equlibrium sq

HKDSE Chemistry A Modern View Part X Chemical Equilibrium

©Aristo Educational Press Ltd. 2010 297

Structured Questions

Chemistry: Chapter 41 Dynamic equilibrium

Section 41.1

1

Ammonium chloride, a white solid, undergoes thermal decomposition to give

ammonia gas and hydrogen chloride gas. However, on cooling, ammonia gas will

recombine with hydrogen chloride gas to form ammonium chloride again.

(a) Write a chemical equation to represent the reversible reaction involved.

(b) Suggest ONE method to distinguish between two gaseous products.

(c) The reaction is an example of ‘sublimation’. Explain the term.

[4M]

2

When a mixture of propan-1-ol and ethanoic acid is heated under reflux in the

presence of an acid catalyst, an ester is formed.

(a) Name the ester formed.

(b) Name the acid catalyst.

(c) Two reactions are taking place at equilibrium.

(i) Write the equations of these two reactions.

(ii) What can you say about the rates of these two reactions?

[5M]

3

For each of the following reactions,

(i) state whether it is reversible or irreversible, and

(ii) write a balanced chemical equation.

(a) Dissolving ammonia in water

(b) Adding iron(II) sulphate solution to potassium hydroxide solution

(c) Mixing acidified silver nitrate solution and magnesium chloride solution

[6M]

Section 41.2

4

When a mixture of ethanoic acid and ethanol is heated under reflux in the presence of

an acid catalyst, an ester is formed.

(a) Is the reaction reversible or irreversible? Explain briefly.

(b) Write a chemical equation for the reaction involved.

(c) Name the ester formed.



(d) Suggest the acid catalyst for the reaction.

(e) State TWO characteristics of dynamic equilibrium.

[8M]

5

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]

6

When dinitrogen tetroxide is introduced into a sealed vessel at a certain temperature,

it decomposes to nitrogen dioxide as shown below:

N2O4(g) 2NO2(g)

(a) What can you say about the rates of the forward and backward reactions just

after N2O4(g) is introduced into a sealed vessel?

(b) What can you say about the rates of the forward and backward reactions at

equilibrium?

(c) What will happen to the reaction mixture if a hole is found in the vessel?

[6M]

7

Consider the following chemical equilibrium:

2NO(g) + O2(g) 2NO2(g)

(a) Write the equations of the forward and backward reactions.

(b) Sketch a graph to show the changes in rates of forward and backward reactions

against time until the reaction mixture reaches equilibrium.

(c) A student stated that the pressure of the reaction mixture at equilibrium is not

constant. Comment on his statement.

[8M]

8



When equal numbers of moles of N2(g) and O2(g) are introduced into a sealed vessel

at a certain temperature, the following reaction occurs:

N2(g) + O2(g) 2NO(g)

(a) (i) What can you say about the rates of the above forward and backward

reactions at equilibrium?

(ii) If the rate of forward reaction at equilibrium can be represented by

combining 1 mol of N2(g) with 1 mol of O2(g) per second, then represent

the rate of backward reaction at equilibrium in a similar way.

(b) Sketch a graph to show the changes in concentrations of N2(g), O2(g) and NO(g)


[6M]

9

A student used the following experimental set-up to study the equilibrium of thermal

decomposition of calcium carbonate.

(a) Write an equation to represent the equilibrium. It is known that calcium

carbonate would decompose to calcium oxide and carbon dioxide upon heating.

(b) Do you think that the above decomposition would reach equilibrium? Explain

briefly.

(c) Calcium carbonate is used as an antacid in the medical field. A student stated that

this is not a good idea. Comment on his statement.

[6M]

10

When ammonium iodide, a solid, is heated in a sealed vessel at a certain temperature,

it decomposes to give ammonia gas, hydrogen gas and iodine gas. A state of dynamic

equilibrium is established after some time.

(a) Two different reactions are taking place at equilibrium.

(i) Write the equations for these two reactions.

solid sodium carbonate

beaker

Bunsen burner



(ii) What can we say about the rates of these reactions?

(b) Write an equation to represent this state of equilibrium of the reaction.

(c) Sketch the changes in concentrations of reactant and products with time until the

reaction mixture reaches equilibrium.

[8M]

11

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) Calculate the concentrations of H2(g) and I2(g) at equilibrium respectively.

(ii) Hence, sketch the changes in concentrations of H2(g), I2(g) and HI(g)


[11M]

12

Phosphorous pentachloride, PCl5, undergoes thermal decomposition to give

phosphorus trichloride, PCl3, and chlorine. When PCl5 is heated in a sealed container

at a certain temperature, a chemical equilibrium is established.

(a) Write an equation to represent this state of equilibrium.

(b) Write the equations of the forward and backward reactions.

(c) Sketch a graph to show the changes in the rates of forward and backward

reactions against time until the reaction mixture reaches equilibrium.

[7M]

13

When sulphur dioxide is heated with oxygen in a sealed vessel at a certain

temperature, the following reaction occurs:

2SO3(g) 2SO2(g) + O2(g)

(a) Two reactions are occurring at equilibrium.

