Chemical Equilibrium

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Demonstration of Chemical Equilibrium:

  • Hydrogen and iodine are heated in a closed vessel. he reacting mixture is deep violet in colour due to the presence of iodine vapours. At 717 K the reaction between the reactants takes place. Gradually the deep violet colour of the mixture becomes faint indicating that iodine is being consumed.
  • After some time it is observed that the intensity of violet colour becomes constant, indicating that the reaction is stopped although both hydrogen and iodine are present. This happens because equilibrium is reached. Thus the reaction is reversible and can be represented by

H2(g) +  I2(g)  ⇌    2HI(g)

Chemical equilibrium:

  • Chemical equilibrium is a state of a system of reacting substances at which the rate of the forward reaction is equal to the rate of backward reaction.

Explanation:

  • Consider general reversible reaction A + B  ⇌  C + D
  • At the start of the reaction, only reactants are present. Hence concentrations of A and B are maximum and that of C and D are minimum (zero). Hence the rate of the forward reaction is maximum and that of the backward reaction is zero.
  • As the reaction proceeds reactants A and B react to produce products C and D. Thus the concentrations of reactants A and B decrease and that of products increases. Thus the rate of the forward reaction decreases and that of the backward reaction increases.
  • A point will be reached when the rate of the forward reaction is equal to the rate of the backward reaction. This state of the of reaction is called the chemical equilibrium.
  • At chemical equilibrium concentration of reactants and product remains unchanged throughout.  It means that at equilibrium neither forward nor backward reaction stops, but are continued at equal rates in opposite directions.  Hence we can say that the equilibrium is dynamic and not static.
  • It is to be noted that the reversible reactions involving gaseous substances are carried out in a closed vessel.

Graphical Representation:

Chemical Equilibrium 02

Chemical Equilibrium a Dynamic Equilibrium:

  • Chemical equilibrium is a state of a system of reacting substances at which the rate of the forward reaction is equal to the rate of the backward reaction
  • At chemical equilibrium concentration of reactants and product remains unchanged throughout.  It means that at equilibrium neither forward nor backward reaction stops, but are continued at equal rates in opposite directions.
  • The concentrations of reactants and products at chemical equilibrium are constant. At the same time, all the observable properties of the system become constant. Hence we can say that the equilibrium is dynamic and not static.


Characteristics of Chemical Equilibrium:

  • Chemical equilibrium exists in reversible reactions only.
  • At equilibrium, the rate of the forward reaction is equal to the rate of the backward reaction.
  • At equilibrium, the concentrations of reactants and products remain constant. These concentrations are called equilibrium concentrations.
  • At equilibrium, both the forward reaction and the backward reaction do not stop. Actually, all the reactants and products are present at the equilibrium
  • At equilibrium, neither the forward reaction nor the backward reaction has ceased. Both the reactions proceed in opposite directions with the equal rate. Thus chemical equilibrium is dynamic in nature.
  • At the chemical equilibrium, the observable properties of the system like pressure, colour, concentrations, etc. become constant and remain unchanged thereafter.
  • The equilibrium can be approached from either direction.
  • Equilibrium can only be attained only if the system is closed.
  • State of chemical equilibrium is unaffected by catalyst because the presence of catalyst equally increases the speed of both the forward and the backward reaction.
  • State of chemical equilibrium changes with the change in the factors like concentration, temperature and pressure.

Factors Affecting Chemical Equilibrium:

Effect of the  Change of Concentration:

  • Effect:
  • If the concentration of reactants increases then there will be a rise in the concentration of the products and thus equilibrium is shifted from left to right.
  • Explanation: 
  • By the law of mass action, the rate of a chemical reaction is directly proportional to the product of active masses of reactants, at given temperature at that instant.
  • As the concentration of reactants is increased, the product of concentrations of reactants increases thus to keep the value of equilibrium constant the same the product of concentrations of products is increased. Thus more products are formed. Hence equilibrium is shifted from left to right.

Effect of the Change of Pressure:

  • Change in pressure plays an important role in gaseous reactions.  There can be three types of gaseous reactions:
    • Chemical reactions accompanied by the increase in volume
    • Chemical reactions accompanied by decrease in volume
    • Chemical reactions accompanied by no change in volume
  • Chemical reactions accompanied by the increase in volume:

Consider reaction.

PCI5(g)    ⇌    PCl3(g)   +     Cl2(g)



1 Vol             1 Vol              1 Vol

1 Vol                                2 Vol

  • In this reaction, 1 volume of reactants gives 2 volumes of products.  Thus in this reaction volume is increased.
  • The chemical reactions involving gases and accompanied by the increase in volume are favoured by a reduction in pressure. Thus by decreasing the pressure at equilibrium, equilibrium is shifted towards the right.
  • Chemical reactions accompanied by the decrease in volume:

Consider reaction.

N2(g) +  3 H2(g)   ⇌    2NH3(g)



1 Vol       3 Vol              2 Vol

4 Vol                        2 Vol

  • In this reaction, 4 volumes of reactants give 2 volumes of products.  Thus in this reaction volume is decreased.
  • Chemical reactions involving gases and accompanied by a decrease in the volume are favoured by an increase in pressure. Thus by increasing the pressure at equilibrium, equilibrium is shifted towards the right.
  • Chemical reactions accompanied by no change in volume:

Consider following reaction.

