Equilibrium: Le Chatelier’s Principle, Henry’s Law, Chemical Equilibrium

Have you ever played tug of war Has a situation arisen where your team and opponent team are pulling the rope with all strength and the rope does not move back and forth This state is the state of Equilibrium. In Chemistry, the state at which both the reactants and products become equal in respect to their concentrations and molecules is called the state of Equilibrium. 

Keyterms: Equilibrium, molecules, concentrations, stoichiometric coefficient, vapou, liquid, solid, chemical equilibrium, Mixture 

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Equilibrium

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Equilibrium is defined as the state when the products, as well as reactants, are restricted to undergo any further change. Thus in the case of Equilibrium, the rate of backward and forward reaction remains constant.

Equilibrium is achieved when molecules returning to the liquid from vapour becomes equal to the molecules leaving the liquid to vapour. There can’t be any net alteration among the number of reactants and products when a reaction comes to chemical equilibrium. It means that the concentration of both remains constant. 

K_c stands for equilibrium constant and is defined as the concentration of products divided by the concentration of reactants where each term is raised to the stoichiometric coefficient.

K_c = [C]^c[D]^d / [A]^a[B]^b

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Equilibrium: Le Chatelier’s Principle

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Equilibrium law or Le Chatelier’s principles are used to predict the effect of changes in temperature or pressure on a system that is in chemical equilibrium. The principle is named after the French chemist Henry Louis Le Chatelier. 

Le Chatelier’s Principle states that equilibrium adjusts the backward and forward reactions in such a way as to accept the effect of a change affecting the equilibrium conditions.

When factors that affect equilibrium like concentration, pressure, temperature, inert gases are changed, the equilibrium will shift in the direction where the effects caused by these changes are not valid.

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Chemical Equilibrium

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Chemical equilibrium in a chemical reaction is defined as the state at which there is no additional change in the concentration of products and reactants with respect to time. 

For example,

N₂ + 3H₂ ↔ 2NH₃

At equilibrium, the rate of backward reaction is equal to the rate of forwarding reaction. 

Equilibrium mixture: 

The mixture of products and reactants is in a state of equilibrium is called equilibrium mixture. On the basis of the extent to which the reactions proceed in order to reach the equilibrium state, these may be further subdivided into three distinct groups:

(i) The reactions which progress towards completion with negligible concentrations of the reactants.

(ii) The reactions in which only small amounts of products are formed and most of the reactants remain unchanged.

(iii) The reactions in which the concentrations of both the products and reactants are comparable when the system is in equilibrium.

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Equilibrium in Physical Processes

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Solid-Liquid Equilibrium: 

The equilibrium is represented as:

H₂O(s) ↔ H₂O (l)

Rate of melting of ice = Rate of freezing of water.

Here, the system is kept in dynamic equilibriums, and from this, it can be inferred that:

(a) The opposing processes occur simultaneously in both. 

(b) The same rate is maintained for both the processes and for that reason the amount of water and ice will remain constant.

Liquid-Vapour Equilibrium: 

The equilibrium can be represented as

H₂O (l) ↔ H₂O (vapour)

Rate of evaporation = Rate of condensation

Liquid-vapour equilibrium can be defined when there is an equilibrium between vapours and liquid.

Solid-Vapour Equilibrium:

Solid-vapour equilibrium is generated when solids sublime to vapour phase. For example, when solid iodine is placed in a closed vessel, violet vapours start appearing in the vessel and the intensity of which keeps on increasing with time till it becomes constant.

The equilibrium can be represented as:

I₂ (S) ↔ I₂ (Vapour)

Rate of sublimation of solid to form vapour = Rate of condensation of vapour to form Solid.

Equilibrium involving Dissolution of Solid in Liquid

The equilibrium between a solid and its solution is represented by the saturated solution and may be represented as

Sugar (in solution) ↔ Solid (solid)

Here precipitation and dissolution take place at the same rate.

Equilibrium between a Gas and its Solution in Liquid

Equilibrium between gas and its solution in liquid can be explained with the following example: 

Let us consider, CO₂ gas inside a sealed soda water bottle that is dissolved under high pressure. A state of equilibrium is attained between vapours of the gas and CO₂ present in the solution.

