304 Chemical Equilibrium
Chapter
8
Chemical Equilibrium
Whenever we hear the word Equilibrium immediately a picture (2) Irreversible reactions : Reaction in which entire amount of the
arises in our mind an object under the influence of two opposing forces. For reactants is converted into products is termed as irreversible reaction.
chemical reactions also this is true. A reaction also can exist in a state of
(i) Characteristics of irreversible reactions
equilibrium balancing forward and backward reactions.
(a) These reactions proceed only in one direction (forward
Reversible and Irreversible reactions direction),
A chemical reaction is said to have taken place when the (b) These reactions can proceed to completion,
concentration of reactants decreases, and the concentration of the products
increases with time. The chemical reactions are classified on the basis of the (c) In an irreversible reaction, G < 0,
extent to which they proceed, into the following two classes;
(d) The arrow () is placed between reactants and products,
(1) Reversible reactions : Reaction in which entire amount of the
reactants is not converted into products is termed as reversible reaction. (ii) Examples of irreversible reactions
(i) Characteristics of reversible reactions (a) Neutralisation between strong acid and strong base e.g.,
(a) These reactions can be started from either side,
NaOH HCl NaCl H 2 O 13.7 kcal
(b) These reactions are never complete,
(c) These reactions have a tendency to attain a state of equilibrium, (b) Double decomposition reactions or precipitation reactions e.g.,
in which Free energy change is zero (G = 0), BaCl2(aq) H 2 SO 4 (aq) BaSO4 (s) 2 HCl(aq)
(d) This sign (⇌) represents the reversibility of the reaction, (c) Thermal decomposition, e.g.,
(ii) Examples of reversible reactions MnO 2 ,
2 KClO3 (s) 2 KCl (s) 3O 2
(a) Neutralisation between an acid and a base either of which or
both are weak e.g., (d) Redox reactions, e.g.,
CH 3 COOH Na OH ⇌ CH 3 COONa H 2 O SnCl 2(aq) 2 FeCl3 (aq) SnCl 4 (aq) 2 FeCl2(aq)
(b) Salt hydrolysis, e.g.,
Equilibrium and Its dynamic nature
Fe Cl 3 3 H 2 O ⇌ Fe OH 3 3 HCl
“Equilibrium is the state at which the concentration of reactants
(c) Thermal decomposition, e.g., and products do not change with time. i.e. concentrations of reactants and
products become constant.”
PCl5 (g ) ⇌ PCl3 (g) Cl 2(g) Q
Products
(d) Esterification, e.g.,
CH 3 COOH C2 H 5 OH ⇌ CH 3 COOC 2 H 5 H 2 O
Concentration
(e) Evaporation of water in a closed vessel, e.g.,
Reactants
H 2 O(l) ⇌ H 2 O(g) Q
Time Equilibrium state
, Chemical Equilibrium 305
The important characteristics of equilibrium state are, K p Kc (RT )n
(1) Equilibrium state can be recognised by the constancy of certain
measurable properties such as pressure, density, colour, concentration etc. K p K x (P)n
by changing these conditions of the system, we can control the extent to n = number of moles of gaseous products – number of moles of
which a reaction proceeds. gaseous reactants in chemical equation.
(2) Equilibrium state can only be achieved in close vessel. As a general rule, the concentration of pure solids and pure liquids
(3) Equilibrium state is reversible in nature. are not included when writing an equilibrium equation.
(4) Equilibrium state is also dynamic in nature. Value of Relation between Kp Units of Kp Units of Kc
(5) At equilibrium state, n and Kc
0 Kp = Kc No unit No unit
Rate of forward reaction = Rate of backward reaction
>0 Kp > Kc (atm)n (mole l–1)n
(6) At equilibrium state, G = 0, so that H = TS. <0 Kp < Kc (atm)n (mole l–1)n
Characteristics of equilibrium constant
(1) The value of equilibrium constant is independent of the original
Rate of reaction
G= 0 concentration of reactants.
(2) The equilibrium constant has a definite value for every reaction
Equilibrium state
at a particular temperature. However, it varies with change in temperature.
(3) For a reversible reaction, the equilibrium constant for the
Time
forward reaction is inverse of the equilibrium constant for the backward
Law of mass action and Equilibrium constant reaction.
On the basis of observations of many equilibrium reactions, two 1
In general, K forward reaction
Norwegian chemists Goldberg and Waage suggested (1864) a quantitative K backward reaction
relationship between the rates of reactions and the concentration of the
reacting substances. This relationship is known as law of mass action. It (4) The value of an equilibrium constant tells the extent to which a
states that reaction proceeds in the forward or reverse direction.
