Download Multiple-choice questions : 1. Which of the following statements

Multiple-choice questions :
1.
Which of the following statements concerning dynamic equilibrium is/are
correct?
(1) A reversible reaction stops when equilibrium is reached.
(2) At equilibrium, the concentrations of reactants and products are constant.
(3) At equilibrium, the concentrations of reactants and products are the same.
A. (1) only
B. (2) only
C. (1) and (3) only
D.
(2) and (3) only
□
2.
Which of the following statements concerning the chemical equilibrium below
is/are correct?
CaCO3(s)
CaO(s) + CO2(g)
(1) The above chemical equilibrium can only be established in a sealed
container.
(2) The reaction occurs spontaneously at room conditions.
(3) Equilibrium can be reached from the backward direction of the reaction.
A. (1) only
B.
C.
D.
(2) only
(1) and (3) only
(2) and (3) only
□
3.
Which of the following statements concerning the effect of catalysts on the
reversible reactions is correct?
A. Catalysts increase either the rate of forward reaction or the rate of backward
reaction of reversible reactions.
B. Catalysts shorten the time for the reversible reactions to reach equilibrium
C.
D.
by increasing the rate of forward reaction and decreasing the rate of
backward reaction.
Catalysts shorten the time for the reversible reactions to reach equilibrium
by increasing both the rates of forward and backward reactions.
Catalysts increase the yield of products for reversible reactions.
□
1
4.
When a small amount of N2O4(l) is introduced into a sealed container at a certain
temperature, the following reaction takes place:
N2O4(g)
2NO2(g)
Which of the following graphs correctly shows the changes in concentrations of
N2O4 and NO2 with time until the reaction reaches equilibrium?
Concentration
B.
Concentration
A.
N2O4
NO2
N2O4
NO2
Time
Concentration
D.
Concentration
C.
Time
N2O4
NO2
N2O4
NO2
Time
Time
□
5.
Equal numbers of moles of I2(g) and H2(g) are introduced in a sealed container at
a certain temperature. The following reaction occurs:
I2(g) + H2(g)
2HI(g)
The reaction mixture is allowed to reach chemical equilibrium.
Which of the following statements concerning the above chemical equilibrium
is/are INCORRECT?
A. The rate of combination between I2(g) molecules and H2(g) molecules is
equal to the rate of decomposition of HI(g) molecules.
B. The colour intensity of the reaction mixture remains constant.
C. The concentrations of I2(g) and H2(g) are always the same.
D.
When 1 mol of HI(g) decomposes, 1 mol of I2(g) and 1mol of H2(g)
combine at the same time.
□
2
6.
In dynamic equilibrium, which of the following properties of a reaction mixture
remain constant?
(1) Concentrations of all involved species
(2) Colour intensity of the reaction mixture
(3) Rates of reactions in both forward and backward directions
A. (1) and (2) only
B. (1) and (3) only
C. (2) and (3) only
D. (1), (2) and (3)
□
7.
Hydrogen sulphide decomposes to give hydrogen gas and sulphur gas:
2H2S(g)
2H2(g) +S2(g)
Which of the following equilibrium constant expressions for the reaction is
correct?
A.
Kc 
B.
Kc 
C.
Kc 
D.
Kc 
2
[H 2S(g)] eqm
2
[S 2 (g)] eqm [H 2 (g)] eqm
[H 2S(g)] eqm
[H 2 (g)] eqm [S2 (g)] eqm
2
[H 2 (g)] eqm
[S 2 (g)] eqm
2
[H 2S(g)] eqm
[H 2 (g)] eqm [S2 (g)] eqm
[H 2S(g)] eqm
□
8.
What is the unit of the equilibrium constant for the following equilibrium
system?
2NO2(g)
N2O4(g)
A.
B.
C.
D.
mol dm3
mol1 dm3
mol2 dm6
mol2 dm6
□
3
9.
What is the unit of the equilibrium constant for the following heterogeneous
equilibrium?
Fe3O4(s) + 4H2(g)
A. no unit
B.
C.
D.
3Fe(s) + 4H2O(g)
mol2 dm6
mol4 dm12
mol1 dm3
□
10. Which of the following is the balanced chemical equation that gives the
equilibrium constant expression shown below?
