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Maharashtra State BoardHSC Science (Computer Science) 12th Standard Board Exam
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अभ्यासक्रम

Overview of Ionic Equilibria

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Estimated time: 34 minutes
Maharashtra State Board: Class 12

Definition: Ionic equilibrium

The equilibrium between ions and unionized molecules in solution is called ionic equilibrium.

Maharashtra State Board: Class 12

Definition: Electrolytes

The substances which give rise to ions when dissolved in water are called electrolytes.

Maharashtra State Board: Class 12

Definition: Nonelectrolytes

The substances which do not ionize and exist as molecules in aqueous solutions are called nonelectrolytes.

Maharashtra State Board: Class 12

Definition: Strong electrolytes

The electrolytes ionizing completely or almost completely are called strong electrolytes.

Maharashtra State Board: Class 12

Definition: Weak electrolytes

The electrolytes which dissociate to a smaller extent in aqueous solution are called weak electrolytes.

Maharashtra State Board: Class 12

Definition: Arrhenius acid

An acid is a substance which contains hydrogen and gives rise to H⁺ ions in aqueous solution is called Arrhenius acid.

Maharashtra State Board: Class 12

Definition: Arrhenius base

A base is a substance which contains OH group and produces hydroxide ions (OH⁻) in aqueous solution is called Arrhenius base.

Maharashtra State Board: Class 12

Definition: Bronsted–Lowry acid

A substance that donates a proton (H⁺) to another substance is called Bronsted–Lowry acid.

Maharashtra State Board: Class 12

Definition: Bronsted–Lowry base

A substance that accepts a proton (H⁺) from another substance is called Bronsted–Lowry base.

Maharashtra State Board: Class 12

Definition: Lewis acid

Any species that accepts a share in an electron pair is called Lewis acid.

Maharashtra State Board: Class 12

Definition: Lewis base

Any species that donates a share in an electron pair is called Lewis base.

Maharashtra State Board: Class 12

Definition: Hydrolysis of salt

The reaction in which cations or anions or both ions of a salt react with ions of water to produce acidity or alkalinity (or sometimes even neutrality) is called hydrolysis of salt.

Maharashtra State Board: Class 12

Definition: Buffer solution

A solution which resists drastic changes in pH when a small amount of strong acid or strong base or water is added to it is called buffer solution.

Maharashtra State Board: Class 12

Definition: Acidic buffer solution

A solution containing a weak acid and its salt with strong base is called acidic buffer solution.

Maharashtra State Board: Class 12

Definition: Basic buffer solution

A solution containing a weak base and its salt with strong acid is called basic buffer solution.

Maharashtra State Board: Class 12

Formula: Degree of Dissociation

\[\alpha=\frac{\text{number of moles dissociated}}{\text{total number of moles}}\]

Percent dissociation:

% dissociation = α × 100

Maharashtra State Board: Class 12

Formula: Acid Dissociation Constant

\[K_a=\frac{[H^+][A^-]}{[HA]}\]

For weak acid:

\[K_a=\frac{\alpha^2c}{1-\alpha}\]

If α is very small:

\[K_a=\alpha^2c\]

\[\alpha=\sqrt{\frac{K_a}{c}}\]

Maharashtra State Board: Class 12

Formula: Base Dissociation Constant

\[K_b=\frac{[B^+][OH^-]}{[BOH]}\]

For weak base:

\[K_b=\frac{\alpha^2c}{1-\alpha}\]

If α is small:

\[K_b=\alpha^2c\]

\[\alpha=\sqrt{\frac{K_b}{c}}\]

Maharashtra State Board: Class 12

Formula: Ionic Product of Water

Kw​ = [H3​O+][OH−]

At 298 K:

Kw = 1.0×10−14

Maharashtra State Board: Class 12

Formula: pH and pOH

pOH = −log⁡[OH−]

Relationship:

pH + pOH = 14

Maharashtra State Board: Class 12

Theory: Arrhenius Theory of Acids and Bases

According to Arrhenius theory, acids and bases are defined on the basis of ion formation in aqueous solution.

  • An acid is a substance which produces H⁺ ions in aqueous solution.
  • A base is a substance which produces OH⁻ ions in aqueous solution.
  • This theory explains the acidic and basic nature only in aqueous medium.

Example of acid:

\[\mathrm{H}\mathrm{C}\mathrm{l}(\mathrm{a}\mathrm{q})\xrightarrow{water}\mathrm{H}^+(\mathrm{a}\mathrm{q})+\mathrm{C}\mathrm{l}^-(\mathrm{a}\mathrm{q})\]

Example of base:

NaOH(aq) → Na⁺(aq) + OH⁻(aq)

Maharashtra State Board: Class 12

Theory: Bronsted–Lowry Theory

Bronsted and Lowry proposed a more general theory based on proton transfer.

  • An acid is a substance that donates a proton (H⁺).
  • A base is a substance that accepts a proton (H⁺).
  • Acid-base reactions involve transfer of proton from acid to base.

Example reaction:

HCl + NH₃ ⇌ NH₄⁺ + Cl⁻

A pair of acid and base differing by a proton is called a conjugate acid–base pair.

Maharashtra State Board: Class 12

Law: Ostwald’s Dilution Law

Ostwald expressed the quantitative relationship between concentration and degree of dissociation of weak electrolytes.

It applies only to weak electrolytes.

The degree of dissociation increases on dilution.

For weak acid HA:

HA ⇌ H+ + A−

The dissociation constant is:

\[K_a=\frac{[H^+][A^-]}{[HA]}\]

For small α:

\[K_a=\alpha^2c\quad\mathrm{and}\quad\alpha=\sqrt{\frac{K_a}{c}}\]

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