(i) Write the equations of these two reactions.

(ii) What can we say about the rates of these two reactions?

(b) What can you say about the concentrations of SO3(g), SO2(g) and O2(g) at

equilibrium?

(c) State TWO characteristics of dynamic equilibrium.

[6M]



14

(a) State THREE characteristics of an equilibrium system.

(b) The industrial preparation of ammonia can be represented by the following

equation:

N2(g) + 3H2(g) 2NH3(g)

The diagram below shows the change in amount of nitrogen with time after

mixing 1 mol of nitrogen and 3 mol of hydrogen in a sealed container. At

equilibrium, the number of moles of N2(g) is 0.5 mol.

At equilibrium, what is the number of moles of

(i) hydrogen?

(ii) ammonia?

(c) On the diagram above, sketch the changes in amounts of hydrogen and ammonia

with respect to time.

[8M]

15

The following equation represents a reaction in dynamic equilibrium.

A(aq) + B(aq) 2C(aq)

(a) Explain the term ‘dynamic equilibrium’.

(b) The diagram below shows the change in concentrations of reactants and product

with time.

Am

ount

(m

ol)

N2(g)

Time

Con

cent

rati

on

[C]

[A] or [B]

Time



(i) On the diagram above, mark with a dotted line the time, t, at which the

equilibrium starts and give your reasons.

(ii) Sketch the rates of forward and backward reactions with respect to time.

[6M]

16

Consider the following reversible reaction describing the decomposition of calcium

carbonate in a closed metal container at 800C:

CaCO3(s) CaO(s) + CO2(g)

The set-up is shown below:

(a) Do you expect all CaCO3 solid decomposes at the end? Explain briefly.

(b) After careful examination on the metal container, it is found that there is a tiny

hole on the surface. What do you expect to observe if the experiment is still to be

carried out? Explain your answer.

[4M]

17

Consider the following reversible chemical reaction:

2NO2(g) N2O4(g)

N2O4 is a colourless gas and NO2 is a brown gas. NO2 is initially injected into a

closed gas jar.

(a) Describe what you can observe when NO2 is just injected into the jar until the

closed system attains equilibrium.

(b) Describe the change in the concentration of NO2 throughout the same period of

time as (a).

(c) Describe the chemical changes occurring at molecular level throughout the same

period of time as (a).

[6M]

metal container

calcium carbonate solid



18

Consider the following reaction:

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

(a) Name the reactants for the forward reaction.

(b) Name the reactants for the backward reaction.

(c) Suppose initially ethanoic acid and ethanol are mixed together. Sketch a diagram

showing the forward and backward reaction rates.

[4M]

19

(a) State FOUR important characteristics of dynamic equilibrium.

(b) Using the reaction: CH3COOH(aq) CH3COO(aq) + H+(aq) to illustrate the

meaning of the term ‘dynamic’.

[7M]

20

Consider the following equilibrium reaction:

SbCl5(g) SbCl3(g) + Cl2(g)

SbCl5(g) is initially placed into a sealed container and is allowed to attain

equilibrium.

(a) Which chemical species are present in the sealed container at equilibrium?

Write the eq(b) uations for the forward reaction and the backward reaction

(c)

5(g) is initially placed into a sealed container until the

equilibrium is attained.

[9M]

hemistry: Chapter 42 Equilibrium constant

tions 42.142.4

respectively.

Describe the rate of forward reaction and the rate of backward reaction when a

certain amount of SbCl

C

Sec

certain temperature.

xture.

the reaction.

(c) Calculate the equilibrium constant for the reaction.

[8M]

21

In an experiment, 1.00 × 103 mol of PCl5(g) was introduced into a 250 cm3 container

and allowed to reach equilibrium with PCl3(g) and Cl2(g) at a

9.65 × 104 mol of Cl2(g) was found in the equilibrium mi

(a) Write a balanced chemical equation for the reaction.

(b) Write the expression for the equilibrium constant for

22



At 460C, the equilibrium constant, Kc, for the reaction

H2(g) + I2(g) 2HI(g)

is 48. In an experiment, 2.0 g of H2(g) was mixed with 508 g of I2(g) in a 5.0 dm3

(b) Calculate the equilibrium concentrations of H2(g), I2(g) and HI(g) in the

ixture.

[9M]

23

container and the mixture was allowed to reach equilibrium.

(a) Calculate the initial number of moles of H2(g) and I2(g) respectively.

m

At a certain temperature, the equilibrium constant, Kc, for the following reaction

2H2S(g) 2H2(g) + S2(g)

is 2.25 × 104 mol dm3. At equilibrium, the concentrations of H2S(g) and S2(g) are

tively.

(a) Write the expression for the equilibrium constant of this reaction.

Calculate the equilibrium concentration of H2(g).

[3M]

24

2.42 × 103 mol dm3 and 1.165 × 103 mol dm3 respec

(b)

At 250C, the equilibrium constant, Kc, for the following equilibrium

PCl5(g) PCl3(g) + Cl2(g)

is 3.8 × 102 mol dm3. In an experiment, 0.55 mol of PCl3(g) and 0.55 mol of Cl2(g)

(a) l concentrations of PCl3(g) and Cl2(g) respectively.

