Chemical Equilibrium Ipe

CHEMICAL EQUILIBRIUM

Prepared by V. Aditya vardhan adichemadi @ gmail.com

1

CHEMICAL EQUILIBRIUM Reversible reaction : A reaction which proceeds through forward and backward directions is called reversible reaction. In a reversible reaction, reactants are converted into products and vice versa. Eg., 1) H2(g) + I2(g)

2HI(g) CaO (s) + CO2 (g)

3) N2(g) + 3H2(g)

2NH3(g)

VA RD HA N

2) CaCO3(s)

2) 2KClO3(s)

CO2 (g) + 2 H2O (l)

Pr ep ar ed

1) CH4 (g) + 2O2 (g)

DE by VI W JU AR N AN IOR G C AL O LL EG E

Irreversible reaction: A reaction that occur only in direction is called irreversible reaction i.e., only reactants are converted to products and conversion of products to reactants is not possible. Eg.,

2KCl(s) + 3O2(g)

V. AD VA I AG T

YA

Chemical equilibrium: The state at which the rate of forward reaction becomes equal to the rate of backward reaction.

rate of forward reaction

state of equilibrium

rate of backward reaction

Characteristics of chemical equilibrium 1) At equilibrium, the rates of forward and backward reactions are equal. 2) The observable properties such as pressure, concentration, colour, density etc., of the system remain unchanged. 3) Chemical equilibrium is a dynamic equilibrium, because both the forward and backward reactions continue to occur. 4) Equilibrium can be reached by starting the reaction either from the reactants side or from the products side. 5) Pressure and concentration affect the state of equilibrium but do not change the equilibrium constant. Where as, temperature can affect both the state of equilibrium and equilibrium constant. 6) A catalyst can increase the rates of both forward and backward reactions and helps in attaining the equilibrium faster. But it does not affect the state of equilibrium.

CHEMICAL EQUILIBRIUM

Prepared by V. Aditya vardhan adichemadi @ gmail.com

2

Types of chemical equilibria Chemical equilibria are classified into two types as homogeneous and heterogeneous. Homogeneous equilibrium: A chemical equilibrium is said to be homogeneous if all the substances (reactants and products) at equilibrium are in the same phase. Eg., 1) H2(g) + I2 (g)

2HI (g)

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

2NH3(g)

DE by VI W JU AR N AN IOR G C AL O LL EG E

2) NH4HS (s)

CaO (s) + CO2 (g)

NH3(g) + H2S(g)

Pr ep ar ed

1) CaCO3(g)

VA RD HA N

Homogeneous equilibrium: A chemical equilibrium is said to be heterogenous if all the substances at equilibrium are not in the same phase. Eg.,

YA

Law of mass action (proposed by Guld berg & Wage) The rate of a reaction at an instant of time is proportional to the product of active masses of the reactants at that instant of time under given conditions.

V. AD VA I AG T

i) For dilute solutions, the molar concentrations are taken as active masses. ii) For gases at low pressures, the partial pressures are taken as active masses. iii) The active mass of a solid is taken as unity. Equilibrium constant (Kc) in terms of concentration Consider the following reaction at equilibrium, aA + bB

cC + dD

By applying law of mass action, The rate of forward reaction 

v f  [A]a [B]b

or

v f =K f [A]a [B]b

The rate of backward reaction  v b  [C]c [D]d or where

v b =K b [C]c [D]d

[A], [B], [C] and [D] are the equilibrium concentrations of A,B,C and D respectively. Kf and Kb are the rate constants of forward and backward reactions respectively.

But at equilibrium, rate of forward reaction (vf) = rate of backward reaction (vb) i.e.,

K f [A]a [B]b  K b [C]c [D]d

CHEMICAL EQUILIBRIUM

3

or

K f [C]c [D]d = K b [A]a [B]b

or

[C]c [D]d Kc = [A]a [B]b Kf and is known as equilibrium constant. Kb

Kc =

In general,

product of concentrations of products at equilibrium product of concentrations of reactants at equilibrium

VA RD HA N

where K c =

Prepared by V. Aditya vardhan adichemadi @ gmail.com

Units of K c  (mole.L-1 ) n

Where n = (c+d)-(a+b) = no. of moles of products - no. of moles of reactants

PC c PD d PA a PB b

Pr ep ar ed

KP =

DE by VI W JU AR N AN IOR G C AL O LL EG E

Relation between Kc and Kp Kp is the equilibrium constant in terms of partial pressures. If A, B, C and D are gases then for above reaction, Kp can be written as

nRT =CRT V

V. AD VA I AG T

P=

YA

where PA, PB, PC and PD are the partial pressures of A, B, C and D at equilibrium. From ideal gas equation, where C=molar concentration

[C]c [D]d x[ RT ](c  d )( a b) Therefore K P = a b [A] [B] or

K P =K c [RT]Δn

where

Δn=(c+d)-(a+b) = no. of moles of gaseous products - no.of moles of gaseous reactants

If

Δn=0

then

K p =K c

and if

Δn>0

then

K p >K c

and if

Δn<0

then

K p
Illustrations 1) H2(g) + I2(g)

2HI(g)

[HI]2 Kc = [H 2 ] [I2 ] PHI2 Kp = PH . PI 2 2

K P =K c [RT]Δn

CHEMICAL EQUILIBRIUM

4

Prepared by V. Aditya vardhan adichemadi @ gmail.com

Δn=(2)-(1+1)=0 

Kp = Kc

2) PCl5(g)

PCl3(g) + Cl2(g) [PCl3 ][Cl 2 ] [PCl5 ]

