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CHAPTER

CHEMICAL EQUILIBRIUM

Contents
❖ Definition of Chemical Equilibrium
❖ Classification of Chemical Equilibrium
❖ Reversible and Irreversible Reactions
❖ Law of Mass Action
❖ Relationship between Equilibrium constants (Kc/Kp/Kx/Kn)
❖ Importance of Kc
❖ Le-Chatelier’s Principle with applications
❖ Solubility Product
❖ Common Ion Effect

Saeed Anwar Lecturer in Chemistry
Government College Peshawar Pakistan
E-mail: saeedchemistry28@gmail
...
e
...
,
H2(g) + F2(g)

2HF

H2(g) + Cl2(g)

2HCl(g)

H2(g) + Br2(g)

2HBr(g)

H2(g) + I2(g)

2HI(g)

REVERSIBLE REACTIONS: Those reactions which can proceed in both forward
direction and backward direction are called reversible reactions
...
g
...
e
...
,
C(s) + O2(g)

CO2(g)

S(s) + O2(g)

SO2(g)

CH4(g)+ 2O2(g)

CO2(g) + 2H2O(g)

TYPES OF CHEMICAL EQUILIBRIUM

i
...
Heterogeneous equilibrium
i
...
e
...
,
N2(g) + O2(g)

2NO(g)

N2(g) + 3H2(g)

2NH3(g)

CH3COOH(l) + C2H5OH(l)

CH3COOC2H5(l) + H2O(l)

ii
...
e
...
,
CaCO3(s)
FeO(s) + CO(g)

CaO(s) + CO2(g)
Fe(s) + CO2(g)
Ag+(aq) + Cl-(aq)

AgCl(s)

THE LAW OF MASS ACTION
The rate of a chemical reaction is directly proportional to the active masses (molar
concentrations) of the reacting substances each raised to the power of its coefficient is called
Law of mass action
...

According to the Law of mass action

Rate of forward reaction [A] [B]
Rate of forward reaction =Kf [A] [B] ::K = proportionality constant or rate constant
Since [C] and [D] are the reactants for the backward reaction so the rate of reverse reaction is
given by

Rate of backward reaction [C] [D]
Rate of backward reaction = Kb [C] [D]
At equilibrium
The rate of forward reaction = The rate of backward reaction
Thus
Or

Kf [A] [B] = Kb [C] [D]
Kf = [C] [D]
Kb [A] [B]

::Kf = Kc
Kb

Kc = [C] [D]
...
It is defined as “the ratio of the product of the
molar concentrations of the products to that of the reactants
...
They are called the coefficients of the balanced chemical equation
...
(ii)
[A]a [B]b

For a gaseous equilibrium, the concentration of gases in terms of their partial pressures are
PA, PB, PC and PD and the equilibrium constant (Kc) can be written as
Kp = [PC]c [PD]d
...
(iv)
[XA]a [XB]b

Example 7
...
3g of H2 and 1
...
Find Kc at 300 C0 for the reaction
N2(g) + 3H2(g)

2NH3(g)

Solution:
[N2] = 7g/dm3/14g = 0
...
3g/dm3/2g =0
...
53g/dm3/17g = 0
...
09]2

= 9
...
25][0
...

We know that PV = n R T
P = n/V R T

::n/V= C= molar concentration

P= C
...
T
...

Kp = [CCRT]c [CDRT]d
[CART]a [CBRT]b
Kp = [CC]c[CD]d x (RT)c+d
[CA]a[CB]b x (RT)a+b

::[CC]c[CD]d = Kc
[CA]a[CB]b

Kp = Kc (RT)(c+d)-(a+b)
Kp= Kc (RT) n
...
of moles of products _ no
...
n may be positive,
negative and zero
...


II) Relation between Kp and Kx : According to the Dalton’s law of partial pressure,
the partial pressures PA, PB, PC and PD of gases is given by
PA = XA P

PB = XB P

PC = XC P PD = XD P ::X = mole fractions

KP= c[XCP]c d[XDP]d
a

[XAP]a b[XB]b

KP =[XC]c [XD]d x P c+d
[XA]a [XB]b x P a+b
KP = Kx x P (c+d) - (a+b)

:: [XC]c[XD]d = Kx
[XA]a[XB]b

KP = Kx x P

n

but P = R T/ V

KP = Kx x (RT/V) n
...
P = (nA X P/N) ::PA = partial pressure A, X= mole fraction, nA =
no
...
of moles
...
P/N]c d[nD
...
P/N]a b[nB
...
(iii)
Eq
...
These are related to each other
as follows;
KP = Kc (RT) n = Kx (RT/V) n = Kn (P/N) n
If n = o, then (RT)0 =1, (RT/V)0 = 1, (P/N)0 = 1

then

KP = Kc = K x = Kn

Example 7
...
82
...

PCl5(g)
Solution:

PCl3(g) + Cl2(g)

T = 523K R= 0
...
82 n = 2mol-1mol=1
KP = Kc (RT) n
Kc = KP/(RT)1 = 0
...
082 x 523 = 1
...
91 x 10-2

IMPORTANCE OF Kc
The equilibrium constant (Kc) can be used to predict the following;
1
...

2
...
) of a reversible reaction
...
The effect of change in conditions upon equilibrium
...
To calculate equilibrium concentrations of reactants and products
...
Qc is used to predict the direction
of a reversible reaction
...
Therefore when
1
...

2
...

3
...


Example 7
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6 x 10-2
...
02M and that of H2 and I2 are 0
...
01M respectively
...
01] [0
...
61 x 10-5
[HI]02

[1
...
61 x 10-5 &
Kc = 1
...


