Chapter 6 - Equilibrium

Equilibrium explains what happens when opposite physical or chemical processes occur at equal rates. Although an equilibrium system may appear unchanged, particles continue reacting in both directions. This is why chemical equilibrium is called dynamic equilibrium.

The chapter has two connected parts: chemical equilibrium and ionic equilibrium. Students first study reversible reactions and equilibrium constants, and then apply similar ideas to acids, bases, buffers and sparingly soluble salts.

Equilibrium in physical processes

Physical equilibrium may exist between:

• Solid and liquid during melting and freezing

• Liquid and vapour during evaporation and condensation

• Solute and solution during dissolution and crystallisation

• A solute distributed between two phases

At equilibrium, the opposing processes continue at equal rates, so macroscopic properties remain constant.

Chemical equilibrium

For a reversible reaction, the forward reaction is initially faster because only reactants are present. As products accumulate, the backward reaction begins. Equilibrium is reached when both rates become equal.

The concentrations of reactants and products become constant, but they do not have to become equal.

Equilibrium constant

Students learn to write equilibrium expressions using the law of mass action.

• (K_c) is expressed using equilibrium concentrations.

• (K_p) is expressed using equilibrium partial pressures.

For gaseous reactions, (K_p) and (K_c) are related through the change in the number of moles of gas.

In heterogeneous equilibrium, pure solids and pure liquids are omitted from the equilibrium expression because their effective concentrations remain constant.

The magnitude of the equilibrium constant indicates the extent of reaction. It does not indicate how quickly equilibrium is reached.

Reaction quotient and direction of reaction

The reaction quotient, (Q), has the same form as the equilibrium expression but can be calculated at any stage.

• If (Q<K), the reaction proceeds forward.

• If (Q>K), the reaction proceeds backward.

• If (Q=K), the system is at equilibrium.

Le Chatelier’s principle

When an equilibrium system is disturbed, it shifts in a direction that reduces the effect of the disturbance.

Students examine changes in:

• Concentration

• Pressure

• Volume

• Temperature

• Addition of an inert gas

• Addition of a catalyst

Temperature can change the value of the equilibrium constant. A catalyst does not change the equilibrium constant or equilibrium composition; it only reduces the time required to reach equilibrium.

Acids, bases and ionic equilibrium

Students compare three important acid-base concepts:

• Arrhenius concept

• Brønsted–Lowry concept

• Lewis concept

They learn conjugate acid-base pairs, amphoteric substances, strong and weak electrolytes, and acid and base ionisation constants.

Ionic product of water, pH and pOH

Water undergoes slight self-ionisation. Its ionic product connects hydrogen-ion and hydroxide-ion concentrations.

Students solve questions involving:

• Strong acids and bases

• Weak acids and bases

• Degree of ionisation

• pH and pOH

• Acid and base ionisation constants

Buffer solutions and common-ion effect

A buffer resists a large change in pH when a small quantity of acid or base is added. Students learn acidic and basic buffers and the principle behind buffer action.

The common-ion effect suppresses the ionisation of a weak electrolyte when a strong electrolyte containing a common ion is added.

Salt hydrolysis and solubility equilibrium

The nature of a salt solution depends on the strengths of its parent acid and base. Students determine whether a salt solution is acidic, basic or neutral.

For sparingly soluble salts, the solubility product (K_{sp}) is used to calculate solubility and predict precipitation.

What can students study on Teachoo?

• Dynamic-equilibrium explanations

• Writing (K_c) and (K_p) expressions

• Equilibrium-constant numericals

• Reaction-quotient questions

• Le Chatelier reasoning

• Acid-base theory

• pH calculations

• Weak acid and weak base numericals

• Buffer and common-ion questions

• Salt hydrolysis

• Solubility-product calculations

• NCERT solutions, MCQs and case-based questions

Common mistakes

• Assuming equilibrium means the reaction has stopped

• Assuming reactant and product concentrations are equal

• Including pure solids in equilibrium expressions

• Using initial concentrations instead of equilibrium concentrations

• Claiming that a catalyst changes equilibrium yield

• Treating pH as directly proportional to hydrogen-ion concentration

• Confusing solubility with solubility product

Best way to study this chapter

Finish chemical equilibrium before beginning ionic equilibrium. In every numerical, write the balanced equation and equilibrium expression first. For pH questions, identify whether the acid or base is strong or weak before selecting a formula.

Frequently asked questions

What is dynamic equilibrium?

It is a state in which forward and backward processes continue at equal rates, producing no net macroscopic change.

Are concentrations equal at equilibrium?

No. They remain constant but may have very different values.

Does a catalyst change the equilibrium position?

No. It speeds up both directions and helps equilibrium establish faster.

What is the difference between (K) and (Q)?

(K) uses equilibrium values. (Q) can be calculated at any time and predicts the direction in which the system will change.

What does pH measure?

pH is a logarithmic quantity related to hydrogen-ion concentration. A smaller pH usually indicates a more acidic aqueous solution.

What is a buffer?

A buffer is a solution that resists a large change in pH when small amounts of acid or base are added.

What is the common-ion effect?

It is the suppression of ionisation or solubility caused by adding an electrolyte containing an ion already present in the equilibrium.

Learn Equilibrium with Teachoo for chemical-equilibrium concepts, pH numericals, buffers, solubility-product questions and complete NCERT solutions.