Kc =

Kp =

PPCl3 .PCl2 PPCl 5

Kp > Kc

Kp =

[NH 3 ]2 [N 2 ][H 2 ]3 PNH3 2 PN2 .PH2 3

K P =K c [RT]Δn



K p
4) 2NH3(g)

V. AD VA I AG T

Δn = (2) - (1+3) = -2

YA

Kc =

2NH3(g)

DE by VI W JU AR N AN IOR G C AL O LL EG E

3) N2(g) + 3H2(g)

Pr ep ar ed



VA RD HA N

Δn = (1+1)-(1) = +1

N2(g) + 3H2(g)

[N 2 ][H 2 ]3 Kc = [NH3 ]2 Kp =

PN2 .PH2 3 PNH3 2

Δn = (1+3) - (2) = +1 

Kp > Kc

5) CaCO3(s)

CaO (s) + CO2 (g)

K c =[CO 2 ] K p =PCO2

Δn=(1)-(0)=+1 

Kp > Kc

Problems : 1) The molar concentrations of A, B & C at equilibrium for the reaction 2A + B & 3 moles / litre respectively calculate Kc

3C are 1,2

CHEMICAL EQUILIBRIUM

Prepared by V. Aditya vardhan adichemadi @ gmail.com

5

2) Kc value of the following reaction is 0.4 H 2(g) + I2(g) 2HI(g) if the equilibrium concentrations of H2 & HI are 0.2 and 0.8 moles/litre respectively. What is the concentration of I2 at equilibrium. 3) The Kc of a reversible reaction is 5. if the rate constant of forward reaction is 2.5 what is the rate constant for backward reaction. 4) Give the relations between Kc and Kp for the following reactions .  CH 3COONa (i) CH 3COOC 2 H 5(aq) + NaOH (aq) 

(aq)

+ C 2 H 5OH (aq)

 NH 3(g) + H 2S(g) (ii) NH 4 HS(s) 

VA RD HA N

5) For the following reversible reaction Kc is 0.5 L2/m2  2NH 3 (g) N 2(g) +3H 2(g)  Calculate the Kp value at 270C.

Pr ep ar ed

DE by VI W JU AR N AN IOR G C AL O LL EG E

Le Chatelier's principle When external stress is applied on a system at equilibrium, the system shifts the position of equilibrium so as to nullify the effect of stress. Stress can be applied by changing the concentration or pressure or temperature. The effects of these factors are discussed below.

V. AD VA I AG T

YA

Effect of concentration: 1) The forward reaction is favored when the concentration of reactants is increased. 2) The backward reaction is favored when the concentration of products is increased. Effect of pressure: 1) When pressure is increased, the system tries to decrease the pressure by favoring the reaction in which the number of moles of gaseous substances are decreased. 2) When pressure is decreased, the system tries to increase the pressure by favoring the reaction in which the number of moles of gaseous substances are increased. Effect of temperature: 1) When the temperature is increased, the endothermic reaction is favored. 2) When the temperature is decreased, the exothermic reaction is favored. Industrial applications Haber's process (synthesis of ammonia) In this process, ammonia is synthesized from nitrogen and hydrogen gases. Le Chatelier's principle can be applied to improve the yield of ammonia as follows.

 2NH3 (g) ; H = -92 kJ N 2 (g) + 3H 2 (g)  Effect of pressure: The number of moles gaseous substances decreases from (4 to 2) in the forward reaction. Hence increase in pressure favors the synthesis of ammonia. In the industrial process, the reaction is carried out at high pressures (200 atm). Effect of temperature: Forward reaction is exothermic. Hence increase in temperature favors the dissociation of ammonia. But at low temperatures the reaction does not occur. Hence the reaction is carried out at moderate temperatures (725 - 775 K). To increase the rate of reaction, finely powdered iron is used as catalyst and Mo is used as promoter.

CHEMICAL EQUILIBRIUM

6

Prepared by V. Aditya vardhan adichemadi @ gmail.com

Favourable conditions: Pressure Temperature Catalyst Promoter

200 atm 725 to 775 K Fe Mo

Contact process (industrial synthesis of SO3) In this process, sulfur trioxide is synthesized by oxidizing sulfur dioxide. Le Chatelier's principle can be applied to improve the yields as follows.  2SO 3 ( g ) ; ΔH = -189 kJ 2SO 2 ( g )+ O 2 ( g ) 

VA RD HA N

Effect of pressure: The number of moles of gaseous substances decreases from (3 to 2) in the forward reaction. Hence increase in pressure favors the formation of sulfur trioxide. But at high pressures, the iron towers used in the reaction are corroded. To avoid this moderate pressures (10 atm) are employed.

Pr ep ar ed

DE by VI W JU AR N AN IOR G C AL O LL EG E

Effect of temperature: Forward reaction is exothermic. Hence low temperatures favor the synthesis of sulfur trioxide. But at low temperatures the reaction does not occur. Hence the reaction is carried out at moderate temperatures (673 K). To increase the rate of reaction, platinized asbestos or V2O5 are used as catalysts. Favorable conditions 10 atm 673 K Pt or V2O5

V. AD VA I AG T

YA

Pressure Temperature Catalyst

TEST YOUR UNDERSTANDING State whether the following statements are true or false. 1) A catalyst alters only the rate of forward reaction at equilibrium. 2) The forward reaction is favoured when pressure is increased on the following reaction system at equilibrium.  2C( g ) A( g )  B( g ) 

3) The synthesis of ammonia is favoured by removing ammonia formed, in Haber's process, by liquefying it. 4) The value of equilibrium constant Kc can be changed by changing the temperature. 5) Kc > Kp for the following reaction.  CaCO3( s )  H 2O( aq ) Ca (OH ) 2  CO2( g ) 