(b) Calculate the equilibrium concentrations of PCl5(g), PCl3(g) and Cl2(g) i

reaction mixture.

[7M]

25

were mixed in a 2.5 dm3 container and allowed to reach equilibrium with PCl5(g).

Calculate the initia

n the

Consider the following equilibrium:

2SO2(g) + O2(g) 2SO3(g)

In an experiment, 2x mol of SO2(g) and y mol of O2(g) were introduced into a V dm3

iner. At econta quilibrium, 2z mol of SO3(g) was found in the reaction mixture.

(b) , the concentrations of SO2(g), O2(g) and SO3(g) are 0.71 mol

dm3, 0.50 mol dm3 and 1.00 mol dm3 respectively. Calculate the equilib

constant, Kc.

[6M]

(a) Write the expression for the equilibrium constant of the reaction in terms of x, y,

z and V.

At equilibrium

rium



26


CO2(g) + H2(g) CO(g) + H2O(g)

State whether the equilibrium is homogeneous or heterogeneous. Explain briefly. (a)

(b) roduced

m constant of the reaction is 0.63 at a certain temperature.

Determine the equilibrium concentrations of all reactants and products

temperature.

[10M]

27

In an experiment, 0.10 mol of CO2(g) and 0.12 mol of H2(g) were int

into a 1 dm3 container and allowed to reach equilibrium with CO(g) and H2O(g).

The equilibriu

at this

Given:

Reaction Equilibr onstantium c

N2(g) + O2(g) + Br2(g) 2NOBr(g) Kc

2NO(g) N2(g) + O2(g) Kc1

2NO(g) + Br2(g) 2NOBr(g) Kc2

(a)

(ii) expression for the equilibrium constant, Kc2.

ms of

Kc1 and Kc2.

Calculate Kc if Kc1 and Kc2 are 2.4 × 1018 and 1.4 mol1 dm3 respectively.

[5M]

28

(i) Write the expression for the equilibrium constant, Kc1.

Write the

(iii) Hence, write the expression for the equilibrium constant, Kc, in ter

(b)


CH4(g) + H2O(g) CO(g) + 3H2(g) ΔH = +x kJ

Com ch of his statements.

be equal to the amount of

(b) lpy change for the reverse reaction is x kJ.

(c) At equilibrium, the rate of forward reaction is higher than the rate of back

reaction.

The expression for the equilibrium constant, Kc of a homogeneous equilibrium is

shown below.

According to the above information, a student made the following statements.

ment on ea

(a) At equilibrium, the amount of carbon monoxide must

methane.

The entha

ward

[5M]

29



Kc = eqm

2eqm

)]g([)] Y

eqm

(a) (i) Write the equatio r reactions respectively.

(b)

2)]g([Z

g([X

ns for the fo ward and backward

(ii) Deduce the units for the equilibrium constants of both reactions.

Consider the following equilibrium,

2NOCl(g) 2NO(g) + Cl2(g)

ed into a 1 dm3 container and

(i) Calculate the initial concentration of NOCl.

i) Calculate the equilibrium concentrations of NOCl and Cl2.

(iii) Write an expression for Kc and calculate its value.

[9M]

ria:

In an experiment, 2 mol of NOCl was introduc

allowed to reach equilibrium with NO and Cl2. At equilibrium, 0.66 mol of NO

was found in the reaction mixture.

(i

30

Consider the following equilib

2P(g) + 3Cl2(g) 2PCl3(g) K1

PCl (g) + Cl3 2(g) PCl5(g) K2

2P(g) + 5Cl2(g) 2PCl5(g) K

(a) 1 and K2.

(b) Write an expression for an

(c)

Write the expressions for K

K in terms of K1 d K2.

In an experiment, PCl3(g), Cl2(g) and PCl5(g) existed in equilibrium as shown

by the following equation:

PCl3(g) + Cl2(g) PCl5(g)

The equilibrium mixture in a 2.5 dm3 vessel contained 1.6 × 103 mol of

0.03 mol of Cl2 and 5.04 × 104 mol of PCl5.

nt for the reaction.

[9M]

PCl3,

Calculate the equilibrium consta

31

Consider the following equilibrium,

CH4(g) + 2H2S(g) CS2(g) + 4H2(g) Kc = 3.6 mol2 dm6

What does K indicate? (a)

f the concentrations

of CH4(g), H2S(g), CS2 2 dm3, 1.2 mol dm3, 0.9 mol

(c)

c

(b) State whether the reaction mixture is at equilibrium or not i

(g) and H (g) are 1.07 mol

dm3 and 1.78 mol dm3 respectively.

The following equation represents a reaction in equilibrium:



ClF3(g) ClF(g) + F2(g)

K, 21

(i) e reaction.

