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Chemical equilibrium practice

QCE Chemistry · Original practice questions with worked solutions

Practise equilibrium expressions, reaction quotients and responses to changing conditions. Explain changes in terms of the reaction and its concentrations, rather than relying only on a memorised direction.

Key ideas

  • At dynamic equilibrium, forward and reverse reactions continue at equal rates. Concentrations remain constant but need not be equal.
  • For aA+bB⇌cC+dDaA+bB\rightleftharpoons cC+dD, the concentration expression is Kc=[C]c[D]d/([A]a[B]b)K_c=[C]^c[D]^d/([A]^a[B]^b), omitting pure solids and liquids.
  • Compare QQ with KK: Q<KQ<K favours net forward reaction; Q>KQ>K favours net reverse reaction. Temperature changes can change KK.

Worked example

For H2(g)+I2(g)⇌2HI(g)\mathrm{H_2(g)+I_2(g)\rightleftharpoons2HI(g)}, Kc=[HI]2/([H2][I2])K_c=[\mathrm{HI}]^2/([\mathrm{H_2}][\mathrm{I_2}]). The squared term follows the balanced coefficient of HI.

A common mistake

A catalyst changes how quickly equilibrium is reached. It does not change the equilibrium constant or equilibrium composition.

Try these questions

Attempt each question before revealing the worked solution. Saved questions and marks also appear in the main bank on this device.

Q1 · Practice questionSimple familiar1 mark

QUESTION 1

A sealed flask contains the equilibrium N2O4(g)⇌2NO2(g) at constant temperature. Its colour remains constant. Which statement is correct?
(A)
Both reactions have stopped.
(B)
The forward and reverse rates are equal and non-zero.
(C)
The concentrations of the two gases must be equal.
(D)
All nitrogen dioxide has been consumed.
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Q31 · Practice questionSimple familiar4 marks

QUESTION 31 (4 marks)

A small sealed display vial contains brown NO2 and colourless N2O4: N2O4(g)⇌2NO2(g). The brown colour deepens when the vial is warmed.

a) Determine whether the forward reaction is endothermic or exothermic. Justify your answer. [2 marks]

b) Explain why the colour becomes constant after the warmed vial is held at a fixed temperature. [2 marks]

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Q32 · Practice questionComplex familiar4 marks

QUESTION 32 (4 marks)

For A(g)⇌B(g), Kc=3.0 at a fixed temperature. Initially a 2.0 L vessel contains 0.80 mol A and no B.

a) Write the equilibrium expression. [1 marks]

b) Calculate the equilibrium concentrations of A and B. [3 marks]

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All chemical equilibrium practice questions