Kc =
4
[HCl(g )]eqm
2
[SiCl 4 (g )]eqm [H 2 (g )]eqm
A.
SiCl4(g) + 2H2(g)
4HCl(g)
B.
C.
D.
SiCl4(g) + 2H2(g)
Si(s) + 4HCl(g)
Si(s) + 4HCl(g)
SiCl4(g) + 2H2(g)
4HCl(g)
SiCl4(g) + 2H2(g)
□
11. Consider the following reaction:
H2(g) + I2(g)
2HI(g)
Suppose the equilibrium constant of the given reaction at 298 K is Kc. Which is
the correct expression of equilibrium constant, Kc’, for the following reaction at
298 K?
2HI(g)
H2(g) + I2(g)
A. Kc’ = Kc
1
B. Kc’ =
Kc
C.
Kc’ = Kc2
D.
Kc’ = K c
□
4
12. Which is the correct expression of the equilibrium constant, Kc, for the following
reaction?
2CrO42(aq) + 2H+(aq)
Cr2O72(aq) + H2O(l)
2
[Cr2O7 (aq)]eqm[H 2O(l)] eqm
A.
Kc =
B.
Kc =
[CrO 4 (aq)]2 eqm[H  (aq)]2 eqm
C.
Kc =
[Cr2O7 (aq)]eqm
D.
Kc =
[CrO 4 2 (aq)] 2 eqm[H  (aq)] 2 eqm
2
[Cr2O7 2 (aq)]eqm[H 2 O(l)] eqm
2
[CrO 4 2 (aq)]2 eqm[H  (aq)] 2 eqm
2
[CrO 4 (aq)]2 eqm[H  (aq)] 2 eqm
[Cr2O7 2 (aq)]eqm
□
13. The value of Kc for the reaction H2(g) + CO2(g)
H2O(g) + CO(g) is 1.45 at
1100C. What is the value of Kc for the reaction H2O(g) + CO(g)
CO2(g) at the same temperature?
A. 0.69
B. 0.83
C.
D.
H2(g) +
1.20
1.45
□
14. Which of the following is the expression for the equilibrium constant, Kc, for the
reaction:
CaCO3(s)
CaO(s) + CO2(g)?
[CaO(s)] eqm[CO 2 (g)] eqm
A. Kc =
[CaCO 3 (s)] eqm
[CaCO3 (s)] eqm
[CaO(s)] eqm[CO 2 (g)] eqm
B.
Kc =
C.
Kc = [CO2(g)]
1
Kc =
[CO 2 (g)] eqm
D.
□
5
15. Which of the following statements concerning the magnitude of equilibrium
constant, Kc, is correct?
A. It is affected by the change in pressure.
B. If Kc is a positive value, the forward rate is faster than the backward rate.
C. It indicates how far the equilibrium reaches.
D. The larger the Kc value, the faster the reaction.
□
16. The equilibrium constants of four different equilibrium systems are shown
below.
Equilibrium system
I
II
5
Kc
3.2 × 10
4.7 × 10
III
17
IV
17
3.2 × 10
4.7 × 105
Which of the equilibrium systems favours the reactants most?
A.
B.
C.
D.
I
II
III
IV
□
17. Consider the following reaction:
2SO2(g) + O2(g)
2SO3(g)
At one equilibrium position, [SO2(g)]eqm, [O2(g)]eqm and [SO3(g)]eqm are 5.33 ×
103 M, 3.70 × 103 M and 5.33 × 103 M respectively. Which of the following
set of concentrations ([SO2(g)]eqm, [O2(g)]eqm and [SO3(g)]eqm respectively)
constitutes another equilibrium position at the same temperature?
A.
B.
C.
D.
4.72 × 102 M, 1.013 × 105 M and 2.47 × 103 M
3.68 × 103 M, 5.284 × 105 M and 3.09 × 103 M
5.99 × 102 M, 1.221 × 105 M and 8.12 × 103 M
7.21 × 103 M, 4.592 × 105 M and 5.83 × 105 M
□
18. Consider the following equilibrium:
C(s) + CO2(g)
2CO(g)
The equilibrium mixture was found to contain 0.42 mol dm3 of CO2(g) and 0.82
mol dm3 of CO(g) at a certain temperature. What was the equilibrium constant
at that temperaure?
A. 1.95
B.
C.
D.