(ii) In another experiment, 39.4 g of ClF3 was introduced into the

container at 700 K, calculate the equilibrium concentrations of all species in

the reaction mixture.

hydr

(a)

(b)

r. The mixture was heated in the presence of finely

divided iron and allowed to reach equilibrium with ammonia. At equilibri

mol of ammonia was found in the reaction mixture. Calculate the equilibrium

ol of I2(g) were introduced into a 3

(g). At equilibrium, 0.24 mol

(a) alanced equation for the reaction.

(b) Calculate the equilibrium constant for the reaction.

Hence, what can you say about the equilibrium position of the reaction? Explain

[8M]

34

chloride, PCl5, was heated at 250C in a

In an experiment, 9.25 g of ClF3(g) was introduced into a 2 dm3 container at 700

.5% of ClF3 decomposed to give an equilibrium mixture.

Calculate the Kc of th

same

[11M]

32

In the Haber Process, ammonia is synthesized by the reaction between nitrogen and

ogen.

Write a balanced equation for the reaction in the process.

In an experiment, 10 mol of nitrogen and 28 mol of hydrogen were introduced

into a 100 dm3 containe

um, 6

constant for the reaction.

[6M]

33

In an experiment, 0.15 mol of H2(g) and 0.15 m

dm3 container and allowed to reach equilibrium with HI

of HI(g) was found in the reaction mixture.

Write a b

(c)

briefly.

In an experiment, 2.5 g of phosphorus penta

2.0 dm3 sealed container until equilibrium was reached.


value of KThe

(a) calculate the concentrations of PCl5(g), PCl3(g) and Cl2(g) at equilibrium.

) calculate the concentrations of PCl5(g), PCl3(g) and Cl2(g) at equilibrium if the

volume of the sealed container is 5.0 dm3.

c at 250C is 0.19 mol dm3,

(b

[15M]



35

heated to a certain temperature.

Consider the following reaction. Hydrogen sulphide is partially decomposed when

2H2S(g) 2H2(g) + S2(g)

Write the expression for the equilibrium constant, K(a)

(b) sk of volume 10.0 dm3 and heated

until equilibrium was reached. At equilibrium, 4.2 dm3 of hydrogen was for

Calculate the value of Kc and state its unit.

[6M]

c, for this reaction at this

temperature.

6.0 dm3 of H2S(g) was placed in a sealed fla

med.

(Assume molar volume of gases = 24.0 dm3)

36

The decomposition of HI(g) can be represented by the equation:

2HI(g) H2(g) + I2(g)

2.56 g of HI(g) was introduced into a sealed vessel and was heated to 730 K. When

and then opened

unde t sodium

a

(a) (i) e observed when HI(g) was heated to 730 K.

?

dium

(ii) What is the number of moles of iodine in the reaction mixture?

(i) Write the expression for the equilibrium constant, Kc for the reaction.

m constant for the reaction.

[10M]

Cons n:

equilibrium was reached, the equilibrium mixture was cooled rapidly

r po assium iodide solution. The mixture required 22.2 cm3 of 0.1 M

thiosulph te solution to react with iodine.

What would b

(ii) Why was the reaction mixture put into potassium iodide solution

(b) (i) Write an ionic equation for the reaction between iodine and so

thiosulphate.

(c)

(ii) Calculate the equilibriu

37

ider the following reactio

CaCO3(s) CaO(s) + CO2(g)

Explain briefl(a) y why concentrations of solids and liquids are not included in the

(b) ction, and

state its unit.

Given that the equilibrium constant of the reaction at 800C is 2.80 mol dm3.

entration of CO2(g) in the reaction tank?

[4M]

equilibrium expression.

Write the expression of equilibrium constant, Kc, for the above rea

(c)

What is the equilibrium conc

38

Consider the following reaction:



H2(g) + I2(g) 2

D nt pro of h gas ine gas are mixed together at a certain

temperature. The results are as follows.

Initial concent 02 Eq ium concentration ×

HI(g)

iffere portions ydrogen and iod

ration × 1 M uilibr 102 M

Trial [H )] [I2 ] [H )] [ [ [ 2(g (g) I(g H2(g)] I2(g)] HI(g)]

1 1 1 0 0.27 0.27 1.47

2 2 1 0 1.10 0.10 1.80

3 4 1 0 3.04 0.04 1.92

(a) stant, Kc, for this reaction.

(b) By referring to the experimental results, calculate the equilibrium constant.

) Predict the equilibrium concentration of each species when 3 × 102 M of H2(g)

is initially mixed with 1 × 102 M of I2(g).

[10M]

Write the expression for the equilibrium con

(c

39

Consider the following esterification reaction:

CH3COOH(l) + C2H5OH(l) CH3COOC2H5(l) + H2O(l)

To determine the equilibrium constant of this reaction at 298 K, 25 cm3 of CH3COOH

and 25 cm3 of C2H5OH are mixed in a beaker. After one week, the reaction mixture

.4 cm3 of 3 M sodium hydroxide is needed to neutralize all

g cm

.

d products including

H2O, assuming that the volume of the equilibrium mixture is still 50 cm3.

) Write the expression for the equilibrium constant, Kc, for this reaction.

Calculate the equilibrium constant of this reaction at 298K.

