CHEMICAL EQUILIBRIUM

REVERSIBLE REACTIONS

Reactions which do not always proceed to completion and may be made to proceed in the opposite direction under suitable conditions are called reversible reactions e.g.

3Fe + 4H2O Fe3O4 + 4H2

H2 + I2 2HI

N2O4 2NO2

N2 + 3H2  2NH3

IRREVERSIBLE REACTIONS

Reactions which always proceed to completion in one direction only are called irreversible reactions.

 

       

         

CHEMICAL EQUILIBRIUM

When a reversible reaction is carried out in a closed vessel a stage reached when the speed of the forward reaction equals the speed of the backward reaction and chemical equilibrium is said to be established.

CHARACTERISTICS OF CHEMICAL EQUILIBRIUM

1. Equilibrium can be attained from either side.
2. Equilibrium is dynamic in nature i.e. at equilibrium, reaction does not stop.
3. At equilibrium there is no change in the concentration of various species.
4. The equilibrium state remains unaffected by the presence of catalyst. Catalyst helps to attain the equilibrium state rapidly.
5. It can be achieved in a closed container.
6. The observable properties of the process become constant and remain unchanged.

EQUILIBRIUM STATE AND FREE ENERGY CHANGE

At equilibrium G is equal to zero and we have

G = H – TS

H = TS

LAW OF MASS ACTION

It was put forward by Guldberg and Waage. The law states that the rate at which a substance reacts is directly proportional to its active mass and the rate of a chemical reaction is directly proportional to the product of the active masses of the reacting substances. For a general reaction

aA + bB   cC + dD

Rate of forward reaction

Rate of backward reaction

where Kf and Kb are velocity constants for forward and backward reactions respectively. At equilibrium point,

Rate of forward reaction = Rate of backward reaction

Kc is called the equilibrium constant.

FACTORS INFLUENCING EQUILIBRIUM CONSTANT

EQUILIBRIUM CONSTANT IS NOT INFLUENCED BY

1. Concentration of reactants and products.
2. Presence of a catalyst.
3. Pressure.
4. Presence of inert materials.
5. The direction from which the equilibrium state is reached.

EQUILIBRIUM CONSTANT IS INFLUENCED BY

1. Temperature : The variation of equilibrium constant is given by Van't Hoff  equation.

where ,   (Kp)1 and (Kp)2 = Equilibrium Constant at temperature T1 & T2

R = Universal Gas Constant

For exothermic reaction : Kp decreases with increase of temperature since Kf decreases.

For endothermic reaction : Kp increases with increase of temperature since Kf increases.

2. The mode of representing the reaction :

The reaction A + B C + D may be written as

   C + D  A + B

            

 

3. Stoichiometric representation of equation :

N2 + 3H2   2NH3

 NH3

          

USE OF PARTIAL PRESSURE INSTEAD OF CONCENTRATIONS

For gaseous reacting substances partial pressures are conveniently used since at any fixed temperature partial pressure is directly proportional to concentration. For a general reaction

aA + bB   cC + dD

RELATION BETWEEN KC AND KP

where

n = [moles of products – moles of reactants] gaseous only.

RELATION BETWEEN KC AND KP FOR DIFFERENT TYPES OF REACTIONS

1. When n = 0, Kp = Kc e.g. for reaction A B.

2. When n = +ve, Kp > Kc e.g. for reaction A  2B.  

3. When n = –ve, Kp < Kc e.g. for reaction 2A  B.  

UNITS OF Kp AND Kc

• Unit of Kp = (atm) n
• Unit of Kc = (mol lit–1) n

CHARACTERISTICS OF EQUILIBRIUM CONSTANT

1. It has definite value for every chemical reaction at a particular temperature
2. The more is the value of Kc or Kp, the more is the completion of reaction or the more is the concentration of products.
3. When the reaction can be expressed as sum of two other reactions, the Kc of overall reaction is equal to the product of equilibrium constants of individual reactions.

HOMOGENEOUS EQUILIBRIUM

In homogeneous equilibrium the reactants and products are present in the same phase (gaseous or liquid).

2SO2 (g) + O2 (g)  2SO3 (g)

HETEROGENEOUS EQUILIBRIUM

In heterogeneous equilibrium the reactants and products are present in two or more phases.