27 original questions · Page 1 of 2

  1. Q1 · Original practice · 1 mark
    A sealed flask contains the equilibrium N2O4(g)⇌2NO2(g)\mathrm{N_2O_4(g)\rightleftharpoons 2NO_2(g)} at constant temperature. Its colour remains constant. Which statement is correct?
    Chemical equilibrium
  2. Q2 · Original practice · 1 mark
    The volume of a flask containing N2(g)+3H2(g)⇌2NH3(g)\mathrm{N_2(g)+3H_2(g)\rightleftharpoons2NH_3(g)} is suddenly halved at constant temperature. After re-equilibration, which change occurs?
    Chemical equilibrium
  3. Q3 · Original practice · 1 mark
    For H2(g)+I2(g)⇌2HI(g)\mathrm{H_2(g)+I_2(g)\rightleftharpoons2HI(g)}, Kc=64K_c=64. A mixture has [H2]=0.10[\mathrm{H_2}]=0.10, [I2]=0.10[\mathrm{I_2}]=0.10 and [HI]=0.40 mol L−1[\mathrm{HI}]=0.40\ \mathrm{mol\,L^{-1}}. It will initially
    Chemical equilibrium
  4. Q4 · Original practice · 1 mark
    Which expression represents KcK_c for CaCO3(s)⇌CaO(s)+CO2(g)\mathrm{CaCO_3(s)\rightleftharpoons CaO(s)+CO_2(g)}?
    Chemical equilibrium
  5. Q5 · Original practice · 1 mark
    For AgCl(s)⇌Ag+(aq)+Cl−(aq)\mathrm{AgCl(s)\rightleftharpoons Ag^+(aq)+Cl^-(aq)}, Ksp=1.8×10−10K_{sp}=1.8\times10^{-10}. Its molar solubility in pure water is closest to
    Chemical equilibrium
  6. Q31 · Original practice · 4 marks
    A small sealed display vial contains brown NO2\mathrm{NO_2} and colourless N2O4\mathrm{N_2O_4}: N2O4(g)⇌2NO2(g)\mathrm{N_2O_4(g)\rightleftharpoons2NO_2(g)}. The brown colour deepens when the vial is warmed.
    Chemical equilibrium
  7. Q32 · Original practice · 4 marks
    For A(g)⇌B(g)\mathrm{A(g)\rightleftharpoons B(g)}, Kc=3.0K_c=3.0 at a fixed temperature. Initially a 2.0 L vessel contains 0.80 mol A and no B.
    Chemical equilibrium
  8. Q33 · Original practice · 4 marks
    A flask contains H2(g)+I2(g)⇌2HI(g)\mathrm{H_2(g)+I_2(g)\rightleftharpoons2HI(g)}. At equilibrium [H2]=0.20[\mathrm{H_2}]=0.20, [I2]=0.30[\mathrm{I_2}]=0.30 and [HI]=1.20 mol L−1[\mathrm{HI}]=1.20\ \mathrm{mol\,L^{-1}}.
    Chemical equilibrium
  9. Q34 · Original practice · 4 marks
    PCl5(g)⇌PCl3(g)+Cl2(g)\mathrm{PCl_5(g)\rightleftharpoons PCl_3(g)+Cl_2(g)}. Initially [PCl5]=0.50 mol L−1[\mathrm{PCl_5}]=0.50\ \mathrm{mol\,L^{-1}} with no products. At equilibrium [Cl2]=0.12 mol L−1[\mathrm{Cl_2}]=0.12\ \mathrm{mol\,L^{-1}}.
    Chemical equilibrium
  10. Q35 · Original practice · 4 marks
    The graph shows A(g)⇌B(g)\mathrm{A(g)\rightleftharpoons B(g)} in a constant-volume vessel. Only the temperature is increased at t=4.0t=4.0 min. The solid line is B and the dashed line is A.
    Chemical equilibrium
  11. Q36 · Original practice · 4 marks
    At 25∘C25^\circ\mathrm C, the solubility product of CaF2\mathrm{CaF_2} is 3.2×10−113.2\times10^{-11}.
    Chemical equilibrium
  12. Q37 · Original practice · 4 marks
    Equal volumes of 2.0×10−4 mol L−12.0\times10^{-4}\ \mathrm{mol\,L^{-1}} silver nitrate and 4.0×10−4 mol L−14.0\times10^{-4}\ \mathrm{mol\,L^{-1}} sodium chloride are mixed. For silver chloride, Ksp=1.8×10−10K_{sp}=1.8\times10^{-10}.
    Chemical equilibrium
  13. Q38 · Original practice · 4 marks
    A saturated solution is in contact with excess Mg(OH)2(s)\mathrm{Mg(OH)_2(s)}. Some sodium hydroxide is added, with negligible volume change.
    Chemical equilibrium
  14. Q86 · Original practice · 7 marks