1.60 mol dm3
0.625 mol1 dm3
0.512
□
6
19. At a certain temperature, the equilibrium constant, Kc, for the reaction
C2H5CO2H(l) + C2H5OH(l)
C2H5CO2C2H5(l) + H2O(l)
is 4. When 1 mol dm3 of propanoic acid (C2H5CO2H) reacted with 1 mol dm3
of ethanol (C2H5OH) in the presence of concentrated sulphuric acid and the
reaction mixture was allowed to reach equilibrium, what was the equilibrium
concentration of propanoic acid in the mixture?
A.
B.
C.
D.
0.33 mol dm3
0.67 mol dm3
1 mol dm3
2 mol dm3
□
dm3
dm3
20. In Haber process, 0.60 mol
of nitrogen is mixed with 1.80 mol
of
hydrogen in a sealed container at a certain temperature to give ammonia.
N2(g) + 3H2(g)
2NH3(g)
At equilibrium, 30% of the original nitrogen is consumed. What is Kc for this
reaction?
A.
B.
C.
D.
0.154 mol2 dm6
0.265 mol2 dm6
0.680 mol2 dm6
1.470 mol2 dm6
□
21. It is known that hydroxyapatite, Ca5(PO4)3OH, is present in our tooth enamel. It
undergoes continuous reversible reactions with the minerals supplied from saliva.
The following equation shows the reaction:
Ca5(PO4)3OH(s)
5Ca2+(aq) + 3PO43(aq) + OH(aq)
At equilibrium, the concentration of OH(aq) is 2 × 104 mol dm3. What is the
value of Kc?
A.
B.
C.
2 × 104 mol dm3
1.20 × 1010 mol3 dm9
2.16 × 1025 mol9 dm27
D.
4.32 × 1029 mol9 dm27
□
7
22. Consider the following equilibrium:
SO2Cl2(g)
SO2(g) + Cl2(g)
In an experiment, 13.5 g of SO2Cl2(g) was introduced into a 4 dm3 container and
allowed to reach equilibrium with SO2(g) and Cl2(g). At equilibrium, 0.069 mol
of Cl2(g) was found in the reaction mixture. What is the equilibrium constant of
the reaction?
A.
B.
C.
D.
0.0384 mol dm3
0.0768 mol dm3
2.23 mol dm3
3.34 mol dm3
□
23. Chemical A ionizes in water and reaches an equilibrium: A(aq)
2B(aq). The
equilibrium constant for the ionization is 180 mol dm–3. Originally there is 0.97
M of A. What is the equilibrium concentration of B?
A.
B.
C.
D.
0.02 mol dm3
0.07 mol dm3
0.95 mol dm3
1.90 mol dm3
□
24. Consider the following chemical equilibrium:
Cr2O72(aq) + H2O(l)
2CrO42(aq) + 2H+(aq)
Which of the following statement(s) is/are correct?
(1) Both Cr2O72(aq) and CrO42(aq) are present in the reaction mixture.
(2) The solution becomes orange when dilute NaOH(aq) is added to the
mixture.
(3) The solution becomes yellow when dilute H2SO4(aq) is added to the
mixture.
A. (1) only
B. (2) only
C. (1) and (3) only
D.
(2) and (3) only
□
8
25. Consider the following equilibrium reaction:
N2(g) + 3H2(g)
2NH3(g)
When extra nitrogen gas is added to the closed container, which of the following
changes would happen on the value of Kc at 298 K?
A. The value of Kc increases.
B. The value of Kc decreases.
C. The value of Kc remains unchanged.
D. The change cannot be determined.
□
26. Consider the following equilibrium reaction:
2CrO42(aq) + 2H+(aq)
Cr2O72(aq) + H2O(l)
The colour of the solution turns yellow by adding
(1) dilute hydrochloric acid.
(2) sodium carbonate solution.
(3)
A.
B.
C.
D.
sodium sulphite solution.
(1) only
(2) only
(1) and (3) only
(2) and (3) only
□
27. Consider the following system which is in equilibrium at a certain temperature:
2CO(g) + O2(g)
2CO2(g)
If some CO(g) is added to the system, which of the following statements is
correct?
A. There is a change in concentrations for the reactants only.
B. There is a change in concentration for the product only.
C. There is a change in concentrations for both reactants and products.
D. There is no change in concentrations for both reactants and products.
□
28. Which of the following statements are correct if an equilibrium system is heated?
(1)
(2)
(3)
A.