[13M]

er the following chemical equilibrium:

has reached equilibrium. 10 cm3 of the mixture is withdrawn to titrate with 3 M

sodium hydroxide. 10

CH3COOH remaining in the sample.

(Given that the densities of CH3COOH(l) and C2H5OH(l) are 1.049 g cm3 and 0.789 3 respectively.)

(a) Calculate the initial concentrations of CH3COOH and C2H5OH present

(b) Calculate the equilibrium concentrations of all reactants an

(c

(d)

40

Consid

SO3(g) + NO(g) SO2(g) + NO2(g) Kc = 0.5

1 mol of SO3(g) and 1 mol of NO(g) were placed in a 10 dm3 flask and allow

act.

ulate the concentration of SO2(g) at equilibrium.

ed to

re

Calc



[4M]

41

In an experiment, 11.31 cm3 of methanoic acid, 36.44 cm3 of ethanol and a small

quantity of catalyst were mixed together.

HCOOH(l) + CH3CH2OH(l) HCOOCH2CH3(l) + H2O(l)

After several hours, equilibrium was reached, 5.0 cm3 of the reaction mixture required

25.0 cm3 of 0.25 M NaOH(aq) for neutralization.

nd 1.22 g 3 g

(a) expression for the equilibrium constant, K , for the reaction.

anol before reaction.

(c) Calculate the number of moles of methanoic acid in 5.0 cm3 of the equil

mixture.

[10M]

42

The following graph shows the change of numbers of moles of the reactants and

products with time for the reaction: 2A(g) + B(g)

(Given that the densities of ethanol and methanoic acid are 0.789 g cm3 a

cm respectively; the molar masses of ethanol and methanoic acid are both 46

mol1.)

Write the c

(b) Calculate the number of moles of methanoic acid and eth

ibrium

(d) Calculate the equilibrium constant, Kc, for the reaction.

A2B(g) at 500C.

(a) t1 represent respectively?

r.

(c) Calculate the equilibrium concentrations of all the chemical species in the

if the volume of the sealed container is 500 cm3.

0C and state its unit.

What do the slopes at point M and point N at time

(b) At which time is the system at equilibrium? Explain your answe

system

(d) Calculate Kc for the above reaction at 50

N

M

A

A(g)

0.425

0.15

B(g)

2B(g)

0.575

t1 t2 Time

Num

ber

of m

oles

(m

ol)



[10M]

43



The value of the equilibrium constant for the reaction is 8.00 × 103 mol dm3 at 200

C. At this temperature, a sample of PCl5(g) is introduced into a sealed container

equilibrium concentration of PCl5(g) is 5.00 × 104 mol dm3. Find the initial

[5M]

until

the

concentration of PCl5(g).

44


2XO(g) + O2(g) 2XO2(g)

The value of the equilibrium constant for this reaction at 398 K is 1.00 × 104 mol1

dm3. If 1.00 mol of XO(g) and 2.00 mol of O2(g) are placed in a 2.00 dm3 vesse

allowed to attain equilibrium at 398 K, find the equilibrium concentrations of each

An industrial process is stu c SO2Cl2(g) in a 100.0 dm3

SO2Cl2(g)

l and

are

chemical species.

[8M]

45

died by pla ing 0.030 mol of

reactor along with 2.00 mol of SO2(g) and 1.00 mol of Cl2(g) at 173C. The equation

is shown as follows:

SO2(g) + Cl2(g)

The value of the equilibrium constant for the reaction at 173C is 0.0810 mol

ilibrium concentrations for all the chemical species in the reaction

ture.

dm3.

Find the equ

mix

[7M]

Section 42.5

46

In an experiment, equal number of moles of ethanol and propanoic acid were mixed

(a)

(b) Suggest a catalyst for the above reaction.

Give an outline to describe how to determine the equilibrium constant, Kc, of the

[8M]

47

A student determined the equilibrium constant, Kc, of the following reaction by an

experiment.

and heated under reflux to give an ester.

Write the balanced equation for the above reaction.

(c)

above reaction with the use of standard sodium hydroxide solution.



Ag+(aq) + Fe2+(aq) Ag(s) + Fe3+(aq)

(c)

(ii) ns of Ag+(aq) and Fe2+(aq)

(iii) ) is required to react completely with the Ag+(aq) in

(1) ionic equation for the reaction between Ag+(aq) and

ultant solution?

(d) Calculate the equilibrium constant, Kc, for the reaction.

[12M]

43 The effect of changes in concentration and temperature on

hemical equilibria

tion 43.1

(a) Why is it necessary to allow the mixture to stand for two hours?

(b) Briefly explain what would be observed when the end point is reached.

(i) Write the expression for the equilibrium constant, Kc, for the reaction.

According to step 1, what are the concentratio

immediately after mixing?

14.2 cm3 of KSCN(aq

the resultant solution.

Write an

SCN(aq).