Prediction of the extent of a chemical reaction
I) If the Kc is very large, this indicates that the reaction is complete
...

II) If the Kc is very small, then the concentrations of reactants are greater than the
concentrations of products at equilibrium
...


Example 7
...

N2(g) + O2(g)

2NO(g)

Since the Kc is very small, therefore the conc
...
of
products
...


LE CHATELIER’S PRINCIPLE
A French Chemist Henry Le Chatelier’s (1884) studied the effect of concentration,
temperature and pressure on a large number of systems in equilibrium
...
OR

“If a system at equilibrium is disturbed by some change in concentration, temperature or
pressure, the system shifts in such a way to cancel (reduce) the effect of that change”
...


APPLICATIONS
1
...

e
...

N2(g) + 3H2(g)

2NH3(g) similarly

N2(g) + O2(g)

2NO(g)

2SO2(g) + O2(g)

2SO3(g)

On the other hand, by increasing concentration of products the equilibrium will shift in the
backward direction and more reactants is formed till equilibrium is restored
...
Effect of change in pressure:
By changing pressure, the equilibrium will shift in that direction in which a decrease in
volume occurs
...

(a) If a decrease in volume occurs then the increase in pressure shifts the equilibrium in the
forward direction
...
g
...
Thus, the equilibrium will shift in the forward direction
(decrease in volume 2vol)
...
e
...
,
N2O4(g)

2NO2(g)

1vol

2vol

Reactants 1vol and products 2vol
...

(c) For a reversible reaction in which no change in volume occurs, the increase or decrease of
pressure will not affect the position of equilibrium
...
g
...
Effect of change in temperature:
The shifting of equilibrium depends whether the reaction is endothermic or exothermic
...
e
...
,
N2(g) + O2(g) + heat

2NO(g)

H2S(g) + heat

H2(g) + S(g)

Both reactions are endothermic
...

(b) An increase in temperature will shift the equilibrium of exothermic reactions in the
backward direction
...
g
...
Thus the equilibrium will shift in the backward direction by
increasing temperature
...
Effect of catalyst:
A catalyst is a substance which increases the rate of reaction only and lowers the activation
energy
...
Addition of a catalyst increases
the speed of forward and backward reaction at the same extent
...

(a) Haber’s process:

Let us consider the synthesis of ammonia which is of great importance in industry used in the
manufacture of nitrogen fertilizers
...
384 KJ/mole

i
...
The equilibrium will shift in the forward
direction by increasing concentration of reactants
...
Effect of pressure: According to Le Chatelier’s principle, an increase in pressure will
shift the equilibrium in that direction in which a decrease in volume occurs
...
Thus, the formation of ammonia favors by
high pressure
...
Effect of temperature: The formation of ammonia is an exothermic reaction and
its reverse is an endothermic reaction
...

(b) Hence the synthesis of ammonia is carried out at 450C0 in the presence of finely divided
iron which act as a catalyst and a high pressure of 200_500atm is used
...

(b) Formation of SO3 for the manufacture of H2SO4:
2SO2(g) + O2(g)

2SO3(g)

H= -94
...

(b) A decrease in volume is favored by a high pressure (1
...
7atm)
...
The optimum temperature for this reaction is 400_500C0
...

When a sparingly soluble salt such as AgCl is shaken with water it dissolves until a saturated
solution is formed
...

AgCl(s)

Ag+(aq) + Cl-(aq)

According to the law of mass action
Kc = [Ag+] [Cl-]
[AgCl]
Since the concentration of a pure solid AgCl is constant, therefore, we can write
Kc [AgCl]= [Ag+] [Cl-]

::Kc[AgCl] = KSP

KSP = [Ag+] [Cl-]
Examples: Write solubility product(KSP) for the following
...
7:

Calculate the concentration of Pb+2 ion when solid PbSO4 is added to water
...
6 x 10-6
...
6 x 10-8
[Pb+2] = [SO4-2]
Let[Pb+2]= x mole/dm3
So

and [SO4-2] = x mole/dm3

KSP = (x) (x) = 1
...
6 x 10-8 by taking square root
[Pb+2] = x = 1
...
265 x 10-4

APPLICATIONS
The solubility product can be used to predict whether precipitation will occur or not under
given concentrations
...

(ii) If the ionic product of a solution is less than KSP, the solution is unsaturated and additional
solid can be dissolved in this solution
...


COMMON ION EFFECT
Definition: It is a phenomenon in which the solubility of one electrolyte is suppressed by the
addition of another strong electrolyte having a common ion is called common ion effect
...
e
...
,

(i) Addition of NH4Cl suppresses the ionization of NH4OH
...
As a result, Ammonium hydroxide (NH4OH)
precipitates
...


NaCl (s)

Na+ + Cl-

HCl

H+ + -Cl

The concentration of NaCl increases by adding HCl gas due to common ions and the
equilibrium shifts in the backward direction and more NaCl precipitates and purifies
...
Calcium oxalate
Ca(COO)2

Ca+2 + 2COO-

CaCl2

Ca+2 + 2Cl-

The concentration of Ca(COO)2 increases by adding CaCl2 due to common ions and the
equilibrium shifts in the backward direction
...

Common ion method is used in many processes to get a substance in pure form
...


Give the concentration units for the following reversible reactions
...


Write down Kc for the following reversible reactions
...


PCl3(g) + Cl2(g)

Kc = [PCl3] [Cl2]
[PCl5]

(ii)

2HI(g)

H2(g) + I2(g)

Ans
...


Kc = [CO] [H2O]

CO(g) + H2O(g)

[H2] [CO2]
(iv) 3Fe(s) + 4H2O(g)
Ans

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