B.
C.
D.
The rate of forward reaction is increased.
The rate of backward reaction is increased.
Kc is increased.
(1) and (2) only
(1) and (3) only
(2) and (3) only
(1), (2) and (3)
□
9
29. When there is a decrease in temperature, what are the changes in Kc value and
equilibrium position for the following equilibrium reaction?
CH4(g) + 4NO(g)
2N2(g) + CO2(g) + 2H2O(g)
Kc value
Change in equilibrium position
A.
increases
to the right
B.
increases
to the left
C.
decreases
to the right
D.
decreases
to the left
ΔH = ve
□
30. For the following reaction:
Cu(OH)2(s)
Cu2+(aq) + 2OH(aq)
ΔH = +ve
2+
The equilibrium concentration of Cu (aq) would be increased by
(1) increasing the temperature.
(2) adding a few drops of dilute hydrochloric acid.
(3) adding a few drops of dilute sodium hydroxide solution.
A. (1) and (2) only
B. (1) and (3) only
C. (2) and (3) only
D. (1), (2) and (3)
□
31. Consider the following equilibrium reaction:
4HCl(g) + O2(g)
2Cl2(g) + 2H2O(g)
ΔH = +ve
Which of the following changes will cause a shift in the equilibrium position of
the above reaction?
(1) Adding Cl2(g)
(2) Removing O2(g)
(3) Adding a catalyst
A. (1) and (2) only
B. (1) and (3) only
C. (2) and (3) only
D.
(1), (2) and (3)
□
10
32. The following graph shows the change in concentrations of the reactant and
products with time for the reversible reaction:
C(aq)
A(aq) + B(aq)
ΔH = +ve
Concentration (mol dm3)
A(aq)
B(aq)
C(aq)
Time
Which of the following changes is done to the system at time t?
A. Adding a small amount of C(aq)
B. Removing some of C(aq)
C. Increase in temperature
D. Decrease in temperature
□
33. Consider the following equilibrium system at constant pressure.
2SO2(g) + O2(g)
2SO3(g)
ΔH = ve
Which of the following statements is/are correct?
(1) An increase in temperature favours the backward reaction.
(2) An increase in concentration of SO3(g) shifts the equilibrium position to the
right.
(3) An increase in temperature decreases the value of Kc.
A. (1) only
B. (2) only
C. (1) and (3) only
D.
(2) and (3) only
□
11
34. When there is an increase in temperature, what are the changes in Kc value and
equilibrium position for the following equilibrium reaction?
I2(g) + C5H8(g)
C5H6(g) + 2HI(g)
ΔH = +ve
Kc value
Change in equilibrium position
A.
Increases
Shifts to the left
B.
Increases
Shifts to the right
C.
Decreases
Shifts to the left
D.
Decreases
Shifts to the right
□
Each question below consists of two separate statements. Decide whether each of
the two statements is true or false; if both are true, then decide whether or not
the second statement is a correct explanation of the first statement. Then select
one option from A to D according to the following table:
A. Both statements are true and the 2nd statement is a correct explanation
of the 1st statement.
B. Both statements are true and the 2nd statement is NOT a correct
explanation of the 1st statement.
C. The 1st statement is false but the 2nd statement is true.
D. Both statements are false.
35.
When a reaction reaches equilibrium,
there are no observable changes of the
reaction mixture.
The rates of forward and backward
reactions are the same at equilibrium.
□
36.
Equilibrium constant does not depend
on the concentrations of reactants and
products.
Equilibrium constant is dependent on
temperature.
□
37.
The larger the equilibrium constant, the
higher is the reaction rate.
If the equilibrium constant is larger than
1, the product side is favoured.
□
12
Conventional questions :
1.
It is known that weak acids ionize slightly in water.
For example,
CH3COOH(aq)
CH3COO(aq) + H+(aq)
(a) State the chemical species that can be found in the reaction mixture at
equilibrium.
(b) (i)
State TWO reactions that are taking place at equilibrium.
(ii) What can you say about the rates of these two reactions at equilibrium?
(c) Explain why 1 M ethanoic acid has a higher pH value than 1 M
hydrochloric acid.
