Chemical Equilibrium

26 free MDCAT Chemistry MCQs on Chemical Equilibrium (PM&DC syllabus), each with the answer, an explanation and the textbook line it comes from. Tap to check, no sign-up.

Tap an answer to check it · 26 questions

1. Question 1

   The law of mass action was first enunciated by Guldberg and Waage in:
   • A1864
   • B1884
   • C1888
   • D1904
   Show answer and explanation

   Answer: A) 1864

   1884 is the year of Le-Chatelier's principle. Source: Chemistry 11 KPK (taleem360.com), p. 217.

2. Question 2

   Human blood is buffered to a pH of about:
   • A7.4
   • B1.5
   • C6.0
   • D8.4
   Show answer and explanation

   Answer: A) 7.4

   Bicarbonates, phosphates and proteins buffer blood; 1.5 is gastric juice. Source: Chemistry 11 KPK (taleem360.com), p. 265.

3. Question 3

   Reactions that do not go to completion and whose products react to re-form the reactants are called:
   • AExothermic reactions
   • BIrreversible reactions
   • CReversible reactions
   • DSpontaneous reactions
   Show answer and explanation

   Answer: C) Reversible reactions

   They are shown with double arrows and reach equilibrium in a closed vessel. Source: Chemistry 11 KPK (taleem360.com), p. 216.

4. Question 4

   Equilibrium in which forward and reverse reactions continue at the same rate, with no change in concentrations, is called:
   • ASaturation
   • BStatic equilibrium
   • CComplete reaction
   • DDynamic equilibrium
   Show answer and explanation

   Answer: D) Dynamic equilibrium

   Reactions do not stop at equilibrium; they occur at equal rates. Source: Chemistry 11 KPK (taleem360.com), p. 215.

5. Question 5

   An equilibrium in which all the components are present in only one phase is:
   • AHomogeneous
   • BHeterogeneous
   • CIonic
   • DStatic
   Show answer and explanation

   Answer: A) Homogeneous

   N2 + O2 <=> 2NO is homogeneous; CaCO3(s) <=> CaO(s) + CO2(g) is heterogeneous. Source: Chemistry 11 KPK (taleem360.com), p. 217.

6. Question 6

   According to Le-Chatelier's principle, a system at equilibrium subjected to a stress adjusts itself so as to:
   • AIncrease the effect of the change
   • BMinimize the effect of the change
   • CStop both reactions
   • DKeep all concentrations equal
   Show answer and explanation

   Answer: B) Minimize the effect of the change

   The stress may be a change in concentration, pressure or temperature. Source: Chemistry 11 KPK (taleem360.com), p. 227.

7. Question 7

   For PCl5 <=> PCl3 + Cl2, Kp = 0.82 at 523 K. Taking R = 0.082 dm3 atm/K mol, Kc is:
   • A5.24 x 10^-3
   • B35.2
   • C0.82
   • D1.91 x 10^-2
   Show answer and explanation

   Answer: D) 1.91 x 10^-2

   Delta n = 1, so Kc = Kp/RT = 0.82/(0.082 x 523) = 1.91 x 10^-2. Source: Chemistry 11 KPK (taleem360.com), p. 223.

8. Question 8

   An equilibrium mixture contains 0.25 mol/dm3 N2, 0.15 mol/dm3 H2 and 0.09 mol/dm3 NH3. Kc for N2 + 3H2 <=> 2NH3 is:
   • A9.6
   • B0.104
   • C2.4
   • D96
   Show answer and explanation

   Answer: A) 9.6

   Kc = (0.09)^2 / (0.25 x 0.15^3) = 0.0081/0.000844 = 9.6. Source: Chemistry 11 KPK (taleem360.com), p. 220.

9. Question 9

   0.5 mol each of H2 and I2 are placed in a 1 dm3 flask. Kc for H2 + I2 <=> 2HI is 54.3. The equilibrium [HI] is:
   • A0.393 M
   • B0.786 M
   • C0.107 M
   • D1.572 M
   Show answer and explanation

   Answer: B) 0.786 M

   Kc = (2x)^2/(0.5-x)^2, so 2x/(0.5-x) = 7.37, x = 0.393, [HI] = 2x = 0.786 M; [H2] = [I2] = 0.107 M. Source: Chemistry 11 KPK (taleem360.com), p. 238.

10. Question 10

    Kp is greater than Kc when the difference in moles of gaseous products and reactants is:
    • AZero
    • BPositive
    • CNegative
    • DIrrelevant
    Show answer and explanation

    Answer: B) Positive

    Kp = Kc(RT)^(delta n); RT > 1, so Kp > Kc when delta n is positive and Kp = Kc when it is zero. Source: Chemistry 11 KPK (taleem360.com), p. 221.

11. Question 11

    At chemical equilibrium:
    • AForward rate exceeds reverse rate
    • BBoth reactions stop at that point
    • CForward and reverse rates are equal
    • DReverse rate exceeds forward rate
    Show answer and explanation

    Answer: C) Forward and reverse rates are equal

    The reactions continue (dynamic equilibrium) at equal rates. Source: Chemistry 11 KPK (taleem360.com), p. 215.

12. Question 12

    Adding a catalyst to a reversible reaction:
    • AIncreases product yield
    • BIncreases the value of Kc
    • CShifts equilibrium forward
    • DHelps attain equilibrium earlier
    Show answer and explanation

    Answer: D) Helps attain equilibrium earlier

    It speeds up forward and reverse reactions equally, so neither Kc nor position changes. Source: Chemistry 11 KPK (taleem360.com), p. 217.

