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Chemical Equilibrium : Equilibrium Constant, Degree of Dissociation, Le-Chatelier's Principle

Equilibrium

This is a state of reversible reaction when all the measurable properties of the reaction becomes constant, and ΔG (Gibbs free energy) becomes zero.

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

Equilibrium Constant

It is the Ratio of rate constant of forward reaction and backward reaction.

Changes in Equilibrium Constant

Keq only depend on -

  1. Reaction coefficient or Stoichiometric coefficient
  2. Temperature of reaction

Changes in Keq when Stoichiometric coefficient changes -

Rules :

  1. When reaction multiplied by n

  2. When reaction divided by n

  3. When reaction is reversed

  4. When two reactions added

Changes in Keq when Temperature changes -

Keq in terms of Concentration (Kc) and Partial pressure (Kp)

Degree of Dissociation

It is the ratio of total dissociated mole of Reactant and initial mole of Reactant.

Degree of Dissociation in terms of Vapour density -

where,

Equilibrium mixture : Calculation of Kc and Kp

Case 1

Case 2

Case 3

Case 4

(I)

(II)

Case 5

(I)

(II) Direct method for Kp

Direction of Reversible Reaction

If any of these variable changes :

then,
Direction of reaction changes to forward or backward, but Equilibrium constant (Keq) does not change.

There are three methods to determine the direction of equilibrium reaction -

Quotient Ratio (Qc)

Δ⁢G=ΔGo+R⁢TlnQc(i)

The value of Qc = Keq and ΔG = 0 at equilibrium.

Gibbs free energy (ΔG)

Le-Chatelier’s Principle

(i) Effect of Concentration

(ii) Effect of Temperature

(iii) Effect of Pressure/Volume

If number of moles same both sides then Reaction always present at equilibrium.

(iv) Effect of Catalyst

No effect of catalyst on Keq or ΔH, it only decreases the time to achieve equilibrium.

It decreases the activation energy and increases the rate of reaction in both forward and backward direction.

(v) Effect of inert gas addition

  • At constant Volume (V) = No effect
  • At constant Pressure (P), V increases = Reaction shifts to more no of gaseous mole side.
# Equilibrium Constant # Degree of Dissociation # Le-Chatelier's Principle # Physical Chemistry