(d) In an experiment, a student accidentally added a small amount of HCl to the
reaction mixture in (a). He found that the pH value of the reaction mixture
did not change much. Explain briefly.
[8M]
13
2.
When 0.4 mol of H2(g) and 0.3 mol of I2(g) are mixed in a 1 L closed vessels,
they react to form HI(g). After 3 minutes, the reaction mixture reaches
equilibrium and the vessel contains 0.5 mol of HI(g).
(a) Write an equation to represent this state of the equilibirum.
(b) (i)
(ii)
Calculate the concentrations of H2(g) and I2(g) at equilibrium
respectively.
Hence, sketch the changes in concentrations of H2(g), I2(g) and HI(g)
against time until the reaction mixture reaches equilibrium.
[11M]
14
3.
In an experiment, 1.00 × 103 mol of PCl5(g) was introduced into a 250 cm3
container and allowed to reach equilibrium with PCl3(g) and Cl2(g) at a certain
temperature. 9.65 × 104 mol of Cl2(g) was found in the equilibrium mixture.
(a) Write a balanced chemical equation for the reaction.
(b) Write the expression for the equilibrium constant for the reaction.
(c) Calculate the equilibrium constant for the reaction.
[8M]
15
4.
At 250C, the equilibrium constant, Kc, for the following equilibrium
PCl5(g)
PCl3(g) + Cl2(g)
is 3.8 × 102 mol dm3. In an experiment, 0.55 mol of PCl3(g) and 0.55 mol of
Cl2(g) were mixed in a 2.5 dm3 container and allowed to reach equilibrium with
PCl5(g).
(a) Calculate the initial concentrations of PCl3(g) and Cl2(g) respectively.
(b) Calculate the equilibrium concentrations of PCl5(g), PCl3(g) and Cl2(g) in
the reaction mixture.
[7M]
16
5.
Consider the following equilibrium:
CO2(g) + H2(g)
CO(g) + H2O(g)
(a) State whether the equilibrium is homogeneous or heterogeneous. Explain briefly.
(b) In an experiment, 0.10 mol of CO2(g) and 0.12 mol of H2(g) were introduced
into a 1 dm3 container and allowed to reach equilibrium with CO(g) and H2O(g).
The equilibrium constant of the reaction is 0.63 at a certain temperature.
Determine the equilibrium concentrations of all reactants and products at this
temperature.
[10M]
17
6.
Given:
Reaction
N2(g) + O2(g) + Br2(g)
2NOBr(g)
2NO(g)
N2(g) + O2(g)
2NO(g) + Br2(g)
2NOBr(g)
(a) (i)
Equilibrium
constant
Kc
Kc1
Kc2
Write the expression for the equilibrium constant, Kc1.
(ii) Write the expression for the equilibrium constant, Kc2.
(iii) Hence, write the expression for the equilibrium constant, Kc, in terms
of Kc1 and Kc2.
(b) Calculate Kc if Kc1 and Kc2 are 2.4 × 1018 and 1.4 mol1 dm3 respectively.
[5M]
7.
Consider the following equilibrium:
CH4(g) + H2O(g)
CO(g) + 3H2(g)
ΔH = +x kJ
According to the above information, a student made the following statements.
Comment on each of his statements.
(a) At equilibrium, the amount of carbon monoxide must be equal to the
amount of methane.
(b) The enthalpy change for the reverse reaction is x kJ.
(c) At equilibrium, the rate of forward reaction is higher than the rate of
backward reaction.
[5M]
18
8.
In an experiment, bromine was dissolved in water to form bromine water.
Br2(aq) + H2O(l)
H+(aq) + Br(aq) + HOBr(aq)
The pale yellow colour of bromine water is due to bromine, as the products are
essentially colourless. Deduce the colour change when a few drops of the
following solutions are added to bromine water.
(a) Concentrated hydrochloric acid
(b) Concentrated potassium hydroxide
(c) Silver nitrate solution
[6M]
19
9.
Consider the following equilibrium system:
2SO2(g) + O2(g)
2SO3(g)
ΔH = ve
If the pressure of the system is kept constant, predict and explain the change in
equilibrium position for the reaction when
(a) the temperature of the system is increased.
(b) the concentration of SO2(g) is decreased.
(c) the concentration of SO3(g) is decreased.