(2) What is the concentration of Ag+(aq) in the res

Chemistry: Chapter

c

Sec

Fe(NO ) (aq) were added to 50 cm3 of distilled water, the

(a)

(ii) ical equation for the reaction between Fe(NO ) (aq) and

(b)

llowing chemical reagents are added to

re.

q)

48

In an experiment, 2 cm3 of 0.1 M potassium thiocyanate, KSCN(aq) and 2 cm3 of 0.2

M iron(III) nitrate, 3 3

resultant solution turned red.

(i) What is the red substance formed?

Write a chem 3 3

KSCN(aq).

When the reaction reaches equilibrium, an orange-yellow solution is formed.

Explain the colour changes when the fo

the orange-yellow equilibrium mixtu

(i) 5 drops of 0.2 M Fe(NO3)3(a

(ii) 5 drops of 0.1 M KSCN(aq)

(iii) 5 drops of 6.0 M KOH(aq)

Procedure:

Mix 25.0 cm1. .10 M AgNO3(aq) and 25.0 cm3 of 0.12 M Fe(NO3)2(aq) in a

3. t solution into a conical flask and titrate with

3 of 0

dry clean bottle.

2. Allow the mixture to stand for two hours.

Pipette 25.0 cm3 of the resultan

0.02 M KSCN(aq) until an end.



(iv) A few pieces of Na2HPO4 crystal

[12M]

Consider the fo h :

49

llowing reaction which as attained dynamic equilibrium

CaCO3(s) + 2HCl(aq) CaCl2(aq) + CO2(g) + H2O(l)

For each of the following changes, describe briefly what would be observed.

(b) hydrogen chloride gas, which is very soluble in water, through the

.

(d) Adding sodium hydroxide solution to the reaction mixture.

[4M]

In an experiment, ter.

(a) Dissolving the salt calcium chloride into the reaction mixture.

Bubbling

mixture.

(c) Letting some carbon dioxide gas escape out of the container

50

bromine was dissolved in water to form bromine wa

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

hydroxide

(c) Silver nitrate solution

[6M]

BiCl3, hydrolyses in water to give

a white precipita

solutions are added to bromine water.

(a) Concentrated hydrochloric acid

(b) Concentrated potassium

51

The following equation shows bismuth trichloride,

te, bismuth oxychloride (BiOCl).

BiCl3(s) + H2O(l) BiOCl(s) + 2H+(aq) + 2Cl(aq)

By Le Châtelier’s Principle, predict the observations when the following solutions are

c acid

(c) Saturated sodium chloride solution

[6M]

tion 43.2

added to the reaction mixture.

(a) Concentrated hydrochlori

(b) A large amount of water

Sec

ide paper.

l(aq)

52

The following equation shows the equilibrium established in cobalt chlor

[Co(H2O)6]2+(aq) + 4C ) ΔH = +ve

(II) exahydrate hlorocobaltate(II)

[CoCl4]2(aq) + 6H2O(l

cobalt h tetrac

pink blue



(a) Account for the colour change when water is added to cobalt chloride paper.

Explain why heating moisture-proof pellets ( 防潮珠 ) containing

tetrachloroco

(b)

baltate(II) and cobalt(II) hexahydrate complexes can restore the

blue colour.

[4M]

ith CO2(g) at 1800 K. The equilibrium is achieved according

to the equation below:

53

When H2(g) is mixed w

H2(g) + CO2(g) H2O(g) + CO(g)

If the value of the equilibrium constant is 6.0, and the concentrations of each chemical

CO2(g)] = 0.30 mol dm3; [H2O(g)] = 0.55 mol dm3;

(b) ixture at 1800 K, show that the

K?

(d) Calculate the equilibrium concentrations of all chemical species at 1800 K

[14M]

tion 43.3

species at 1800 K are as follows:

[H2(g)] = 0.20 mol dm3; [

[CO(g)] = 0.55 mol dm3,

(a) Write an expression for the equilibrium constant, Kc, for the above reaction.

By finding the reaction quotient of the reaction m

system has not reached the state of equilibrium.

(c) In which direction will the system shift in order to attain equilibrium at 1800

.

Sec

Consider the followin

54

g 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

.

(c) the concentration of SO3(g) is decreased.

[6M]

Consider the follo s

equilibrium position for the reaction when

(a) the temperature of the system is increased

(b) the concentration of SO2(g) is decreased.

55

wing equilibrium ystem:

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)

temperature

Change in

Effect on [CO2(g)]



Effect on [CO(g)]

Effect on equilibrium

position

Effect on equilibrium

constant

[12M]

56

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) Fe3O4(s) + 4H2(g) H2(g) is removed

(c) NH4Cl(s) NH3(g) + HCl(g) pressure is decreased

(d) 2NO(g) N2(g) + O2(g) volume is decreased

(e) BiCl3(aq) + H2O(l) BiOCl(s) + 2HCl(aq) NaOH(aq) is added

[10M]

57

The following equation represents a reaction at equilibrium:

2A(g) + B(g) 4C(g)

(a) State the effect on the equilibrium position when the volume of the vessel is

decreased at constant temperature. Explain briefly.

(b) A decrease in temperature can increase the yield of product C. Predict whether

the forward reaction is endothermic or exothermic. Explain briefly.