“The rate of a chemical reaction is directly proportional to the (5) The equilibrium constant is independent of the presence of
product of the molar concentrations of the reactants at a constant catalyst.
temperature at any given time.” (6) The value of equilibrium constant changes with the change of
The molar concentration i.e. number of moles per litre is also called temperature. Thermodynamically, it can be shown that if K1 and K 2 be
active mass. It is expressed by enclosing the symbols of formulae of the the equilibrium constants of a reaction at absolute temperatures T1 and
substance in square brackets. For example, molar concentration of A is expressed
as [A]. T2 . If H is the heat of reaction at constant volume, then
Consider a simple reversible reaction H 1 1
log K 2 log K1 (Van’t Hoff equation)
aA bB ⇌ cC dD (At a certain temperature) 2 .303 R T2 T1
According to law of mass action The effect of temperature can be studied in the following three cases
(i) When H 0 i.e., neither heat is evolved nor absorbed
Rate of forward reaction [ A]a [B]b k f [ A]a [B]b
log K 2 log K1 0 or log K 2 log K1 or K 2 K1
Rate of backward reaction [C]c [D]d kb [C]c [D]d
Thus, equilibrium constant remains the same at all temperatures.
At equilibrium ,
(ii) When H = +ve i.e., heat is absorbed, the reaction is endothermic. The
Rate of forward reaction = Rate of backward reaction
temperature T2 is higher than T1 .
k f [ A] [B] k b [C] [D]
a b c d
log K 2 log K1 ve or log K 2 log K1 or K 2 K1
kf [C]c [D]d
Kc The value of equilibrium constant is higher at higher temperature in
kb [ A]a [B]b
case of endothermic reactions.
Where, K c is called equilibrium constant.
(iii) When H = – ve, i.e., heat is evolved, the reaction is exothermic. The
In terms of partial pressures, equilibrium constant is denoted by
temperature T2 is higher than T1 .
K p and
PCc PDd log K 2 log K1 ve or log K1 log K 2 or K1 K 2
Kp
PAa PBb The value of equilibrium constant is lower at higher temperature in
the case of exothermic reactions.
In terms of mole fraction, equilibrium constant is denoted by K x
(7) The value of the equilibrium constant depends upon the
and
stoichiometry of the chemical equation.
( X C )c ( X D )d
Kx For the reaction
( X A )a ( X B )b
Relation between K , K and K 2SO 3 (g) ⇌ 2SO 2 (g) O2 (g) and SO 3 (g) ⇌ SO 2 (g) O2 (g)
p c x
Chapter
8
Chemical Equilibrium
Whenever we hear the word Equilibrium immediately a picture (2) Irreversible reactions : Reaction in which entire amount of the
arises in our mind an object under the influence of two opposing forces. For reactants is converted into products is termed as irreversible reaction.
chemical reactions also this is true. A reaction also can exist in a state of
(i) Characteristics of irreversible reactions
equilibrium balancing forward and backward reactions.
(a) These reactions proceed only in one direction (forward
Reversible and Irreversible reactions direction),
A chemical reaction is said to have taken place when the (b) These reactions can proceed to completion,
concentration of reactants decreases, and the concentration of the products
increases with time. The chemical reactions are classified on the basis of the (c) In an irreversible reaction, G < 0,
extent to which they proceed, into the following two classes;
(d) The arrow () is placed between reactants and products,
(1) Reversible reactions : Reaction in which entire amount of the
reactants is not converted into products is termed as reversible reaction. (ii) Examples of irreversible reactions
(i) Characteristics of reversible reactions (a) Neutralisation between strong acid and strong base e.g.,
(a) These reactions can be started from either side,
NaOH HCl NaCl H 2 O 13.7 kcal
(b) These reactions are never complete,
(c) These reactions have a tendency to attain a state of equilibrium, (b) Double decomposition reactions or precipitation reactions e.g.,
in which Free energy change is zero (G = 0), BaCl2(aq) H 2 SO 4 (aq) BaSO4 (s) 2 HCl(aq)
(d) This sign (⇌) represents the reversibility of the reaction, (c) Thermal decomposition, e.g.,
(ii) Examples of reversible reactions MnO 2 ,
2 KClO3 (s) 2 KCl (s) 3O 2
(a) Neutralisation between an acid and a base either of which or
both are weak e.g., (d) Redox reactions, e.g.,
CH 3 COOH Na OH ⇌ CH 3 COONa H 2 O SnCl 2(aq) 2 FeCl3 (aq) SnCl 4 (aq) 2 FeCl2(aq)
(b) Salt hydrolysis, e.g.,
Equilibrium and Its dynamic nature
Fe Cl 3 3 H 2 O ⇌ Fe OH 3 3 HCl
“Equilibrium is the state at which the concentration of reactants
(c) Thermal decomposition, e.g., and products do not change with time. i.e. concentrations of reactants and
products become constant.”