[6M]
10. Consider the following equilibrium system:
H2O(g) + CO(g)
H2(g) + CO2(g)
ΔH = ve
According to Le Châtelier’s Principle, fill in the following table for the
disturbance to the equilibrium system:
Addition of
H2O(g)
Addition of
H2(g)
Change in
temperature
Effect on [CO2(g)]
Effect on [CO(g)]
Effect on equilibrium
position
Effect on equilibrium
constant
[12M]
20
11. For the following equilibrium reactions, state whether, and in which direction,
the equilibrium is shifted by the change applied. In each case, explain your
answer.
(a) 3O2(g)
2O3(g)
volume is increased
(b) 3Fe(s) + 4H2O(l)
(c) NH4Cl(s)
(d) 2NO(g)
Fe3O4(s) + 4H2(g)
NH3(g) + HCl(g)
pressure is decreased
N2(g) + O2(g)
(e) BiCl3(aq) + H2O(l)
H2(g) is removed
volume is decreased
BiOCl(s) + 2HCl(aq) NaOH(aq) is added
[10M]
21
12. The following equilibrium was originally established in a 1 dm3 container at a
certain temperature:
CO2(g) + H2(g)
CO(g) + H2O(g)
The equilibrium concentrations of of CO2(g), H2(g), CO(g) and H2O(g) are
0.0517 mol dm3, 0.0717 mol dm3, 0.0483 mol dm3 and 0.0483 mol dm3
respectively.
(a) If 0.02 mol dm3 of CO2(g) is added to the equilibrium system, determine
the new equilibrium concentrations of all species in the mixture.
(b) State the effect on the equilibrium position if the reaction mixture is
introduced into a 2 dm3 container. Explain briefly.
[9M]
22
Answers :
Multiple-choice questions :
1
2
3
4
5
6
7
8
9
10
B
C
C
A
D
D
C
B
A
B
11
12
13
14
15
16
17
18
19
20
B
C
A
C
C
B
A
B
A
A
21
22
23
24
25
26
27
28
29
30
D
A
D
A
C
B
C
A
A
A
31
32
33
34
35
36
37
38
39
40
A
C
C
B
A
B
C
Conventional questions :
1.
(a) CH3COOH(aq) ions, CH3COO(aq) ions and H+(aq) ions [1]
(b) (i) The two reactions are:
CH3COOH(aq)  CH3COO(aq) + H+(aq) [1]
CH3COO(aq) + H+(aq)  CH3COOH(aq) [1]
(ii) The rates of these two reactions are the same at equilibrium. [1]
(c) Ethanoic acid is a weak acid that ionizes slightly in water while hydrochloric
acid is a strong acid that ionizes completely in water. [1] Therefore, the
concentration of H+(aq) in hydrochloric acid is higher and the pH value is lower.
[1]
(d) The newly added small amount of H+ reacted with CH3COO(aq) in the reaction
mixture, forming CH3COOH(aq). [1] Therefore, the H+ concentration which
determines the pH value did not change much. [1]
23
2.
(a) H2(g) + I2(g)
(b) (i)
2HI(g) [1]
0.4 mol
= 0.4 mol L1
1L
The initial concentration of H2(g) =
The initial concentration of I2(g) =
0.3 mol
= 0.3 mol L1
1L
The concentration of HI(g) at equilibirum =
0.5 mol
= 0.5 mol L1 [1]
1L
According to the equation, the mole ratio of H2 : I2 : HI = 1 : 1 : 2
0.5 mol
No. of moles of H2(g) and I2(g) reacted =
= 0.25 mol [1]
2
∴ the concentration of H2(g) at equilibrium = 0.4 mol L1  0.25 mol L1
= 0.15 mol L1 [1]
the concentration of I2(g) at equilibrium = 0.3 mol L1  0.25 mol L1
= 0.05 mol L1 [1]
(ii)
Concentration
HI
H2
I2
Time
Correct drawing [3]
Correct labelling [3]
3.