3Fe(s) + 4H2O(g)  Fe3O4 (s) + 4H2 (g)

CHEMICAL EQUILIBRIUM APPLIED TO HOMOGENEOUS SYSTEM

GASEOUS SYSTEM

They are of two types

1. Gaseous reactions in which the number of moles of products remain the same as that of reactants

 

Hydrogen - iodine equilibrium : Suppose, a moles of H2 and b moles of I2 are present in a container of V litres. At equilibrium x moles of each have combined to form HI.

           

Initial molar conc.             0

Eqb. molar conc.         

Applying the law of chemical equilibrium

 ..........(i)

The equilibrium constant written as Kc indicates that active masses are expressed in terms of molar concentrations.

The eq. (i) does not contain the volume term. Thus equilibrium is independent of volume and therefore of pressure.

2. Gaseous reactions in which the number of moles of products and reactants are different.

        

Dissociation of PCl5 : Suppose  'a' moles of PCl5 are present in a container of V litres. At equilibrium x moles have dissociated.

              

Initial molar Conc.         0         0

Eqb. molar Conc.                        

Applying the law of chemical equilibrium

 .......(i)

The eq.(i) contains the V term in denominator . If volume increases, the dissociation of PCl5 must also increase to keep Kc constant. The decrease of pressure will cause increase in volume and so the dissociation.

If the value of x is small then   ; 

LIQUID SYSTEM

Examples are :

1. Esterification of acetic acid

At equilibrium 2/3rd of acetic acid is converted into ester. Hence alcohol consumed will also be 2/3rd.

2. Reaction between amylene and tricholoroacetic acid

CHEMICAL EQUILIBRIUM APPLIED TO HETEROGENEOUS SYSTEM

1. Dissociation of calcium carbonate

Applying the law of chemical equilibrium

The active mass of a solid reactant and product is assumed to have a constant value and is taken as unity. The equilibrium  constant is determined by gaseous substances only. Therefore,

2. Reaction of steam on heated iron

Partial pressures of solid is taken unity.

3. Water gas reaction

Since partial pressure of carbon (solid) is taken as unity, the equilibrium constant is given by

VAN'T HOFF ISOCHORE

A relationship between the equilibrium constant Kp, at  any temperature T and constant pressure P, and heat of reaction H°.

The enthalpy change DH does not vary appreciably with change in partial pressures of reactants and products. Therefore DHº can be taken as DH whatever may be the partial pressures of reactants and products

The integrated form of the equation is

Three important conditions may arise

1. when H = 0 no heat is evolved or absorbed

Equilibrium constant does not change when no change in temperature.

2. when H = +ve i.e. heat is absorbed

Equilibrium constant increases with increase of temperature.

3. when H = –ve i.e. heat is evolved

  ;   Equilibrium constant

decreases with increase of temperature

4. when n = 0 i.e. there is no change in volume during a reaction KP = Kc. The variation of equilibrium constant with temperature is given by

, E heat of reaction at constant volume.

VAN'T HOFF REACTION ISOTHERM

It gives the free energy change of a reaction at any given temperature, pressure and composition of the reacting system.

G = G° + RT ln J

At equilibrium G = 0 then G° = –RT ln Jeq

J stands for reaction quotient of partial pressure of products and reactants.

viz.

Jeq means the partial pressure of the products and the reactants at the equilibrium. Hence Jeq can be replaced by Kp.

G° = –RT ln

HENRY'S LAW

The mass of a gas dissolved per unit volume of solvent is proportional to the pressure of the gas in equilibrium with the solution at constant temperature.

• The volume of the gas dissolved remains the same inspite of increase in pressure.
• The dissolution of a gas in a liquid is spontaneous process (G = 0), accompanied by decrease in entropy (S = –ve). Since G = H – TS, G can only be negative if H is –ve. Therefore dissolution of a gas in a liquid is always exothermic in nature.

FACTORS ALTERING THE STATE OF EQUILIBRIUM - LE CHATELIER'S PRINCIPLE

There are three main factors which alter the state of equilibrium. They are (I) Concentration, (II) Temperature, and (III) Pressure.

Le Chatelier's principle states that if a system at equilibrium is subjected to a change of concentration, pressure or temperature, the equilibrium shifts in the direction that tends to undo the effect of the change.