CO₂ (g) ↔ CO₂ (present in solution)

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Equilibrium: Henry’s Law 

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The solubility of a gas in liquid at a certain temperature is governed by Henry's law. Henry’s Law states that the mass of gas dissolved in a given mass of solvent at any temperature is directly proportional to the gas pressure on the surface of the solvent.

m ∝ p

m = K_Hp (where, K_H = Henry’s Constant)

Characteristics of Equilibrium Involving Physical Processes

• At a given temperature, the equilibrium can be achieved only when the system is closed. 
• The equilibrium is dynamic since both the backward and forward processes occur at an equal rate.
• At constant temperature, the concentrations of substances become constant when it is under equilibrium.
• The value of the equilibrium constant represents the progress of the process before equilibrium is achieved.

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Equilibrium in Homogeneous System

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Homogeneous system is when the reactants and products are in the same phase in a system involving reversible reaction.

For Example,

H₂ (g) + I₂ (g) ↔ 2HI (g)

It can be observed after some time that equilibrium is formed. The equilibrium can be observed by constancy in the colour of the reaction mixture.

Equilibrium in Homogeneous System

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Law of Chemical Equilibrium

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Law of Chemical Equilibrium states that at a constant temperature, the rate of a chemical reaction is directly proportional to the product of the molar concentrations of the reactants each raised to a power equal to the corresponding stoichiometric coefficients as represented by the balanced chemical equation. 

Let us consider a general reversible reaction;

A + B ↔ C + D

After some time, there is a reduction in reactants A and B and an accumulation of the products C and D. As a result, the rate of the forward reaction decreases and that of backward reaction increases. 

Eventually, the two reactions occur at the same rate and a state of equilibrium is attained. 

By applying the Law of Mass Action;

The rate of forward reaction;

R_f = K_f [A]^a [B]^b

The rate of backward reaction;

R_b = K_b [C]^c [D]^d

Where,

[A], [B], [C] and [D] are the concentrations of A, B, C and D at equilibrium respectively.

a, b, c, and d are the stoichiometric coefficients of A, B, C and D respectively.

Kf and Kb­ are the rate constants of forward and backward reactions.

However, at equilibrium,

Rate of forward reaction = Rate of backward reaction.

Rate of forward reaction = Rate of backward reaction.

K_c is called the equilibrium constant expressed in terms of molar concentrations. 

The above equation is known as the equation of Law of Chemical Equilibrium.

Relationship between Equilibrium Constant K, reaction Quotient Q and Gibbs energy G

A mathematical expression of equilibrium in relation to thermodynamics can be described by the following equation.

ΔG = ΔG^Θ + RT ln Q

Where, 

G^Θ is standard Gibbs energy.

At equilibrium, when ΔG= 0 and Q = Kc

ΔG = ΔG^Θ + RT ln K = 0

ΔG^Θ = -RT ln K

ln K = ΔG^Θ / RT

Taking antilog of both sides, we get,

K = e^{-ΔGΘ /RT}

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Factors Affecting Equilibrium

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Effect of Change of Concentration: 

If we change the concentration of any of the products or reactants in a reaction at the state of equilibrium, the configuration of the equilibrium changes so that the effect is minimized.

Effect of Pressure Change: 

There is no effect of pressure if the number of moles of gaseous products and reactants are equal.

The total number of moles per unit volume increases with an increase in pressure. Thus the equilibrium will shift to the direction having a lesser number of moles per unit volume.

Effect of Inert Gas Addition:

There is no effect on equilibrium after the addition of an inert gas if the volume is kept constant. This is because there is no change in the partial pressure or the molar concentration on the addition of inert gas at constant volume.

Effect of Temperature Change:

When the temperature of the system is decreased or increased, the equilibrium shifts in a direction that is opposite in order to neutralize the effect that will be occurring due to the change. In exothermic reaction forward reaction is favored by low temperature e.g.,

N₂ (g) + 3H₂ (g) ↔ 2NH₃ (g)

ΔH = – 92.38 kJmol^-1

In the case of an endothermic reaction, there will be a shift in the equilibrium in the direction of the endothermic reaction with rising in temperature. 

Effect of a Catalyst:

In a reaction mixture, a catalyst has no effect on the equilibrium composition. This is because the catalyst increases with the speed of both forward and backward reactions to the same extent in a reversible reaction.

Equilibrium: Electrolytes

Electrolytes are substances that are responsible for the conduction of electricity in an aqueous solution. Electrolytes include salts, bases and acids. In an aqueous solution, the electricity is conducted due to anions and cations which are ignited by the dissociation or ionization of electrolytes present in the solution.

