H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 1 Chapter 17 Acid-base Equilibrium I : The Basic Concepts 17.1Concept of Acid and Base.

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H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 1 Chapter 17 Acid-base Equilibrium I : The Basic Concepts 17.1Concept of Acid and Base 17.2Dissociation of Water 17.3The pH Scale 17.4Strength of Acids and Bases

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book Concept of Acid and Base (SB p.128) Arrhenius Acid-base Theory Arrhenius Acid-base Theory suggested that 1)An acid is a substance which ionizes to give hydrogen ions (H + ) and its corresponding negative ions when dissolved in water e.g. HCl(g) H 2 O H + (aq) + Cl - (aq) 2)A base is a substance which gives hydroxide ions (OH - ) and its corresponding positive ions when dissolved in water. e.g. NaOH(s) H 2 O Na + (aq) + OH - (aq)

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 3 Acid - a proton (H + ) donor Base - a proton (H + ) acceptor acidbase 17.1 Concept of Acid and Base (SB p.130) Bronsted-Lowry Acid-base Theory HCl(aq) + H 2 O(l) H 3 O + (aq) + Cl - (aq)

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 4 Acid - a proton (H + ) donor Base - a proton (H + ) acceptor acid’base’ Bronsted-Lowry Acid-base Theory HCl(aq) + H 2 O(l) H 3 O + (aq) + Cl - (aq) 17.1 Concept of Acid and Base (SB p.130)

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 5 Acid - a proton (H + ) donor Base - a proton (H + ) acceptor acid’base’ acidbase 17.1 Concept of Acid and Base (SB p.131) Bronsted-Lowry Acid-base Theory HCl(aq) + H 2 O(l) H 3 O + (aq) + Cl - (aq)

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 6 acid’base’ acidbase conjugate acid conjugate base Cl - is the conjugate base of the acid HCl. H 3 O + is the conjugate acid of the base H 2 O Concept of Acid and Base (SB p.131) Bronsted-Lowry Acid-base Theory HCl(aq) + H 2 O(l) H 3 O + (aq) + Cl - (aq)

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 7 Other examples of conjugate acid-base pairs acid base conjugate acid conjugate base acid base conjugate acid conjugate base 17.1 Concept of Acid and Base (SB p.132) CH 3 COOH(aq) + H 2 O(l) H 3 O + (aq) + CH 3 COO - (aq) H 2 SO 4 (aq) + H 2 O(l) H 3 O + (aq) + HSO 4 - (aq)

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 8 Other examples of conjugate acid-base pairs acid base conjugate acid conjugate base acidbase conjugate acid conjugate base 17.1 Concept of Acid and Base (SB p.132) H 2 CO 3 (aq) + H 2 O(l) H 3 O + (aq) + HCO 3 - (aq) NH 3 (aq) + H 2 O(l) NH 4 + (aq) + OH - (aq)

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 9 Dissociation of Water H 2 O(l) + H 2 O(l) H 3 O + (aq) + OH - (aq) Note that [H 2 O(l)] remains nearly constant. Thus we have: ionic product of water Do you think K w varies with temp? 17.2 Dissociation of Water (SB p.133) K w = [H 3 O + (aq)] [OH - (aq)] Or 2H 2 O(l) H 3 O + (aq) + OH - (aq) K c = This equation holds for any aqueous solution.

