Chapter 7
Chemical Equilibrium
Question (1) Select the suitable word or phrase to complete the following sentences
1. The equilibrium system involves two --------------- processes
(a) identical
(b) simultaneous
(c) reversible
(d) both answers (b) and (c) are right.
2. Addition of sodium chloride solution to silver nitrate solution is----------------a reaction.
(a) complete (irreversible) (b) instantaneous
(c) reversible
(d) answers (a) & (b) are right.
3. The reaction of hydrochloric acid with magnesium is a complete (irreversible) reaction as
(a) it occurs at high temperature
(c) hydrogen gas is evolved
products.
(b) it occurs under high pressure
(d) there is an equilibrium between the reactants and the
4. The solution of acetic acid and ethanol in the estrification reaction turns the blue litmus paper to
red as ---------(a) ethanol has no effect on litmus
(b) a dynamic equilibrium is established, where the rates of both the forward and backward
reactions are equal.
(c) the reaction is reversible and acetic acid is still occurring in the reaction solution.
(d) both answers (b) and (c) are right.
5. During the complete (irreversible) reaction, the graph between the concentration and time
indicates-(a) an equilibrium is established between reactants and products.
(b) the concentration of the reactants decreases until it is completely consumed.
(c) the concentration of the products increases until the end of the reaction.
(d) both answers (b) and (c) are right
6. During the reversible reaction, the graph relating concentration and time (reaction rate)
indicates----(a) the concentration of the reactants decreases until it is completely consumed.
(b) the concentration of the products increases and the concentration of the reactants
decreases until they reach equilibrium.
(c) the concentrations of both the reactants and the products increase until they reach
equilibrium.
(d) there is no change in the concentration of both the reactants and the products since the
start of the reaction.
7. Some reactions occur instantaneously such as--------------
(a) the reaction of acetic acid and ethanol to produce ethyl acetate ester and water.
(b) insertion of a strip of magnesium metal in hydrochloric acid solution.
(c) the reaction of silver nitrate solution and sodium chloride solution.
(d) both answers (b) and (c) are right.
8. Some reactions are slow such as ---------------(a) the reaction of silver nitrate solution with sodium chloride solution.
(b) the reaction of oil and caustic soda (NaOH) to produce soap and glycerol.
(c) insertion of a strip of magnesium metal in hydrochloric acid solution.
(d) the reaction of sodium hydroxide solution with hydrochloric acid solution.
9. If the value of the equilibrium constant is less than one i.e .this means that -------------(a) the reaction is reversible
(b) the concentration of the products are less concentration of the reactants.
(c) the reaction is complete and instantaneous
(d) both answers (a) and (b) are right than the
10. If the value of the equilibrium constant is high, this means that------------------(a) the reaction proceeds almost to its end.
(b) the concentration of the reactants is larger than the concentration of the products.
(c) the concentration of the products is larger ‘than the concentration of the reactants.
(d) both answers (a) and (c) are right
11. The increase of temperature increases the rate of reaction as it-------------------(a) increases the proportion of activated molecules.
(b) the kinetic energy of the activated molecules is high enough to break the bonds within the
molecules.
(c) supply the energy needed for endothermic reactions.
(d) all the previous answers are right.
12. The increase of pressure increases the rate of the reactions which are characterized by-------(a) both the reactants and products of the reaction are in the gas- phase.
(b) the reaction is accompanied by a reduction in the volume of the produced gases.
(c) the reaction is reversible.
(d)
all the previous answers are right
13. The catalyst is a substance that-----------------------(a) increases the rate of the slow chemical reactions.
(b) save much of the heat energy needed to enhance the reaction.
(c) does not affect the position of equilibrium.
(d) all the previous answers are right.
14. The strong electrolytic solutions are characterized by -----------------------(a) contain completely ionized substances.
(b) the dissolved substances are ionized instantaneously and conducts the electric current.
(c) the dissolved substances ionize slowly and are bad conductors of the electric current.
(d) both answers (a) and (b) are right.
15. The solution of hydrogen chloride (HCI) gas in benzene----------------------(a) contains ions and illuminates the lamp which is connected to two poles dipped in the
solution.
(b) does not contain ions and does not illuminate the lamp.
(c) the chemical bond between the hydrogen atom and chlorine atom in gaseous HC1 is ionic.
(d) both answers (a) and (c) are right.
16. The solution of pure acetic acid in water--------------------------(a) contains ions and illuminate the lamp which is connected to two poles dipped in the
solution.
(b) does not contain ions and does not illuminate the lamp.
(c) contain ions that increase in number on dilution with water.
(d) both answers (a) and (c) are right.
