Law of Chemical Equilibrium

The Law of Chemical Equilibrium (also called the Law of Mass Action) is a fundamental principle of chemistry that relates the concentrations of reactants and products in a reversible reaction at equilibrium. It is key to predicting the direction, yield, and balance of chemical reactions.

Statement of the Law

For a general reversible reaction:

aA + bB ⇌ cC + dD

where A and B are reactants, C and D are products, and a, b, c, d are stoichiometric coefficients, the law states:

At equilibrium, the ratio of the product of the concentrations of products raised to their stoichiometric powers to that of the reactants is constant at a given temperature.

Equilibrium Constant Expression

Mathematically, the equilibrium constant expression (K_c) is:

K_c = [C]^c[D]^d / [A]^a[B]^b

Here, square brackets [] denote molar concentrations at equilibrium.

Key Points

• K_c is constant only at a particular temperature.
• Only gases and aqueous species are included; solids and pure liquids are omitted.
• Large K_c (>10³) → products favored.
• Small K_c (<10^-3) → reactants favored.

Example

For the Haber process:

N₂(g) + 3H₂(g) ⇌ 2NH₃(g)

The equilibrium expression is:

K_c = [NH₃]² / [N₂][H₂]³

Significance of the Law

• Predicts the direction of a reaction.
• Calculates equilibrium concentrations.
• Helps optimize industrial processes (e.g., Haber process).
• Explains reaction yields under different conditions.

Quiz: Test Your Understanding

1. State the Law of Chemical Equilibrium in simple words.
2. Write K_c for: 2SO₂ + O₂ ⇌ 2SO₃.
3. What does a large K_c value indicate?
4. Why are solids excluded from K_c?
5. How does temperature affect equilibrium constant?

Quiz Answers

1. At equilibrium, product-to-reactant concentration ratio (with powers) is constant.
2. K_c = [SO₃]² / [SO₂]²[O₂]
3. Large K_c → product-favored.
4. Because their concentrations remain constant.
5. K_c changes as equilibrium shifts with temperature.

Multiple Choice Questions (MCQs)

1. The Law of Chemical Equilibrium is also known as:
   • a) Law of Conservation of Mass
   • b) Law of Definite Proportions
   • c) Law of Mass Action ✔
   • d) Hess’s Law
   Explanation: It is based on the Law of Mass Action proposed by Guldberg and Waage.
2. For H₂ + I₂ ⇌ 2HI, the expression is:
   • a) [H₂][I₂] / [HI]²
   • b) [HI]² / [H₂][I₂] ✔
   • c) [H₂][HI] / [I₂]
   • d) [HI] / [H₂]
   Explanation: Products go in numerator, reactants in denominator with powers.
3. Which is NOT included in K_c?
   • a) Gaseous reactants
   • b) Aqueous ions
   • c) Pure solids ✔
   • d) Gaseous products
   Explanation: Concentration of pure solids/liquids is constant.
4. A very small K_c (<1) indicates:
   • a) Reaction favors products
   • b) Reaction favors reactants ✔
   • c) Reaction is complete
   • d) Reaction is irreversible
   Explanation: Small K_c means reactants are predominant at equilibrium.

FAQs on Law of Chemical Equilibrium

Q1. What is the Law of Chemical Equilibrium?

It states that at equilibrium, the ratio of product concentrations to reactant concentrations (raised to their stoichiometric powers) is constant at a given temperature.

Q2. Why are solids and liquids not included in K_c?

Because their concentrations remain constant and do not affect the equilibrium position.

Q3. What factors affect the equilibrium constant?

Only temperature. Pressure or concentration changes shift equilibrium but do not change K_c.

Q4. Where is this law applied in industry?

In processes like the Haber Process for ammonia synthesis and Contact Process for sulfuric acid production.

Q5. What does a large value of K_c signify?

It means the reaction strongly favors product formation at equilibrium.