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 10 Dissociation of Water Thus the ionic product is represented as: Sometimes the above equation is simplified as: K w = [H + (aq)][OH - (aq)] H+H+ Bare H + cannot exist in solution (charge/size  ) H(H 2 O) 5 + (solvated H 3 O + ) H 3 O + (stabilized) 17.2 Dissociation of Water (SB p.133) 2H 2 O(l) H 3 O + (aq) + OH - (aq) H 2 O(l) H + (aq) + OH - (aq)

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 11 The pH Scale For pure water at 25 0 C, 17.3 The pH Scale (SB p.134) pH = -log[H 3 O + (aq)] pOH = -log[OH - (aq)] At 25 o C, [H 3 O + (aq)] = [OH - (aq)] = 1 x mol dm -3 pH = -log [H 3 O + (aq)] = - log(1 x ) = 7

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book The pH Scale (SB p.134) pH = -log[H 3 O + (aq)] pOH = -log[OH - (aq)] For acidic solutions at 25 o C, [H 3 O + (aq)] > [OH - (aq)] [H 3 O + (aq)] > 1 x mol dm -3 -log [H 3 O + (aq)] < 7 pH < 7

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 13 pH = -log[H 3 O + (aq)] pOH = -log[OH - (aq)] For alkaline solutions at 25 o C, [H 3 O + (aq)] < [OH - (aq)] [H 3 O + (aq)] < 1 x mol dm -3 -log [H 3 O + (aq)] > 7 pH > The pH Scale (SB p.134)

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book The pH Scale (SB p.135)

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 15 Universal Indicator Universal indicator is a mixture of dyes which shows different colours at different pH. pH paper is a piece of filter paper soaked with universal indicator The pH Scale (SB p.136) Measurement of pH

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 16 pH meter pH meter is an instrument which can measure the pH values of solutions directly. The meter is connected to a glass electrode which is sensitive to [H 3 O + ] together with a reference electrode. Measurement of pH 17.3 The pH Scale (SB p.136)

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 17 [H 2 O(l)] remains nearly constant. Dissociation constant 17.4 Strength of Acids and Bases (SB p.138) Strong and Weak Acids HA(aq) + H 2 O(l) H 3 O + (aq) + A - (aq) Kc =Ka =

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 18 Remarks 1. K a is an equilibrium constant and is independent of concentration. Thus it can be used to describe the strength of the acid. (Note that pH varies with concentration.) 17.4 Strength of Acids and Bases (SB p.138) 2. Sometimes the K a expression is simplified as follows: HA(aq) H + (aq) + A - (aq) Ka =

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book Strength of Acids and Bases (SB p.139)

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 20 pK a Values pK a = -log K a pK a   strength of acid  17.4 Strength of Acids and Bases (SB p.139) AcidKa( mol dm -3 )pKaRelative strength HI HBr HNO 3 Citric acid H 2 O 3 x x x x Very strong Strong Weak Very weak

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 21 [H 2 O(l)] remains nearly constant. Dissociation constant 17.4 Strength of Acids and Bases (SB p.140) Strong and Weak Bases B(aq) + H 2 O(l) HB + (aq) + OH - (aq) K b =

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 22 pK b = -log K b pK b   strength of base  17.4 Strength of Acids and Bases (SB p.140) BaseK b (mol dm -3 )pK b Relative strength (C 2 H 5 ) 2 NH NH 3 C 6 H 5 NH x x x weak Strong and Weak Bases

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book Strength of Acids and Bases (SB p.146) Polyprotic Acids H 2 CO 3 (aq) + H 2 O(l) H 3 O + (aq) + HCO 3 - (aq) HCO 3 - (aq) + H 2 O(l) H 3 O + (aq) + CO 3 2- (aq) H 2 CO 3 (aq) + 2H 2 O(l) 2H 3 O + (aq) + CO 3 2- (aq) K a = x

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 24 Remarks Each successive dissociation constant is smaller than the previous one. (Increased difficulty of removing an hydrogen ion from the more negative species.) H 2 A \ == \ H + + HA - HA - \ == \ H + + A 2- Ka 1 Ka Strength of Acids and Bases (SB p.147) AcidK a1 (mol dm -3 )K a2 (mol dm -3 )K a3 (mol dm -3 ) Carbonic acid (H 2 CO 3 (aq)) Selenious acid (H 2 SeO 3 (aq)) Phosphoric acid (H 3 PO 4 (aq)) 4.5 x x x x x x x

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 25 The END