2nd Lecture on Chemical Equilibrium Chemistry Part II11th Standard by Rizwana Mohammed

1. The Malegaon High School & Jr. College
Malegaon, (Nashik), 423203
2nd Lecture on Chemical
Equilibrium
Chemistry Part II, 11th Science
By
Rizwana Mohammad

2. Law of mass action and equilibrium constant:
Rate of chemical reaction:
• As any reaction proceeds the concentration of the reactants
decreases and the concentration of the products increases.
• The rate of reaction thus can be determined by measuring the
extent to which the concentration of a reactant decreases in the
given time interval or extent to which the concentration of product
increases in the given time interval.
• The rate of reaction is expressed as:
Rate = −
d[Reactant]
dT
=
d[Product]
dT
• Where d[Reactant] and d[Product] are the small decrease or
increase in concentration during the small time interval dT.

3. Law of mass action:
• The law states that, “the rate of chemical reaction at each instant is
proportional to the concentration product of all the reactants.”
• Consider the reaction,
A+B → C
Rate ∝ [A][B]
Rate = k[A][B] …1
Equation 1 is called the rate equation and the proportionality constant,
k is called the rate constant of the reaction.
Equilibrium constant:
Consider reversible reaction,
A+B ⇌ C+D
We can write the rate equations for the forward and reverse reactions:
Rateforward ∝ [A][B]
∴ Rateforward = kf[A][B] …1
Ratereverse ∝ [C][D]
Ratereverse = kr[C][D]

4. At equilibrium,
Rateforward = Ratereverse
∴ kf[A][B] = kr[C][D]
∴
kf
kr
= Kc =
C [D]
A [B]
Kc is called the equilibrium constant
Consider reversible reaction:
aA + bB ⇌ cC + dD
∴ Kc =
𝐶 𝑐
𝐷 𝑑
𝐴 𝑎
𝐵 𝑏
Equilibrium constant with respect to partial pressure (Kp):
For reactions involving gases, the equilibrium constant is expressed in
terms of partial pressure.
For the reaction,
aA(g) + bB(g) ⇌ cC(g) + dD(g)
Kp =
𝑃 𝐶
𝑐
𝑃 𝐷
𝑑
𝑃 𝐴
𝑎
𝑃 𝐵
𝑏 …1

5. Where PA, PB, PC, PD are equilibrium partial pressures of A, B, C and D
respectively.
For a mixture of ideal gases, the partial pressures of each component
is directly proportional to its concentration at constant temperature.
For component A
PAV = nART
nA
V
is molar concentration of A in moldm-3
PA =
nA
V
RT ;
nA
V
= [A]
PA = [A]RT
Similarly for B, PB = [B]RT
PC = [C]RT, PD = [D]RT

6. Relationship between Kp and Kc:
Consider a general reversible reaction:
aA(g) + bB(g) ⇌ cC(g) + dD(g)
Now substituting equations for PA, PB, PC, and PD an in equation
KP =
PC
c
PD
d
PA
a
PB
b =
C c
RT c
D d
RT d
A a
RT a
B b
RT b
KP =
C c
D d
RT c+d
A a
B b
RT a+b
∴ KP =
C c
D d
A a
B b x (RT)(c+d)-(a+b)
∴ KP =
C c
D d
A a
B b x (RT)Δn
ButKC =
C c
D d
A a
B b
∴ KP = KC(RT)Δn
Where Δn = number of moles of gaseous products - number of moles
of gaseous reactants in balanced chemical equation.
R = 0.08206 L atm K-1mol-1

7. Homogeneous and heterogeneous equilibria:
In a homogeneous equilibrium state all the reactants and products are
same phase.
e.g. 2HI(g) ⇌ H2(g) + I2(g)
In a heterogeneous equilibrium state all the reactants and products
are in different phases.
e.g. NH3(g) + Cl2(g) ⇌ NH4Cl(s)
Equilibrium constant for heterogeneous equilibria:
Consider, C2H5OH(l) ⇌ C2H5OH(g)
KC =
[C2
H5
OH g
]
[C2
H5
OH(l)
]
But[C2H5OH(l)] = 1
∴ KC = [C2H5OH(g)]
Similarly, I2(s) ⇌ I2(g)
∴ KC = [I2(g)]

8. Characteristics of equilibrium constant:
1. The value of equilibrium constant is independent of initial
concentrations of either the reactants or products.
2. Equilibrium constant has a characteristic value for a particular
reversible reaction represented by a balanced equation at a given
temperature.
3. Equilibrium constant is temperature dependent. Hence KC, KP
change with change in temperature.
4. Higher value of KC or KP means more concentration of products is
formed and the equilibrium point is more towards right hand side
vice versa.