(a) PCl5(g)
(b) Kc =
PCl3(g) + Cl2(g) [1]
[PCl3 (g )]eqm [Cl2 (g )]eqm
[PCl5 (g )]eqm
[1]
(c) Initial concentration of PCl5(g)
=
1.00  103 mol
= 4 × 103 mol dm3 [1]
0.25 dm3
Equilibrium concentration of Cl2(g)
=
9.65  104 mol
= 3.86 × 103 mol dm3 [1]
3
0.25 dm
Consider the equilibrium,
24
Concentr
ation
(mol
dm3
)
Initial
Change
Equilibriu
m
Kc =
PCl5(g)
PCl3(g)
4 × 103
3.86 × 103
4 × 103
3.86 × 103
= 1.4 × 104
[1]
+
Cl2(g)
0
+3.86 × 103
0 + 3.86 ×
103
= 3.86 × 103
[1]
0
+3.86 × 103
0 + 3.86 ×
103
= 3.86 × 103
[1]
[3.86  103 mol dm 3 ][3.86  103 mol dm 3 ]
[1.4  10 4 mol dm 3 ]
= 0.106 mol dm3 [1]
4.
(a) Initial concentration of PCl3(g) =
Initial concentration of Cl2(g) =
0.55 mol
= 0.22 mol dm3 [1]
2.5 dm 3
0.55 mol
= 0.22 mol dm3 [1]
2.5 dm 3
(b) Let x mol dm3 be the change in concentration of PCl3(g).
Concentration (mol
dm3)
Initial
Change
Equilibrium
PCl3(g) + Cl2(g)
0.22
x
0.22
x
0.22 
x
0.22 
x
The equilibrium constant for PCl3(g) + Cl2(g)
1
=
= Kc = 26.3 mol1 dm3 [1]
Kc
Kc =
26.3 =
[PCl5 (g)]eqm
[PCl3 (g)]eqm [Cl2 (g)]eqm
PCl5(g)
0
+x
0+x
=
x
PCl5(g)
[1]
x
(0.22  x)(0.22  x)
Solving for x,
26.3(0.22  x )(0.22  x ) = x
x = 0.146 or x = 0.332 (rejected)
∴ the equilibrium concentration of PCl3(g) = 0.22  0.146 mol dm3
= 0.074 mol dm3 [1]
the equilibrium concentration of Cl2(g) = 0.22  0.146 mol dm3
25
= 0.074 mol dm3 [1]
the equilibrium concentration of PCl5(g) = 0.146 mol dm3 [1]
5.
(a) It is a homogeneous equilibrium [1] because all the reactants and products are in
the same phase. [1]
0.10 mol
(b) Initial concentration of CO2(g) =
= 0.10 mol dm3 [1]
1 dm3
0.12 mol
= 0.12 mol dm3 [1]
1 dm3
Initial concentration of H2(g) =
Let x mol dm3 be the change in concentration of CO2(g).
Concentration
(mol
dm3)
Initial
Change
Equilibrium
Kc =
CO2(g)
0.10
x
0.10 
x
[CO(g)]eqm [H 2O(g)]eqm
[CO2 (g)]eqm [H 2 (g )]eqm
+
H2(g)
H2O(g)
0.12
x
0.12 
x
0
+x
0+x=
x
[1]
x2
[1]
(0.10  x)(0.12  x)
Solving for x,
0.63 =
0.63(0.012  0.22x + x2) = x2
x = 0.0483 or x = 0.423 (rejected)
∴ equilibrium concentrations:
[CO2(g)]eqm = 0.10  0.0483 mol dm3 = 0.0517 mol dm3 [1]
[H2(g)]eqm = 0.12  0.0483 mol dm3 = 0.0717 mol dm3 [1]
[CO(g)]eqm = 0.0483 mol dm3 [1]
[H2O(g)]eqm = 0.0483 mol dm3 [1]
6.
(a) (i)
Kc1 =
(ii) Kc2 =
(iii) Kc =
[ N 2 (g)]eqm [O2 (g)]eqm
2
[ NO(g)]eqm
2
[ NOBr(g )]eqm
2
[ NO(g )]eqm
[Br2 (g )]eqm
[1]
[1]
2
[ NOBr(g)]eqm
[ N 2 ]eqm [O2 (g)]eqm [Br2 (g)]eqm
26
[1]
CO(g)
+
0
+x
0+x=
x
=
(b) Kc =
K c2
[1]
K c1
1.4 mol 1 dm3
= 5.83 × 1017 mol1 dm3 [1]
18
2.4  10
7.