(c) A student stated that adding a suitable catalyst can increase the yield of product

C. Comment on his statement.

[7M]

58

The equilibrium constant, Kc, for the following reaction is 190 mol2 dm6 at 1000 K.

CO(g) + 3H2(g) CH4(g) + H2O(g) ΔH = ve

(a) State the effect of an increase in temperature on the equilibrium position. Explain

briefly.

(b) The above equilibrium has been disturbed and the concentrations of CO(g),

H2(g), CH4(g) and H2O(g) are now 4.0 mol dm3, 8.0 mol dm3, 6.0 mol dm3

and 6.0 mol dm3 respectively.

(i) Calculate the reaction quotient, Qc, at the point when the equilibrium is

disturbed.

(ii) To which direction will the equilibrium position shift? Explain briefly.



[6M]

59

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 dm3, 0.0717 mol dm3, 0.0483 mol dm3 and 0.0483 mol dm3 respectively.

(a) If 0.02 mol dm3 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]

60

At a certain temperature, the equilibrium constant, Kc, for the following equilibrium

system is 2.98 mol1 dm3:

Ag+(aq) + Fe2+(aq) Ag(s) + Fe3+(aq)

In an experiment, solid silver is added to a solution containing 0.200 mol dm3 of

Ag+(aq), 0.100 mol dm3 of Fe2+(aq) and 0.300 mol dm3 of Fe3+(aq). Then, the above

reaction occurs.

(a) Determine the concentrations of Ag+(aq), Fe2+(aq) and Fe3+(aq) in the reaction

mixture at equilibrium.

(b) State the effect on the equilibrium position if a certain amount of sodium

chloride solution is added to the equilibrium mixture. Explain briefly.

[6M]

61

Hydration of ethene can be used as one of the ways to produce ethanol, according to

the equation below.

C2H4(g) + H2O(g) C2H5OH(g)

(a) Write an expression for the equilibrium constant.

(b) (i) If you are in charge of the production of ethanol, would you use a high or

low pressure for the above reaction? Explain briefly.

(ii) Give one advantage and one disadvantage of using such level of pressure.

(c) (i) Suggest another way of production of ethanol from yeast.

(ii) Write a balanced equation to represent the reaction.

(Hint: Sugar is metabolized to become ethanol and carbon dioxide.)

[7M]



62

The reaction below is responsible for the production of sulphuric acid in the Contact

process.

2SO2(g) + O2(g) 2SO3(g)

(a) Suggest a catalyst for the above reaction.

(b) A student claims that the catalyst in (a) would cause a change in the

concentrations of all species at equilibrium, thus the equilibrium constant is also

changed. Comment on his statement.

(c) Suggest a way to determine whether the above forward reaction is exothermic or

endothermic by using the equilibrium constants. Explain briefly.

[6M]

63

Pure nickel can be obtained by heating nickel carbonyl which is prepared by the

reaction between impure nickel and carbon monoxide.

Ni(s) + 4CO(g) Ni(CO)4(g) ΔH = ve

According to Le Châtelier’s Principle, predict the effect of the following changes on

the equilibrium positions and the indicated quantities.

Change

Effect on the

equilibrium position

Indicated

quantity Effect

(a) Adding CO(g) Ni(s)

(b) Removing CO(g) Kc

(c) Removing

Ni(CO)4(g)

CO(g)

(d) Increasing the

temperature at

constant pressure

Kc

(e) Decreasing the

temperature at

constant pressure

CO(g)

(f) Decreasing the

volume at constant

temperature

Ni(CO)4(g)

[12M]

64

Consider the equilibrium established by CO(g), I2O5(s), I2(g) and CO2(g) in a closed

container at a particular temperature.

5CO(g) + I2O5(s) I2(g) + 5CO2(g) ΔH = ve



(a) According to Le Châtelier’s Principle, predict the effect on the equilibrium

position of the system if

(i) I2O5(s) is added.

(ii) CO2(g) is added.

(iii) I2(g) is removed.

(iv) the temperature is increased at constant pressure.

(v) the volume is decreased at constant temperature.

(b) In the graphs below, sketch the changes in concentrations of the reactants and

products if

(i) the pressure of the equilibrium system at constant temperature is increased

at 10 min.

(ii) a certain amount of CO(g) is added to the equilibrium system at 10 min.

(iii) the temperature of the equilibrium system at constant pressure is decreased

at 10 min.

10 Time (min)

I2(g)

CO2(g)

CO(g)

Con

cent

rati

on (

mol

dm

3)

10 Time (min)

I2(g)

CO2(g)

CO(g)

Con

cent

rati

on (

mol

dm

3)



[9.5M]

65

HA is a weak acid and it slightly ionizes in water as shown in the following equation:

HA(aq) + H2O(l) H3O+(aq) + A(aq) ΔH = +ve

Using the Le Châtelier’s Principle, predict and explain the effect on the equilibrium

position of the above reaction if

(a) the concentration of HA(aq) is increased.

(b) the pH of the reaction mixture is decreased.

(c) the temperature of the system is increased.