13. Question 13

    The value of Kc for a reaction changes only with change in:
    • ATemperature
    • BConcentration
    • CPressure
    • DCatalyst
    Show answer and explanation

    Answer: A) Temperature

    Kc is independent of initial concentrations. Source: Chemistry 11 KPK (taleem360.com), p. 219.

14. Question 14

    A mixture with Kc = 1 x 10^-30 for N2 + O2 <=> 2NO at 25 C contains at equilibrium:
    • ANo N2 or O2
    • BMostly NO
    • CEqual N2 and NO
    • DVery little NO
    Show answer and explanation

    Answer: D) Very little NO

    A very small Kc means only a tiny amount of product forms. Source: Chemistry 11 KPK (taleem360.com), p. 225.

15. Question 15

    For N2 + O2 <=> 2NO at equilibrium, removing NO from the vessel:
    • ACauses the forward reaction
    • BCauses the reverse reaction
    • CHas no effect
    • DDecreases Kc
    Show answer and explanation

    Answer: A) Causes the forward reaction

    Part of the removed NO is reproduced as the system restores equilibrium. Source: Chemistry 11 KPK (taleem360.com), p. 227.

16. Question 16

    Increasing the pressure on N2O4 <=> 2NO2 at equilibrium:
    • AFavours the forward reaction
    • BFavours the reverse reaction
    • CHas no effect
    • DIncreases Kc
    Show answer and explanation

    Answer: B) Favours the reverse reaction

    Higher pressure favours the side with fewer gas volumes (1 volume of N2O4 vs 2 of NO2). Source: Chemistry 11 KPK (taleem360.com), p. 228.

17. Question 17

    Changing the pressure has no effect on the equilibrium 2HI <=> H2 + I2 because:
    • AThe reaction is exothermic
    • BHI turns into a liquid
    • CNo change in volume occurs
    • DKc is very large
    Show answer and explanation

    Answer: C) No change in volume occurs

    Two gas volumes give two gas volumes. Source: Chemistry 11 KPK (taleem360.com), p. 228.

18. Question 18

    Thermal decomposition of PCl5 into PCl3 and Cl2 is favoured by:
    • AAdding PCl3
    • BHigh pressure
    • CAdding Cl2
    • DLow pressure
    Show answer and explanation

    Answer: D) Low pressure

    One mole of gas gives two, so lower pressure favours decomposition; adding a product shifts it back. Source: Chemistry 11 KPK (taleem360.com), p. 228.

19. Question 19

    For an exothermic reaction at equilibrium, raising the temperature:
    • AShifts it backward
    • BShifts it forward
    • CHas no effect
    • DIncreases Kc
    Show answer and explanation

    Answer: A) Shifts it backward

    The reverse (endothermic) direction absorbs the added heat; Kc decreases. Source: Chemistry 11 KPK (taleem360.com), p. 228.

20. Question 20

    Formation of NO from N2 and O2 (delta H = +180.5 kJ/mol) is favoured by:
    • ALow temperature
    • BHigh temperature
    • CHigh pressure
    • DA catalyst
    Show answer and explanation

    Answer: B) High temperature

    Endothermic forward reaction; pressure has no effect as gas volumes are equal. Source: Chemistry 11 KPK (taleem360.com), p. 229.

21. Question 21

    Ammonia synthesis is exothermic, yet it is not run at a low temperature, because at low temperature the:
    • ACatalyst becomes poisoned
    • BYield of ammonia decreases
    • CReaction proceeds too slowly
    • DAmmonia dissociates more
    Show answer and explanation

    Answer: C) Reaction proceeds too slowly

    A compromise (optimum) temperature gives a reasonable yield at a reasonable rate. Source: Chemistry 11 KPK (taleem360.com), p. 230.

22. Question 22

    High pressure favours ammonia formation in Haber's process because:
    • AKc increases with pressure
    • BThe reaction is exothermic
    • CThe catalyst works better
    • DForward reaction decreases volume
    Show answer and explanation

    Answer: D) Forward reaction decreases volume

    1 volume N2 + 3 volumes H2 give 2 volumes NH3. Source: Chemistry 11 KPK (taleem360.com), p. 229.

23. Question 23

    Pure NaCl is precipitated from its saturated solution by passing:
    • AHCl gas
    • BCl2 gas
    • CCO2 gas
    • DNH3 gas
    Show answer and explanation

    Answer: A) HCl gas

    Cl- is the common ion, so the equilibrium shifts back and NaCl precipitates. Source: Chemistry 11 KPK (taleem360.com), p. 234.

24. Question 24

    When a little NaOH is added to an acetic acid-sodium acetate buffer, the extra OH- ions:
    • APrecipitate as a salt
    • BRemain free in solution
    • CCombine with Na+ ions
    • DCombine to form water
    Show answer and explanation

    Answer: D) Combine to form water

    OH- combines with H+; more acetic acid ionizes to replace it, so pH stays nearly constant. Source: Chemistry 11 KPK (taleem360.com), p. 263.

25. Question 25

    Le-Chatelier's principle applies to:
    • APhysical and chemical equilibria
    • BMechanical and electrical systems
    • COnly physical equilibria
    • DOnly chemical equilibria
    Show answer and explanation

    Answer: A) Physical and chemical equilibria

    It is a general principle for all equilibria. Source: Chemistry 11 KPK (taleem360.com), p. 227.

26. Question 26

    Endothermic reactions are favoured in the forward direction by:
    • ADecrease of temperature
    • BIncrease of temperature
    • CAddition of a catalyst
    • DFreezing the mixture
    Show answer and explanation

    Answer: B) Increase of temperature

    Added heat is absorbed by the endothermic direction. Source: Chemistry 11 KPK (taleem360.com), p. 228.

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