PCl5 (g ) ⇌ PCl3 (g) Cl 2(g) Q
Products
(d) Esterification, e.g.,
CH 3 COOH C2 H 5 OH ⇌ CH 3 COOC 2 H 5 H 2 O
Concentration
(e) Evaporation of water in a closed vessel, e.g.,
Reactants
H 2 O(l) ⇌ H 2 O(g) Q
Time Equilibrium state
, Chemical Equilibrium 305
The important characteristics of equilibrium state are, K p Kc (RT )n
(1) Equilibrium state can be recognised by the constancy of certain
measurable properties such as pressure, density, colour, concentration etc. K p K x (P)n
by changing these conditions of the system, we can control the extent to n = number of moles of gaseous products – number of moles of
which a reaction proceeds. gaseous reactants in chemical equation.
(2) Equilibrium state can only be achieved in close vessel. As a general rule, the concentration of pure solids and pure liquids
(3) Equilibrium state is reversible in nature. are not included when writing an equilibrium equation.
(4) Equilibrium state is also dynamic in nature. Value of Relation between Kp Units of Kp Units of Kc
(5) At equilibrium state, n and Kc
0 Kp = Kc No unit No unit
Rate of forward reaction = Rate of backward reaction
>0 Kp > Kc (atm)n (mole l–1)n
(6) At equilibrium state, G = 0, so that H = TS. <0 Kp < Kc (atm)n (mole l–1)n
Characteristics of equilibrium constant
(1) The value of equilibrium constant is independent of the original
Rate of reaction
G= 0 concentration of reactants.
(2) The equilibrium constant has a definite value for every reaction
Equilibrium state
at a particular temperature. However, it varies with change in temperature.
(3) For a reversible reaction, the equilibrium constant for the
Time
forward reaction is inverse of the equilibrium constant for the backward
Law of mass action and Equilibrium constant reaction.
On the basis of observations of many equilibrium reactions, two 1
In general, K forward reaction
Norwegian chemists Goldberg and Waage suggested (1864) a quantitative K backward reaction
relationship between the rates of reactions and the concentration of the
reacting substances. This relationship is known as law of mass action. It (4) The value of an equilibrium constant tells the extent to which a
states that reaction proceeds in the forward or reverse direction.
“The rate of a chemical reaction is directly proportional to the (5) The equilibrium constant is independent of the presence of
product of the molar concentrations of the reactants at a constant catalyst.
temperature at any given time.” (6) The value of equilibrium constant changes with the change of
The molar concentration i.e. number of moles per litre is also called temperature. Thermodynamically, it can be shown that if K1 and K 2 be
active mass. It is expressed by enclosing the symbols of formulae of the the equilibrium constants of a reaction at absolute temperatures T1 and
substance in square brackets. For example, molar concentration of A is expressed
as [A]. T2 . If H is the heat of reaction at constant volume, then
Consider a simple reversible reaction H 1 1
log K 2 log K1 (Van’t Hoff equation)
aA bB ⇌ cC dD (At a certain temperature) 2 .303 R T2 T1
According to law of mass action The effect of temperature can be studied in the following three cases
(i) When H 0 i.e., neither heat is evolved nor absorbed
Rate of forward reaction [ A]a [B]b k f [ A]a [B]b
log K 2 log K1 0 or log K 2 log K1 or K 2 K1
Rate of backward reaction [C]c [D]d kb [C]c [D]d
Thus, equilibrium constant remains the same at all temperatures.
At equilibrium ,
(ii) When H = +ve i.e., heat is absorbed, the reaction is endothermic. The
Rate of forward reaction = Rate of backward reaction
temperature T2 is higher than T1 .
k f [ A] [B] k b [C] [D]
a b c d
log K 2 log K1 ve or log K 2 log K1 or K 2 K1
kf [C]c [D]d
Kc The value of equilibrium constant is higher at higher temperature in
kb [ A]a [B]b
case of endothermic reactions.
Where, K c is called equilibrium constant.
(iii) When H = – ve, i.e., heat is evolved, the reaction is exothermic. The
In terms of partial pressures, equilibrium constant is denoted by
temperature T2 is higher than T1 .
K p and
PCc PDd log K 2 log K1 ve or log K1 log K 2 or K1 K 2
Kp
PAa PBb The value of equilibrium constant is lower at higher temperature in
the case of exothermic reactions.
In terms of mole fraction, equilibrium constant is denoted by K x
(7) The value of the equilibrium constant depends upon the
and
stoichiometry of the chemical equation.
( X C )c ( X D )d
Kx For the reaction
( X A )a ( X B )b
Relation between K , K and K 2SO 3 (g) ⇌ 2SO 2 (g) O2 (g) and SO 3 (g) ⇌ SO 2 (g) O2 (g)
p c x