H2(g) + Cl2(g)  ⇌  2HCl(g)

1 Vol        1 Vol             2 Vol



2 Vol                     2 Vol

  • In this reaction, 2 volumes of reactants give 2 volumes of products.  Thus in this reaction volume is not changed.
  • Chemical reactions involving gases and accompanied by no change in volume are not affected by the change in pressure.

Effect of the  Change of Temperature:

  • If the temperature of the exothermic chemical reaction is increased, then the concentration of products reduces and thus the equilibrium is shifted towards left. Hence the reduction in temperature favours exothermic reaction at equilibrium. and increase in temperature favours endothermic reaction.
  • It is to be noted that in a reversible reaction if one reaction is exothermic then another reaction is endothermic. Thus the effect of change of temperature on the two reactions is different.

Effect of Presence of a Catalyst:

  • A catalyst is a substance which increases or decreases the rate of a reaction without taking part in the chemical reaction.
  • In a reversible reaction at equilibrium, catalyst affects the rate of both the forward reaction and the backward reaction to the same extent. Hence catalyst at equilibrium does not affect chemical equilibrium.

Addition of inert gas at constant volume:

  • If the inert gas is added to the reaction at constant volume It will result in the increase of the total pressure of the system. At the start, the partial pressures of the reactants and pressure will get changed. But in short time they will attain their equilibrium values before addition of the inert gas. Thus partial pressures of reactants and products are not getting affected. Hence there is no effect of the addition of inert gas on equilibrium at constant volume.

Addition of inert gas at constant pressure:

If the inert gas is added to the reaction at constant pressure It will result in an increase in the volume of the system. Thus the concentration of the reactants and products will decrease. To counterbalance this stress (change) the equilibrium will shift to the side where the number of moles are increased.

 



Gibb’s Energy and Chemical Equilibrium:

Gibb’s Energy or Free Energy:

  • Gibb’s energy or free energy is a thermodynamic function which helps us in the development of a criterion of spontaneity or a feasibility of a process. It refers to the capacity of the system to do useful work.
  • Gibb’s energy is defined as the amount of energy available from a system at given set of conditions that can be put into useful work.

Mathematically,   G = U + PV – TS

Where      G = Gibb’s energy



U = Internal energy of the system

P = Pressure of the system

V = Volume of the system

T = Absolute temperature of the system

S = Entropy of the system



The absolute value of Gibb’s energy cannot be calculated. But the change in it can be calculated as

ΔG = ΔU + Δ(PV) – Δ(TS)

For constant pressure and constant temperature process

ΔG = ΔU + PΔV – TΔS



But ΔU + PΔV = ΔH

∴ ΔG = ΔH – TΔS

This relation is known as Gibb’s Helmholtz equation.

Where,      ΔG = Change in Gibb’s energy

ΔH = Change in  enthalpy of the system

T = Absolute temperature of the system



ΔS = Change in  entropy of the system

The spontaneity of a Reaction w.r.t. Gibb’s Energy and Total Entropy:

  • Let ΔSTotal be the total enthalpy of the system and ΔGT,P is Gibb’s energy at constant temperature and pressure.
  • If ΔSTotal > 0 (positive) or ΔGT,P < 0 (negative), the process will be spontaneous and proceeds in a backward direction. It nears to completion. (K > 1)
  • If ΔSTotal < 0 (negative) or ΔGT,P > 0 (positive), the process will be non-spontaneous. and favoured in backward direction. (K < 1)
  • If ΔSTotal = 0 or ΔGT,P = 0, the process will be in equilibrium state. (K = 1)

Relation Between Gibb’s Energy and Chemical Equilibrium:

  • For spontaneous process Gibb’s energy is negative.
  • For a reversible reaction, there is a decrease in the Gibb’s energy during the course of reaction whether we start from reactants or products.

Let us consider a hypothetical reaction

A  ⇌ B

  • A graph is drawn by taking Gibb’s energy on the y-axis and the change in the composition of the reacting mixture with time on the x-axis. The graph is as follows.

Factors affecting chemical Equilibrium 01



  • The minima in the curve correspond to the composition of the reaction mixture at the equilibrium state at which Gibb’s energy is minimum.
  • From graph following points should be noted.
    • In reaching the equilibrium state whether from A or from B, the ΔG is negative.
    • At equilibrium state, there is no change in Gibb’s energy. i.e. ΔG at equilibrium = 0.
    • If minima of the curve lie very close to the products, then it means that the equilibrium composition strongly favours the products and hence K >> 1. i.e. reaction will proceed to completion.
    • If minima of the curve lie very close to the reactants, then it means that the equilibrium composition strongly favours the reactants and hence K < 1. i.e. the reaction hardly proceeds.

Relation Between Standard Gibb’s Energy change and Equilibrium Constant:

Let us consider a hypothetical reaction

A + B  ⇌  C + D

  • The relation between Gibb’s energy change (ΔG)and standard Gibb’s energy change ΔGis given by

ΔG = ΔGo  + RT ln Q

Where      R = Universal gas constant

T = Absolute temperature of the system

Q = Concentration coefficient



At equilibrium ΔG = 0 and Q = Kc

∴   0 = ΔGo  + RT ln Kc

∴    ΔGo  = – RT ln Kc

∴    ΔGo  = – 2.303 RT log 10 Kc

This is the relation between standard Gibb’s energy change and equilibrium constant.

In exponential form, the expression can be written as

Factors affecting chemical Equilibrium 02

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