Equilibrium: Lewis Acid and Lewis Base

A Lewis acid is a species that contains an empty orbital and is capable of accepting an electron pair from a Lewis base to form a Lewis adduct.

A Lewis base is a species in which the orbitals are filled and contains an electron pair that might donate electrons to Lewis acid but generally is not involved in the bonding process.

Electron deficient species like AlCl₃, BH₃, H⁺ etc. and act as Lewis acids while species like H₂O, NH₃ etc. can donate a pair of electrons and act as Lewis bases.

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Things to Remember

• At the state of equilibrium, the rate of forward reactions and the rate of backward reactions are equal. 
• Equilibrium can be established for both physical and chemical changes. 
• Equilibrium constant (K_c) is expressed as the concentration of products divided by reactants each term raised to the stoichiometric coefficients. 
• Le Chatelier’s principle states that the change in temperature, pressure, concentration, etc., will shift the equilibrium in such a direction that the effect of the change is reduced.
• Substances that conduct electricity in aqueous solutions are called electrolytes.
• Conjugate pair of acid and conjugate pair of base differs only by one proton.
• Lewis acids are the species that are capable of accepting an electron pair from a Lewis base to form a Lewis adduct.
• Lewis bases are a species that donate electrons to Lewis acid but generally is not involved in the bonding process.

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Previous Year Questions

1. What will be the equilibrium constant of the given reaction carried out in a 5L5L vessel and having equilibrium amounts of A2A2 and AA as 0.50.5 mole and 2×10−62×10−6 mole respectively?
2. What is Kp for the given equilibrium ? 2HI(g)⇌H2(g)+I2(g)
3. What will be the expression of KpKp for the given reaction if the total pressure inside the vessel is PP and degree of dissociation of the reactant is aa ?​
4. Assume ΔH⊖ and ΔS⊖ are independent of temperature and ratio of lnK at T1 to lnK at T2 is greater than T2/T1. Here H,S,G and K are enthalpy, entropy, Gibbs energy and equilibrium constant, respectively.
5. Now a certain quantity of extra SO2 is introduced into the container, the volume remaining the same. Which of the following is/are true?..[BITSAT 2012]
6. The increase of pressure on ice ? water at a constant temperature will cause...[VITEEE 2006]
7. Which of the following is correct?...[JIPMER 2009]
8. Find out the solubility of Ni(OH)2 in 0.1 M NaOH. Given that the ionic product of Ni(OH)2 is 2×10−15...[NEET 2020]
9. 0.1M solution of which one of these substances will be basic?...[NEET 1992]
10. The value for the equilibrium constant for this reaction is given by the expression...[NEET 2010]
11. Conjugate base for Bronsted acids H2O and HF are:….[NEET 2019]
12. At certain temperature, 50% of HI is dissociated into H2 and I2, the equilibrium constant is….[NEET 1994]
13. If the equilibrium reaction is A+2B→2C+D,A+2B→2C+D, the value of equilibrium constant is...[NEET 1994]
14. The equilibrium constant for the equilibrium CN−+CH3COOH⇌HCN+CH3COO− would be :...[NEET 2009]
15. The maximum volume of the container, when pressure of CO2 attains its maximum value, will be (Given that :SrCO3(s)⇌SrO(s)+CO2(g) Kp=1.6atm)...[NEET 2017]
16. The equilibrium constant for the oxidation of the NH3 by oxygen to give NO is:...[NEET 2017]
17. Then, find out initial concentration of CH3COOH molecules….[NEET 2001]
18. Hydrolysis of sucrose is given by the following reaction. Sucrose+H20⇌Glucose+FructoseSucrose+H20⇌Glucose+Fructose If the equilibrium constant (Kc)(Kc) is 2×10132×1013 at 300K, the value of ΔrΔr G⊖G⊖ at the same temperature will be :...[NEET 2020]
19. K1 and K2 are equilibrium constant for reactions (i) and (ii) N2(g)+O2(g)⇋2NO(g)  ... ( i ) NO(g)⇋12N2(g)+12O2(g)  ... ( ii ) Then,...[NEET 1989]
20. The solubility of BaSO4 in water is 2.42×10−3gL−1 at 298K. The value of its solubility product (Ksp) will be (Given molar mass of BaSO4=233gmol−1 )..[NEET 2018]