in, meaning the volume is reduced to 50 cm3 at constant temperature.

        Which of the following are true?

        1

        The value of Kc increases
        2

        More N2O4 is formed
        3

        The ratio of begin mathsize 16px style fraction numerator left square bracket NO subscript 2 right square bracket over denominator left square bracket straight N subscript 2 straight O subscript 4 right square bracket end fraction end style decreases

        • 1 and 2 only

        • 1 and 3 only

        • 2 and 3 only

        • 1, 2 and 3

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        91 mark

        One of the characteristics of a state of equilibrium, is that equilibria are said to be dynamic. What is the meaning of dynamic in this context?

        • The position of equilibrium is constantly changing

        • The rates of forward and backward reactions change

        • The reactants and products are continually reacting

        • The concentrations of the reactants and products continue to change

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        101 mark

        The reaction shown below has a value of Kc = 1.0 x 10-4 at 25 degreeC.

        2NOBr (g)   ⇌    2NO (g)   +  Br2 (l)

        Which of the following relationships is correct about this equilibrium at 25 degreeC?

        • [NO] ≫ [NOBr]

        • [NOBr] ≫ [Br2]

        • 2 x [NOBr] = [Br2]

        • [NO] = [NOBr]

        Choose your answer

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