[8M]

66

Contact Process is an important industrial process which involves the following

reversible reaction:

2SO2(g) + O2(g) 2SO3(g) ΔH = ve

The table below shows the percentage of sulphur trioxide in equilibrium mixture at

different temperatures.

Temperature (C) Percentage of sulphur trioxide in equilibrium (%)

400 99.2

500 93.5

600 73.7

(a) Explain why a lower temperature results in a higher percentage of sulphur

trioxide in the equilibrium mixture.

(b) Explain why a temperature of 500C is used in the Contact Process rather than a

lower temperature.

(c) What happens to the position of equilibrium when the following conditions of

the system are changed?

(i) The volume of container is reduced.

(ii) Some sulphur trioxide produced is removed.

10 Time (min)

I2(g)

CO2(g)

CO(g)

Con

cent

rati

on (

mol

dm

3)



[5M]

67

When nitrosyl chloride (NOCl) is heated, it forms an equilibrium mixture with

nitrogen monoxide and chlorine

according to the following equation:

2NOCl(g) 2NO(g) + Cl2(g) ΔH = +76 kJ mol1

(a) Write the expression for the equilibrium constant, Kc, for this reaction.

(b) In an experiment, 1 mol of nitrosyl chloride was placed in a 5 dm3 sealed

container and heated to 500C. At this temperature, 20% of nitrosyl chloride was

dissociated. Calculate the value of Kc at this tempearture.

(c) State the change in the degree of dissociation, when temperature is increased.

(d) State the change in the degree of dissociation, when pressure is increased.

[9M]

68


2SO2(g) + O2(g) 2SO3(g) ΔH = 197 kJ mol1

This reaction takes place in a closed container. For each of the following changes,

state the subsequent change in the equilibrium concentration of SO3(g).

(a) More SO2(g) is added to the container.

(b) Some O2(g) is removed from the container.

(c) The temperature is lowered.

(d) The reaction mixture is transferred into a smaller container.

[4M]

69

Nowadays, hydrogen can be obtained from natural gas by partial oxidation with steam.

This involves the following endothermic reaction:

CH4(g) + H2O(g) CO(g) + 3H2(g)

(a) Write an expression for the equilibrium constant, Kc, for this reaction.

(b) State how the equilibrium position of the system is affected by

(i) increasing the pressure,

(ii) increasing the temperature,

(iii) using a catalyst.

(c) How will the value of Kc be affected by:

(i) increasing the pressure,

(ii) increasing the temperature,

(iii) using a catalyst?

[10M]



70

A certain amount of solid ammonium hydrogensulphide is placed in a sealed flask

already containing 0.60 mol dm–3 of ammonia gas at room temperature. It dissociates

as follows:

NH4HS(s) H2S(g) + NH3(g)

At equilibrium, the concentration of ammonia gas is found to be 0.87 mol dm3.

(a) Write the expression for the equilibrium constant, Kc, for the above dissociation

at room temperature.

(b) Calculate the equilibrium constant for the above dissociation at room

temperature and state its unit.

(c) If the volume of the flask used was doubled, calculate the equilibrium

concentration of ammonia gas.

[10M]

71


Cl2(g) + 3F2(g) 2ClF3(g) ΔH = 329 kJ mol1

(a) Write an expression for the equilibrium constant, Kc, for the above reaction.

(b) State three methods to increase the amount of ClF3(g) in the equilibrium

mixture.

(c)

n more ClF3(g) would be formed

xture.’

Comment on the student’s conclusion.

[7M]

equilibrium in an enclosed reaction

vessel at 298 K

A student wrote the following conclusion in his notebook.

‘As the forward reaction was exothermic, the

than Cl2(g) and F2(g) in the reaction mi

72

C(graphite) and H2(g) were allowed to attain

. The equation is shown below:

C(graphite) + 2H2(g) CH4(g) ΔH = 75 kJ mol1

State, with explanation, the effect of each of the following changes on the equilibrium

sel.

l.

(c) Adding C(graphite) to the reaction vessel.

[6M]

tion 43.4

position of the above system and the value of Kc.

(a) Increasing the temperature of the reaction ves

(b) Increasing the volume of the reaction vesse

Sec

synthesized by the exothermic reaction between

73

In Harber process, ammonia is



nitrogen and hydrogen at 450C.

N2(g) + 3H2(g) 2NH3(g)

(a) Wri e an expression for the equilibrium constant t of the reaction.

(b)

ixture?

(c)

(ii) the temperature of the system is increased to 1000C at constant press

[12M]

hydrogen combine to give ammonia as shown in the

following reversible r

(i) What is the reaction quotient, Qc?

(ii) What information does Qc tell about the reaction m

Predict and explain the effect on the yield of NH3(g) if

(i) the volume of the system is decreased at constant temperature.

ure.

74

In Haber process, nitrogen and

eaction:

N2(g) + 3H2(g) 2NH3(g) ΔH = ve

(a) Write an expression for the equilibrium constant, Kc, of the reaction.

(b)

(ii) the volume of the system under constant temperature is increased.

[7M]

Predict and explain the effect on the equilibrium position if

(i) the temperature of the system under constant pressure is decreased.