[H2 CHEM] Chapter 4 - Theories of Acids Bases

ELAINE PEH (COPYRIGHTED) © Chapter 4 Theories of Acids and Bases (Acid-Base Equilibria)

CHAPTER ANALYSIS FOCUS EXAM WEIGHTAGE ELAINE PEH (COPYRIGHTED) ©

Introduction to Acid & Base Ionic Product of Water (Kw) Dissociation Constant of Acid & Base KEY CONCEPT ELAINE PEH (COPYRIGHTED) ©

Methods to Calculate pH of Acid/Base Methods to Calculate pH of a Salt Solution Buffer Solutions Titration Curves KEY CONCEPT ELAINE PEH (COPYRIGHTED) ©

ARRHENIUS THEORY • Arrhenius Acid dissolves in water to produce H+ (aq)/H3O+ (aq) ions • Arrhenius Base dissolves in water to produce OH- (aq) ions BRONSTED-LOWRY THEORY • A Bronsted Acid is any species that donates a proton H+ • A Bronsted Base is any species that accepts a proton H+ LEWIS THEORY • Lewis Acid accepts electron pair to form a dative covalent bond. • Lewis Base donates electron pair to form a dative covalent bond. . ELAINE PEH (COPYRIGHTED) © Introduction to Acid & Base

CONJUGATE ACID-BASE PAIR • The product formed from an acid is known as the conjugate base of the acid and the product formed from a base is known as the conjugate acid of the base. • The stronger the acid, the weaker its conjugate base is • The stronger the base, the weaker its conjugate acid is TYPES OF ACID • Monoprotic or monobasic acids can donate only one H+ per molecule. • Polyprotic acids have more than one H+ per molecule. They dissociate in water to form H+ in a stepwise manner. TYPES OF BASE • Monoprotic or monoacidic bases can accept only one H+ per molecule. • Polyprotic base can accept more than one H+ per molecule. They accepts H+ in a stepwise manner. . ELAINE PEH (COPYRIGHTED) © Introduction to Acid & Base

STRENGTH OF ACID & BASE • Strong Acid/Base dissociates completely in aqueous solution. • Weak Acid/Base dissociates partially in aqueous solution. Note: → is use to represent the dissociation of strong acid/base while ⇌ is use to represent the dissociation of weak acid/base DEGREE OF DISSOCIATION • The greater the degree of dissociation, the stronger the acid/base is • For strong acid/base, α = 1 • For weak acid/base, 0 < α < 1 ELAINE PEH (COPYRIGHTED) © Introduction to Acid & Base

IONIC PRODUCT OF WATER(Kw) • Water undergoes auto-ionisation when a water molecule donates a proton to another water molecule VARIATION OF Kw WITH TEMPERATURE • When temperature increase, the equilibrium will shift to the right to absorb heat, producing more H3O+ and OH- ions. • Hence both [H3O+] and [OH-] increases, which result in an increase in Kw. • pH of water also decreases when temperature increase, however this does not mean that water is becoming more acidic. • Water remains neutral as there is still an equal concentration of H3O+ and OH- ions. Note: Linking Kw with Chapter 9: Chemical Equilibria ELAINE PEH (COPYRIGHTED) © Ionic Product of Water (Kw)

ELAINE PEH (COPYRIGHTED) © Dissociation Constant of Acid & Base ACID DISSOCIATION CONSTANT (Ka) HA (aq) + H2O (l) ⇌ H3O+ (aq) + A- (aq) • Units = mol dm-3 • It measures the extent of dissociation of a weak acid • The larger the Ka, the greater degree of dissociation, hence the stronger the acid • Concentration of Water/Solvent are excluded from the equilibrium expression The smaller the pKa, the larger the Ka, the stronger the acid BASE DISSOCIATION CONSTANT (Kb) B (aq) + H2O (l) ⇌ BH+ (aq) + OH- (aq) • Units = mol dm-3 • It measures the extent of dissociation of a weak base • The larger the Kb, the greater degree of dissociation, hence the stronger the base • Concentration of Water/Solvent are excluded from the equilibrium expression The smaller the pKb, the larger the Kb, the stronger the base Note: pKa and Ka is the BEST indicator of the strength of the acid/base as it measure the degree of dissociation of the acid/base and it is a constant at constant temperature and does not vary with concentration.

ELAINE PEH (COPYRIGHTED) © Dissociation Constant of Acid & Base RELATIONSHIP BETWEEN Ka, Kb AND Kw HA (aq) + H2O (l) ⇌ H3O+ (aq) + A- (aq) B (aq) + H2O (l) ⇌ BH+ (aq) + OH- (aq)