1. Effect of change of concentration :

If at equilibrium the concentration of one of the reactants is increased, the equilibrium will shift in the forward direction and vice versa. Consider the following equilibrium

 Fe3+ (aq)      + SCN– (aq)         [Fe(SCN)]2+ (aq)

Pale yellow           Colourless       Dark brown

If ferric salt is added the colour of the solution darkens immediately i.e. Fe3+ ions are consumed and more [Fe(SCN)]2+ are formed. If some sulphocyanide salt is added the colour also darkens. If Potassium ferrisulphocyanide capable of giving complex ion [Fe(SCN)]2+ is added the colour lightens to pale yellow.

2. Effect of change in pressure :
1. No effect of pressure on equilibria having same moles of reactants and products e.g. N2 + O2 2NO
   H2 + I2  2HI
2. When there is change in the number of moles the equilibrium will shift in the direction having smaller number of moles when the pressure is increased and vice versa e.g.

N2 + 3H2 2NH3

More pressure more ammonia

PCl5 PCl3 + Cl2

The more the pressure, the lesser the dissociation of PCl5.

3. Effect of temperature :
1. When process is exothermic - Low temperature favours the formation of products.
2. When process is endothermic - High temperature favours the formation of products

e.g. N2 + 3H2  2NH3 + 24.0 kcal.

Since the production of NH3 is exothermic low temperature favours its formation.

4. Effect of addition of inert gas :
1. Addition of Inert gas at constant volume : The total pressure of the system is increased, but the partial pressure of each reactant and product remains the same. Hence no effect on the state of equilibrium.
2. Addition of Inert gas at constant pressure : The total volume is increased, the number of moles per unit volume of each reactant and product is decreased. Hence equilibrium will shift to the side where number of moles are increased e.g.

PCl5 (g)    PCl3 (g) + Cl2 (g)

Introduction of inert gas at constant pressure will shift the equilibrium to right hand side.

5. Effect of catalyst : The presence of catalyst does not change the position of equilibrium. It simply fastens the attainment of equilibrium.

LE CHATELIER'S PRINCIPLE APPLICABLE TO PHYSICAL EQUILIBRIUM

1. Effect of pressure on solubility : The increased pressure, will increase the solubility of a gas and vice versa.
2. Effect of temperature on solubility : The substances which dissolve with the absorption of heat, their solubility will increase with increase of temperature and vice versa e.g. dissolution of NH4Cl, KCl, KNO3 is endothermic which increases with increase of temperature. The dissolution of calcium acetate and Calcium hydroxide is exothermic, their solubility is lowered at higher temperature.
3. Effect of pressure on the melting point of ice :

Ice  liquid water

The ice occupy the more volume than liquid water, so increased pressure will result in melting of ice according to Le Chatelier's principle.

FAVOURABLE CONDITIONS FOR SOME IMPORTANT REACTIONS

Synthesis of ammonia (Haber's process)

N2 (g) + 3H2 (g)  2NH3 (g) + 22.4 kcal

• Low temperature (500°C)
• High pressure (200 – 1000 atm.)
• Excess of N2 and H2

Synthesis of NO (nitric acid birkland eyde process)

N2 (g) + O2 (g)  2NO(g)– 43.2 kcal

• High temperature
• Excess of N2 and O2
• No effect of pressure

Formation of SO3 (sulphuric acid contact process)

2SO2 (g) + O2 (g)  2SO3 + 42.0 kcal

• Low temperature
• High pressure
• Excess of SO2 and O2

Formation of nitrogen dioxide

2NO + O2 2NO2 + 27.8 kcal

• Low temperature
• High pressure
• Excess of NO and O2

Dissociation of nitrogen tetraoxide

N2O4  2NO2 – 14 kcal

• High temperature
• Low pressure
• Excess of N2O4

Oxidation of CO by steam (Bosch process)

CO + H2O  CO2 + H2 + x kcal

• Low temperature
• Excess of steam and CO
• No effect of pressure

Dissociation of PCl5

PCl5 PCl3 + Cl2 – 15 kcal

• High temperature
• Low pressure
• Excess of PCl5

TRIPLE POINT

The temperature and pressure at which the three states of a substance can exist in equilibrium is known as triple point e.g.