17. The ionization constants, Ka, of sulphurous acid, H2SO3 hydrofluoric acid, HF, acetic acid,
CH3COOH and carbonic acid, H2CO3 are 1.7 x 10.2, 6.7 x iO 1.8 x i0 and 4.4 x i0 respectively. The
strengths of these weak acids can thus be arranged as follows-------------(a) sulphurous > hydrofluoric > acetic > carbonic
(b) carbonic> acetic > hydrofluoric> sulphurous
(c) carbonic > hydrofluoric> sulphurous > acetic
(d) hydrofluoric> acetic> sulphurous > carbonic
18. The following solution is acidic (i. e. its pH < 7)--------------------pure water
(b) sea water
(c) vinegar
(d) ammonia solution
19. The following solution is alkaline (i.e. its pH> 7)--------------------(a) vinegar solution
(b) pure water
(c) sodium hydroxide solution
(d) both answers (a) and (c) are right
20. The following solution is neutral
(a) sea water
(b) pure water
(c) orange juice
(d) hydrochloric acid aqueous solution
21. Solution of sodium carbonate in water-----------------(a) turns litmus red
(b) turns litmus blue
(c) has an alkaline effect on litmus
(d) both answers (b) and (c) are right
22. Solution of sodium chloride in water ----------------------(a) turns litmus red
(b) turns litmus blue
(c) has a neutral effect on litmus
(d) both answers (b) and (c) are right
23. Hydrolysis of salt solutions ---------------------(a) is the reverse reaction of neutralization.
(b) occurs when salts are derived from a weak acid and a strong base or the reverse, i. e.
derived from a strong acid and a weak base.
(c) occurs for salts derived from a strong acid and a strong base or the reverse, i. e. derived
from a weak acid and a weak base.
(d) both answers (a) and (b) are right.
Question (2)
Choose the correct answer
1- Which is the correct equilibrium constant expression for the reaction below?
N2( g )  3H 2( g )
2 NH3( g )
[ NH 3 ]2
a- K eq 
[ N 2 ][ H 2 ]3
C - K eq 
[ N 2 ][ H 2 ]3
b- K eq 
[ NH 3 ]2
[2 NH 3 ]
[ N 2 ][3H 2 ]
d- K eq 
[ N 2 ][3H 2 ]
[2 NH 3 ]
2- Given the following reaction at equilibrium
CH 4( g )  2O2( g )
CO2( g )  2H 2O( g )
An increase in the concentration of O2gas at constant temperature and pressure will result in ---a- an increase in the concentration of CH4 (g)
b- an increase in the concentration of CO2 (g)
c-
a decrease in the concentration of CO2 (g)
d- a decrease in the concentration of H2O (g)
3- Which pH value represents a solution with the lowest OH-1 ion concentration?
a-1
b- 7
c-10
d-14
4- Given: KW = [H+] [OH-] = 1x 1014 at 298 k.
What is the concentration of H+ ion in pure water at 298 K?
a- 1 x 10-7 mole/ liter
b- 1 x 107 mole / liter
c- 1 x 10-14 mole/liter
d- 1 x 1014 mole / liter
5- Which compound is an electrolyte?
a- C6H12O6 glucose
b- C 6H6 benzene
c- CH3OH methanol
d- CH3COOH acetic acid
6- The ionization constants (Ka) of four weak acids are shown below, which Ka represents the weakest of
these acids?
a- Ka = 1 x 10-5
b- Ka = 1.0x 10-4
c- K a= 7.1x10-3
d- Ka = 1.7 x 10--2
7- Which change would most likely increases the rate of chemical reaction?
a- decreasing a reactant’s concentration.
b- decreasing a reactant’s surface area.
c- cooling the reaction mixture.
d- adding a catalyst to the reaction mixture.
8- Given the system at equilibrium
AgCl (s) —> Ag+ (aq) + Cl- (aq)
When 0.1 M HCI is added to the system, the equilibrium will shift to the
a- right and the concentration of Ag (aq) will decrease.
b- right and the concentration of Ag (aq) will increase.
c- left and the concentration of Ag (aq) will decrease
d- left and the concentration of Ag (aq) will increase
9- Given the reaction at equilibrium
N2 (g) +3 H2 (g) 
 2NH3 (g) + heat
Which change would shift the equilibrium to right?
a- Increase of the temperature
b- Increase of the pressure
c- Decrease of the nitrogen gas concentration
d- Decrease of the hydrogen gas concentrations.
Question (3)
1- equilibrium constant expression is only valid if the reaction is balanced. Balance the following
equation, and then write the equilibrium constant expression for the balanced equation.
HCl  O2
Cl2  H 2O
2- A value of Keq should include proper units such as mole/liter or liter/mole so that further
calculations using Keq will have the correct units. Calculate the value of the equilibrium constant
for the following reversible reaction
N2( g )  3H 2( g )
2 NH3( g )
The concentrations of the equilibrants (reactants and products) at equilibrium at 400°C are as follows
N2 (g) = 1.2 mole / liter, H2 (g) = 0.80 mole / liter, NH3 (g) = 0.28 mole /liter
3- Write the equilibrium constant expression for the following reversible reaction
CuO(s)+H2
Cu(s)+H2O
(note that H2O is a vapor that behaves like a gas and not a liquid in this reaction)
4- The following forward reaction is endothermic. Which equation is correctly written?
a- heat + CaCO3(S)
CaO (s) + CO2 (g)
CaO( s )  CO2( g ) + heat
b- CaCO3( s )
5- In the previous question, if the temperature is raised, indicate whether the concentration of the
following reactant and products increases or decreases.
a- [CaCO3]
b- [CaO]
c- [CO2]
6- In the following reaction
PCl5 (g)
PCl3 (g) + Cl2 (g)
a- How many moles of gas are on the reactant side of this reaction?
b- How many moles of gas are on the product side of the reaction?
c- Which side of the reaction would be favoured by an increase in pressure?
d- Which side of the reaction would be favoured by a decrease in pressure?
