Types of Chemical Equilibrium and Law of Mass Action

There are two types of chemical equilibrium. Homogeneous equilibrium and Heterogeneous equilibrium. Explain on-base chemical equilibrium class 11 and the chemical equilibrium definition of this topic.

Chemical Equilibrium

1. Homogeneous equilibrium: In an inhomogeneous equilibrium all the components occur only in one phase. For Example:- A system containing gases or totally miscible liquids

Nᆯ (g)   +   Oᆯ (g)   ⇌  2NO (g)

2. Heterogeneous equilibrium:- A heterogeneous equilibrium is one in which two or more phases are involved. For example:- CaCOз (S) ⇌ CaO (S) + CO2 (g)

Representation:- reversible reactions can be represented by two arrows ( ⇌ ) between the reactants and products. A + B  ⇌  C + D

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Dynamic Equilibrium

The stage at which the rate of forwarding reactions becomes equal to the rate of backward reaction is known as dynamic equilibrium.

Explanation of dynamic equilibrium

Consider a general reaction that takes place in a gaseous state in a closed vessel. A + B ⇌ C + D. Let the initial concentration of A and B be the same as the forward reaction proceeds, the concentration of reaction ( A and B) decrease, and those of the product increase continuously. Therefore, the rate of forwarding reaction goes on decreasing while that of backward reaction keeps on increasing.

Types of Chemical Equilibrium

Ultimately a stage is reached when the rate of forwarding reaction backward reaction. At this stage concentration of reactants and products becomes constant. This is called the stage of chemical equilibrium.

Law of Mass Action Chemical Equilibrium 

This law was first enunciated by C.M Guldberg and P. Wage in 1864 which helped us to find the reaction between the concentration of the reactions and products at equilibrium in chemical reactions.

Statement of the law of mass action

The rate at which a substance reacts is proportional to its active mass and the rate of a chemical reaction is proportional to the product of the active masses of the reacting substances.

Rate of reaction Active masses of reacting substances

Active Mass of chemical equilibrium

Active mass means the molar concentration or the number of moles per dm૩in a dilute solution. The derivation of the active mass equation.

Consider a general reaction:   A  +  B  ⇌  C  +  D 

A + B  = Reactant,  C + D = Products.

The equilibrium concentrations in mol/dm3 of A, B, C, and D are represented in square brackets like [A], [B], [C], and [D].

law of mass action chemical equilibrium

Forward Reaction:- The rate of forwarding reaction is proportional to the product of the molar concentration of A and B. Rate of forwarding reaction ∝ [A] [B] =  rf ∝  [A] [B]. remaining sign of proportionality rf = kf  [A] [B]. kf is the proportionality constant known as the rate constant for forwarding reaction. 

Backward reaction:- C and D are the reactants for the backward reaction so the rate of reverse reaction is given by. Rate of reverse reaction ∝ [C] [D] = Rr ∝ [C] [D] = Rr =Kr [C] [D].

At Equilibrium: Rate of forward reaction = Rate of reverse reaction

                              kf [A] [B]  =  kr [C] [D]

On rearranging 

                             kf / kr [A] [B] = [C] [D]

                            kf / kr =  [C] [D] / [A] [B]

                            As. kf / kr = Kc

                           Kc = [C] [D] / [A] [B]

Kc = is the ratio of the rate constants ( kf / kr). The ratio of the product of the molar concentration of the products to that of the reactant.

For general reaction. aA + bB  ⇌ cC + dD

Where a, b, c, d, represent the number of moles of species taking part in a chemical reaction. They are called the coefficients of the balanced chemical equation.

                         Kc = [C]č [D]d / [A]a [B]b

Value of Kc:- the value of Kc is independent of the initial concentrations of the reactant and product but changes only with a temperature change.

For Gases equilibrium 

For gaseous equilibrium, it is convenient to express the concentrations of gases in terms of their partial pressure at any given temperature. let Pa, Pb, Pc, and Pd be the partial pressure of gaseous species than the equilibrium. Constant Kp may be expressed as:

                                            Kp = Pc / Pa X  Pd / Pa

Mole fraction 

If the concentration is expressed in terms of mole fraction, then the equilibrium constant Kx can be expressed as Kx = Xc / Xa ㄨ Xd / Xb

>Fundamentals of Chemistry notes


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