    A saturated solution of Mg(OH)2\mathrm{Mg(OH)_2} is filtered to remove all undissolved solid. At 25∘C25^\circ\mathrm C, Ksp=1.8×10−11K_{sp}=1.8\times10^{-11} and Kw=1.0×10−14K_w=1.0\times10^{-14}. Assume dissolution is the only significant source of hydroxide.
    Chemical equilibrium
  15. Q92 · Original practice · 6 marks
    Ethanoic acid reacts with ethanol in a non-aqueous homogeneous mixture of constant volume 100.0 mL. Initially each reactant has concentration 0.100 mol L⁻¹ and neither product is present. For this model, Kc=[ester][H2O][acid][ethanol]=4.00K_c=\dfrac{[\text{ester}][\mathrm{H_2O}]}{[\text{acid}][\text{ethanol}]}=4.00. The ester molar mass is 88.1 g mol⁻¹.
    Chemical equilibrium
  16. Q94 · Original practice · 6 marks
    A mixture initially contains 0.0100 mol L⁻¹ each of Cu²⁺ and Mg²⁺. Hydroxide is slowly added at 25∘C25^\circ\mathrm C with negligible volume change. Ksp(Cu(OH)2)=2.2×10−20K_{sp}(\mathrm{Cu(OH)_2})=2.2\times10^{-20}; Ksp(Mg(OH)2)=1.8×10−11K_{sp}(\mathrm{Mg(OH)_2})=1.8\times10^{-11}; Kw=10−14K_w=10^{-14}. Ignore complex ions.
    Chemical equilibrium
  17. Q99 · Original practice · 7 marks
    The energy profile shows a reversible reaction. Reactants have energy 40 kJ mol⁻¹, products 10 kJ mol⁻¹, and the uncatalysed transition state 110 kJ mol⁻¹. The dashed catalysed pathway peaks at 85 kJ mol⁻¹.
    Chemical equilibrium
  18. Q101 · Original practice · 1 mark
    Solid iodine and iodine vapour are present in a sealed tube at constant temperature. The colour of the vapour is constant. Which statement explains this observation?
    Chemical equilibrium
  19. Q102 · Original practice · 1 mark
    A mixture is at equilibrium for A(g)⇌2B(g)\mathrm{A(g)\rightleftharpoons 2B(g)}. The vessel volume is instantly halved at constant temperature. Immediately after compression, before any net reaction, the reaction quotient is
    Chemical equilibrium
  20. Q119 · Original practice · 5 marks
    A flask contains liquid ethanol and ethanol vapour at equilibrium at constant temperature. Some liquid remains throughout the experiment. The stopper is then removed and vapour disperses into the surrounding air.
    Chemical equilibrium
  21. Q120 · Original practice · 6 marks
    The graph shows concentrations for A(g)⇌2B(g)\mathrm{A(g)\rightleftharpoons2B(g)}. The temperature is constant. A single change is made at 4.0 min. The solid curve is A and the dashed curve is B. At the new equilibrium, [A]=0.450 mol L−1[A]=0.450\ \mathrm{mol\,L^{-1}} and [B]=0.300 mol L−1[B]=0.300\ \mathrm{mol\,L^{-1}}.
    Chemical equilibrium
  22. Q121 · Original practice · 5 marks
    A closed system is at equilibrium for X(g)⇌Y(g)\mathrm{X(g)\rightleftharpoons Y(g)}. A catalyst is added without changing temperature, volume or composition. The rates just before and just after addition are shown.
    Chemical equilibrium
  23. Q122 · Original practice · 5 marks
    For X(aq)⇌Y(aq)+Z(aq)\mathrm{X(aq)\rightleftharpoons Y(aq)+Z(aq)}, Kc=3.0×10−6K_c=3.0\times10^{-6} at a fixed temperature. Initially [X]=0.300 mol L−1[X]=0.300\ \mathrm{mol\,L^{-1}} and no Y or Z is present.
    Chemical equilibrium
  24. Q123 · Original practice · 5 marks
    A student mixes 30.0 mL of 2.00×10−3 mol L−12.00\times10^{-3}\ \mathrm{mol\,L^{-1}} Pb(NO₃)₂ with 20.0 mL of 4.00×10−3 mol L−14.00\times10^{-3}\ \mathrm{mol\,L^{-1}} KI. Volumes are additive. At this temperature Ksp(PbI2)=8.0×10−9K_{sp}(\mathrm{PbI_2})=8.0\times10^{-9}.
    Chemical equilibrium