Ice (s) water (l)  vapour (g) can exist at 0.0098°C and 4.58 mm.

DEGREE OF DISSOCIATION FROM DENSITY MEASUREMENT

The density of one mole of gas is given by

D = where M = Mol. wt of gas; P = Total pressure.

The volume of the gas increases on dissociation in proportion to increase in the total number of moles, but total weight remains constant. Hence density decreases in the same proportion. Consider dissociation of PCl5. Let x be degree of dissociation

PCl5        PCl3   + Cl2

1 – x  x             x Total moles (1 + x)

where D is the theoretical vapour density and

D= Molecular mass

d is observed vapour density at temperature tºC.

If nx moles of products are formed, then total number of moles after dissociation

1 – x + nx = 1 + x (n – 1)

Tags:

equilibrium meaning equilibrium meaning in hindi equilibrium constant equilibrium movie equilibrium pronunciation equilibrium economics equilibrium state equilibrium price equilibrium and non equilibrium test equilibrium and disequilibrium equilibrium arrow equilibrium antonyms equilibrium and disequilibrium in balance of payment equilibrium analysis equilibrium and its types equilibrium all formulas a equilibrium is set up equilibrium between demand and supply equilibrium biology equilibrium biomechanics equilibrium balance equilibrium body equilibrium byju's equilibrium board equilibrium brain equilibrium constant formula equilibrium chemistry equilibrium class 11 equilibrium constant depends on equilibrium condition equilibrium constant expression equilibrium constant for the reaction equilibrium definition equilibrium definition chemistry equilibrium definition physics equilibrium dialysis equilibrium diagram equilibrium definition in hindi equilibrium distillation is also known as equilibrium definition economics equilibrium equation equilibrium engineering consultancy equilibrium exchange rate equilibrium examples equilibrium expression equilibrium equations in structural analysis equilibrium exercises equilibrium force equilibrium formula equilibrium film equilibrium filmyzilla equilibrium full movie equilibrium flash vaporization equilibrium full movie download equilibrium full movie download in hindi 480p equilibrium graph equilibrium gdp equilibrium gdp formula equilibrium graph chemistry equilibrium graph economics equilibrium gif equilibrium gdp means equilibrium gibbs free energy g equilibrium constant equilibrium hindi meaning equilibrium hindi equilibrium hindi dubbed movie download equilibrium how to pronounce equilibrium hole concentration formula equilibrium handwritten notes equilibrium hindi dubbed movie download filmyzilla equilibrium human body hw equilibrium hw equilibrium equation hw equilibrium calculator hw equilibrium conditions hw equilibrium practice problems equilibrium in hindi equilibrium in economics equilibrium in chemistry equilibrium index of an array equilibrium in physics equilibrium in balance of payment equilibrium in sports equilibrium in hindi meaning equilibrium jee equilibrium jee mains questions equilibrium jee notes equilibrium jee advanced questions equilibrium jee mains equilibrium jee mains notes equilibrium jee mains questions 2021 equilibrium jewellery equilibrium kinetics equilibrium ka hindi meaning equilibrium kya hota hai equilibrium kc equilibrium kp and kc equilibrium khan academy equilibrium kya hai equilibrium katmoviehd k equilibrium formula k equilibrium constant k equilibrium equation k equilibrium calculator k equilibrium potential k equilibrium units k equilibrium and gibbs free energy k equilibrium expression equilibrium level of income equilibrium level equilibrium law equilibrium line altitude equilibrium level of national income equilibrium length of muscle equilibrium level of income can be determined at equilibrium level of output equilibrium moisture content equilibrium meaning in economics equilibrium meaning in english equilibrium meaning in chemistry equilibrium meaning in physics equilibrium ncert equilibrium ncert solutions equilibrium notes equilibrium neet questions equilibrium notes for neet equilibrium ncert solutions pdf equilibrium notes pdf equilibrium neet questions pdf in equilibrium in equilibrium state in equilibrium algebraic sum of moment of force is in equilibrium sum of all forces in equilibrium state of a pn junction diode in equilibrium