Mastering Keq & Equilibrium (HSC Module 5)

• Keq >> 1 → products favoured
• Keq << 1 → reactants favoured
• Keq ≈ 1 → significant amounts of both

• Q uses initial concentrations.
• Q < Keq → forward reaction favoured
• Q > Keq → reverse reaction favoured
• Q = Keq → system at equilibrium

• Write dissociation equation
• Express ion concentrations in terms of s (molar solubility)
• Substitute into Ksp expression
• Solve for s

• Exothermic reaction ↑ temperature → reaction shifts left, Keq decreases
• Endothermic reaction ↑ temperature → reaction shifts right, Keq increases
• Only temperature changes Keq.

• A dynamic process in which the forward and reverse reactions occur at the same rate.
• Concentrations of reactants & products remain constant (not equal).
• Occurs in a closed system.

• MX → M⁺ + X⁻ → Ksp = [M⁺][X⁻]
• MX₂ → M²⁺ + 2X⁻ → Ksp = [M²⁺][X⁻]²
• M₂X₃ → 2M³⁺ + 3X²⁻ → Ksp = [M³⁺]²[X²⁻]³
• Stoichiometry must be applied carefully.

• Solubility: amount of solute that dissolves in given solvent.
• Saturated solutions reach equilibrium between dissolved ions and undissolved solid.
• Basis for Ksp.

• Higher pressure favors fewer moles of gas.
• Lower pressure favors more moles of gas.
• Increasing reactant concentration shifts right.
• Increasing product concentration shifts left.

• Describes the ratio of products to reactants at equilibrium.
• Depends only on temperature.
• Shows the extent of reaction, not rate.

• Equilibrium constants guide Haber Process NH₃ production
• Equilibrium constants optimize Contact Process H₂SO₄ production
• Ksp controls solubility in pharmaceuticals
• Ksp enables water hardness removal
• Ksp facilitates selective precipitation in mining

Chemical Equilibrium & Constants (Keq & Ksp)

HSC Chemistry – Module 5: Equilibrium and Acid Reactions

Keq quantifies equilibrium composition Temperature is the only factor that changes Keq Ksp describes solubility equilibrium Q vs Keq/Ksp predicts direction of reaction or precipitation Equilibrium principles guide industrial optimisation

Master Equilibrium for HSC Success!

• Ratio of products to reactants concentrations
• Exponents match stoichiometric coefficients
• Solids and liquids excluded
• Concentrations in mol L⁻¹

• Q > Ksp → precipitate forms
• Q = Ksp → saturated solution
• Q < Ksp → no precipitate
• Used in selective precipitation and water treatment.

• A system at equilibrium will adjust to minimise the effect of a disturbance.
• Key disturbances: Concentration, Temperature, Pressure/Volume (gases), Catalysts (no effect on position)

• I: Initial concentrations of all species.
• C: Changes based on stoichiometry (use x).
• E: Equilibrium concentrations (I + C values).
• Essential for setting up and solving Keq algebraically.

• Adding common ion reduces solubility of salt.
• Shifts dissolution equilibrium left (Le Chatelier's principle).
• Example: NaCl increases [Cl⁻], precipitating AgCl.
• Lowers ion product below Ksp value.
• Used to control precipitation in analysis.

• Closed system
• Constant temperature
• Reaction must be reversible
• No external disturbance (Le Chatelier conditions)

• Dynamic: continual molecular reaction, no net change.
• Static: no movement — NOT chemical equilibrium.
• Chemical systems are always dynamic.

• Equilibrium constant for sparingly soluble salt dissolution.
• Equals product of aqueous cation and anion concentrations.
• Lower Ksp indicates lower salt solubility.
• Precipitation occurs when ion product exceeds Ksp.
• Excludes pure solid from concentration expression.