(a) It is incorrect [1] because the equilibrium concentration of each species is
dependent on the equilibrium constant of the reaction. [1]
(b) It is correct. [1]
(c) It is incorrect [1] because the rates of forward and backward reactions are the
same at equilibrium. [1]
8.
(a) When concentrated hydrochloric acid is added, this increases the concentration
of H+(aq), thus the equilibrium position of the system shifts to the left. [1] The
colour of bromine water becomes darker. [1]
(b) When concentrated potassium hydroxide is added, the OH(aq) removes H+(aq)
from bromine water, thus the equilibrium position of the system shifts to the
right. [1] The colour of bromine water becomes paler. [1]
(c) When silver nitrate solution is added, Ag+(aq) removes Br(aq) from bromine
water, the equilibrium position of the system shifts to the right. [1] The colour of
bromine water becomes paler. [1]
9.
(a) The equilibrium position will shift to the left. [1] Since the forward reaction is
exothermic, [1] increasing the temperature shifts the equilibrium position to the
left.
(b) The equilibrium position will shift to the left. [1] According to Le Châtelier’s
Principle, the equilibrium position will shift to the left producing more SO2(g).
[1]
(c) The equilibrium position will shift to the right. [1] According to Le Châtelier’s
Principle, the equilibrium position will shift to the right producing more SO3(g).
[1]
27
10.
Addition of
H2O(g)
Addition of H2(g)
Increases [1]
Decreases [1]
Effect on [CO2(g)]
Decreases [1]
Increases [1]
Effect on [CO(g)]
Shifts to the
right [1]
Shifts to the
left [1]
Effect on equilibrium
position
No change
[1]
No change
[1]
Change in
temperature
Decreases
[1]
Increases
[1]
Shifts to
the
left
[1]
Decreases
[1]
11.
(a) The equilibrium position shifts to the left. [1]
Since an increase in volume can decrease the pressure, the equilibrium position
shifts to the left, raising the number of gas molecules and bringing the pressure
back up. [1]
(b) The equilibrium position shifts to the right. [1]
Since the concentration of hydrogen decreases, the equilibrium position shifts to
the right to produce more hydrogen. [1]
(c) The equilibrium position shifts to the right. [1]
Since the pressure is decreased, the equilibrium position shifts to the right to
produce more gases. [1]
(d) There is no change in equilibrium position. [1]
It is because the numbers of moles of gas molecules are equal on both sides of
chemical equation. [1]
(e) The equilibrium position shifts to the right. [1]
Since sodium hydroxide removes HCl once it is formed, the concentration of
HCl decreases. Thus the equilibrium position shifts to the right to produce more
HCl. [1]
28
12.
(a) The equilibrium constant, Kc =
=
[CO(g)]eqm [H 2O(g)]eqm
[CO2 (g)]eqm [H 2 (g)]eqm
[1]
(0.0483 mol dm3 )(0.0483 mol dm3 )
(0.0517 mol dm 3 )(0.0717 mol dm 3 )
= 0.629 [1]
dm3
Let x mol
be the change in concentration of CO2(g).
Concentr
ation
(mol
CO2(g) +
H2(g)
CO(g) +
3
dm
)
0.0517 +
0.02
0.0717
0.0483
Initial
= 0.0717
+x
Change
x
x
0.0717
0.0483
Equilibri
0.0717 
+

um
x
x
x
0.629 =
H2O(g)
0.0483
+x
0.0483
+
x
(0.0483  x mol dm3 )(0.0483  x mol dm3 )
(0.0717  x mol dm3 )(0.0717  x mol dm3 )
Solving for x,
x = 4.78 × 103 or x = 0.508 (rejected)
 new equilibrium concentrations
[CO2(g)]eqm = 0.0717  4.78 × 103 mol dm3 = 0.0669 mol dm3 [1]
[H2(g)]eqm = 0.0717  4.78 × 103 mol dm3 = 0.0669 mol dm3 [1]
[CO(g)]eqm = 0.0483 + 4.78 × 103 mol dm3 = 0.0531 mol dm3 [1]
[H2O(g)]eqm = 0.0483 + 4.78 × 103 mol dm3 = 0.0531 mol dm3 [1]
(b) The equilibrium position will remain unchanged. [1] Since the numbers of moles
of gas molecules are equal on both sides of reaction, [1] the change in volume
does not affect the equilibrium position. [1]
29