the average velocity of gas molecules is in equilibrium state the thermodynamic probability of a system is in equilibrium ch3cooh + h2o equilibrium of forces equilibrium of a firm under monopolistic competition equilibrium of firm equilibrium of concurrent forces equilibrium of firm under perfect competition equilibrium of floating bodies equilibrium of a particle equilibrium of forces in engineering mechanics equilibrium point equilibrium position equilibrium potential for an ion is calculated using equilibrium point in array equilibrium physics equilibrium price is also known as equilibrium quantity equilibrium questions equilibrium questions for neet equilibrium quantity formula equilibrium questions and answers equilibrium quotes equilibrium questions class 11 equilibrium quotient q equilibrium constant equilibrium reaction equilibrium reception equilibrium receptor equilibrium relative humidity equilibrium rate equilibrium reaction example equilibrium relates to which of these equilibrium rate of interest equilibrium synonym equilibrium symbol equilibrium solutions equilibrium state in thermodynamics equilibrium superelevation equilibrium solubility equilibrium slideshare equilibrium tablet equilibrium test equilibrium theory equilibrium temperature equilibrium thermodynamics equilibrium tablet uses in hindi equilibrium types equilibrium test for coordination equilibrium under perfect competition equilibrium under monopoly equilibrium under monopolistic competition equilibrium under price discrimination equilibrium under discriminating monopoly equilibrium under oligopoly equilibrium unemployment equilibrium unit equilibrium vapour pressure equilibrium vector equilibrium vs steady state equilibrium value equilibrium vibrational frequency equilibrium vapour equilibrium vedantu equilibrium vapour pressure of water equilibrium weightage in neet equilibrium wage equilibrium wikipedia equilibrium weight equilibrium weightage in jee mains equilibrium wage rate formula equilibrium watch online equilibrium weight formula equilibrium xc boots equilibrium xl massage pad equilibrium xcel hd equilibrium xenon concentration equilibrium yoga equilibrium yaw equilibrium yoga india equilibrium yaw ballistics equilibrium yield equilibrium yoga peterborough equilibrium yield meaning equilibrium youtube equilibrium zone equilibrium znacenje equilibrium zen gym greenville sc equilibrium znaczenie equilibrium 10mg equilibrium 11th chemistry ncert equilibrium 11th chemistry notes equilibrium 123movies equilibrium 11 chemistry equilibrium 18 decillion equilibrium 1984 equilibrium 100 rule 1 equilibrium ball valve equilibrium 2002 equilibrium 2002 movie download in hindi equilibrium 2002 filmyzilla equilibrium 2002 subtitles equilibrium 2002 full movie download equilibrium 2002 dual audio 480p equilibrium 2002 full movie in hindi dubbed equilibrium 2 rs3 2 equilibrium ball valve equilibrium 300mb dual audio equilibrium 3 duluth equilibrium 3 therapeutic equilibrium 3 forces 3 equilibrium equations 3 equilibrium of perfectly competitive firm equilibrium 480p equilibrium 480p download equilibrium 480p dual audio download equilibrium 4 ruthless 3 equilibrium 4k equilibrium 4 vs aftershock 4 equilibrium 4k blu ray equilibrium 4 vs aftershock 3 equilibrium 5 rule equilibrium 5 ruthless 3 equilibrium 500 equilibrium 6 rs3 equilibrium 720p download equilibrium 720p dual audio equilibrium 7 letters equilibrium 7 day detox equilibrium 7 day detox reviews equilibrium 725 equilibrium 7 spa equilibrium 88

equilibrium class 11 notes for neet

equilibrium class 11 notes pdf download

equilibrium class 11 notes byju's

equilibrium class 11 notes pdf download ncert

equilibrium class 11 notes topperlearning

equilibrium class 11 notes physics wallah

equilibrium class 11 notes examfear

equilibrium class 11 notes chemistry chapter 7

equilibrium class 11 notes pdf

equilibrium class 11 short notes

notes of equilibrium class 11

equilibrium class 11 notes chemistry pdf download

equilibrium ncert notes

ch equilibrium class 11 notes

notes on equilibrium class 11

equilibrium notes for jee mains

notes of equilibrium class 11 pdf

equilibrium class 11 ncert notes

notes for equilibrium class 11

equilibrium class 11th

equilibrium class 11 pdf

short notes on equilibrium class 11

chapter 7 equilibrium notes