7- What is the effect of an increase of pressure on the equilibrium of the following reaction?
heat + CaCO3 (s)
CaO (s) + CO (g)
8- For the following reversible reaction, increasing the pressure will favour which side of the reaction?
H 2( g )  I 2( g )
2HI ( g )
9- The reversible reaction below is presently at equilibrium
2CO( g )  O2( g )
2CO( g )  heat
In order to increase the concentration of the produced CO2 (g), indicate whether you would increase or
decrease the following conditions:
a- pressure
b- temperature
c- concentration of O2 (g)
10- Write the equilibrium constant expression for the solubility product of the very slightly soluble salt
AgBr
AgBr(s) ---------------> Ag+ (aq) + Br- (aq)
11- The solubility product equation represents the dynamic equilibrium between a solid and its
dissociated ions in a saturated solution.
Bi2 s3( s )
2Bi 3( aq )  3S 2( aq )
Write the solubility product expression for the saturated aqueous solution of Bi 2S3
12- Solid barium sulphate (BaSO4) is shaken in contact with pure- water at 25°C for several days. Each
day a sample is withdrawn and analyzed for its barium concentration. After several days, the value of
[Ba+2] was constant, indicating that the equilibrium
BaSO4 (s)
Ba+2 (aq) + SO-2 (aq) has been reached. The concentration of Ba+2 is 1.04 x 10-5
mole/liter. Calculate KSP for BaSO4
Question (4)
Solve the following problems
I- Find the pH value of the following solutions, their [H+] are
a- 10-5
b- 10-12
c- 10-7
and indicate whether each of these solutions is acidic, alkaline or neutral.
2- The following equation indicates the ionization of acetic acid, a weak acid, its concentration C = 0.05M
in its aqueous solution CH 3COOH ( aq )  H 2O( l )
(1 )C
H 3O ( aq)  CH 3COO ( aq) .Where α is the degree of
C
C
ionization of the acid – if the ionization constant of the acid is 1.8×10-5, find
a- the degree of the ionization of the acid.
b- the concentration of the H3O+ ion in the acid solution
c- the pH of the acid solution
3- The following equation indicates the ionization of a weak base, ammonium hydroxide (ammonia
solution), its concentration C = 0.1M. NH 4OH ( aq )
(1 )C
NH 4  ( aq )  OH  ( aq )
C
C
α is the degree of ionization of the base- if the ionization constant of the base, Kb = 1.6 ×10-5, calculate:
a- the degree of ionization of the base.
b- The hydroxyl ion concentration in the alkaline solution
c- The POH value of the solution
d- The pH value of the solution
4- The degree of solubility of aluminium hydroxide is 10-6 mole/liter. Calculate the solubility product of
Al(OH)3
5- Calculate the equilibrium constant KP for the reaction N2( g )  3H 2( g )
2 NH3( g ) ∆H= -92Kj
The pressure of the gases are: 2.3 atmosphere for N2, 7.1 atmosphere for H2, and 0.6 atmosphere for
NH3. Comment on the value of Kp and how could the product of the reaction be increased? And why?
6- The KSP for CaF2 is 3.9 x 10-11. What is the solubility of CaF2 in water in grams per liter?
Question (5)
Define the following
1- The complete reaction.
2- The reversible reaction.
3- Chemical equilibrium in
reversible reactions.
4- The rate of chemical reaction.
5- The law of mass action.
6- Complete ionization.
7- Incomplete (weak) ionization.
8- Ionic equilibrium
9- Ostwald’s law for dilution.
10- Ionic product of water.
11- The pH value.
12- The pOH value.
13- Hydrolysis of salt solutions.
14- Solubility product.
Question (6)
Answer the following:
1- What are the factors affecting the rate of chemical reactions?
2- What is meant by the effect of the nature of the reactants on the rate of the reaction?
3- State the law of mass action referring to the equation
FeCl3 + 3 NH4SCN
Fe (SCN)3 + 3 NH4Cl
and find the equilibrium constant of the reaction. What is the effect of adding an excess of ammonium
thiocyanate to the above reaction.
4- Define the energy of activation- Mention an experiment to show the effect of increasing temperature
on the rate of a reaction at an equilibrium state.
5- State Le Chatelier’ principle and its application in the following reaction with respect to the change in
concentration, pressure and temperature.
2SO2 (g) + O2 (g)
2SO3 (g)
∆H = -ve value
What is the effect of adding a catalyst such as V2O5 to the previous reaction?
6- Write two chemical equations for the hydrolysis of salts indicating the acidic effect of one of the salts
and the basic effect of another salt on litmus.