Ch. 7 - Acids and Bases cont. Last Week Arrhenius vs. Bronsted-Lowry Acids and Bases Conjugate Acid-Base Pairs Water: Acid or Base ph and Related Simple Calculations Today Acid Strength More ph Calculations Bases (more detail) Polyprotic Acids (more detail) Week 10 CHEM 1310 - Sections L and M 1 Via K a Value The larger the K a value the stronger the acid Week 10 CHEM 1310 - Sections L and M 2 Via Position of Equilibrium Strong Acids fully dissociate! Week 10 CHEM 1310 - Sections L and M 3 1
Via Equilibrium [H + ] vs. Original [HA] [HA] 0 [H + ] equilibrium for Strong Acids Week 10 CHEM 1310 - Sections L and M 4 Via Position of Equilibrium Weak Acids do not fully dissociate! Week 10 CHEM 1310 - Sections L and M 5 Via Equilibrium [H + ] vs. Original [HA] [HA] 0 >> [H + ] equilibrium for Weak Acids! Week 10 CHEM 1310 - Sections L and M 6 2
Via Strength of A - vs. H 2 O Strong acid Weak Conjugate Base Weak acid Strong Conjugate Base Week 10 CHEM 1310 - Sections L and M 7 1. Weak Acid 2. Strong Acid 3. HF is not an acid Is HF a strong or a weak acid? 4. Cannot be determined from information given Week 10 CHEM 1310 - Sections L and M 8 1. Weak Acid 2. Strong Acid 3. HF is not an acid Is HF a strong or a weak acid? The larger the K a value the stronger the acid 4. Cannot be determined from information given Week 10 CHEM 1310 - Sections L and M 9 3
ph: Strong vs. Weak Acids What is the ph of a 1.0M HCl solution? For a strong acid: [HA] [H + ] = 1.0 M Thus, ph = -log (1.0) = 0 ph = -log [H + ] Week 10 CHEM 1310 - Sections L and M 10 ph: Strong vs. Weak Acids What is the ph of a 1.0M HF solution? HF H + + F - ph = -log [H + ] K a = [H + ] x [F - ] [HF] K a = X 2 [HF] 0 - X [HF] 0 >> X Week 10 CHEM 1310 - Sections L and M 11 ph: Strong vs. Weak Acids What is the ph of a 1.0M HF solution? HF H + + F - Let X = [H + ] = [F - ] ph = -log [H + ] K a = X 2 1 - X K a = X 2 X = a =.2 x 10-4 X = 0.0268 [HF] 0 >> X ph = -log (0.0268) = 1.6 Week 10 CHEM 1310 - Sections L and M 12 4
Another Sample Problem A solution is prepared by dissolving 0.56-g of benzoic acid (C 6 H 5 COOH; K a = 6.4x10-5 ) into enough water to make 1 L. Calculate the ph of the solution. Determine the MW of benzoic acid Write the chemical equation describing the dissolution of this acid Is this a weak acid? Solve problem on board! Week 10 CHEM 1310 - Sections L and M 13 Which of the following K a values belongs to 1. 6.6 x 10-4 2. 4.6 x 10-4 3. 9.1 x 10-8 4. 3.0 x 10-8 Week 10 CHEM 1310 - Sections L and M 14 Which of the following K a values belongs to 1. 6.6 x 10-4 2. 4.6 x 10-4 The stronger the acid, the higher the K a 3. 9.1 x 10-8 4. 3.0 x 10-8 Week 10 CHEM 1310 - Sections L and M 15 5
Which of the following pk a values belongs to 1. 9.25 2. 7.46 3. 3.40 4. 1.92 Week 10 CHEM 1310 - Sections L and M 16 Which of the following pk a values belongs to 1. 9.25 2. 7.46 3. 3.40 The stronger the acid, the higher the K a and the lower the pk a! 4. 1.92 Week 10 CHEM 1310 - Sections L and M 17 Percent Dissociation % Dissociation = [A - ] [HA] x 100 Week 10 CHEM 1310 - Sections L and M 18 6
Bases and Base Strength B (aq) + H 2 O (l) BH + (aq) + OH (aq) B = Base K b + - [BH ][OH ] = [B][H O] + - [BH ][OH ] = [B] 2 pk = -log K b 10 b K b is the Basicity constant Week 10 CHEM 1310 - Sections L and M 19 K a1 > K a2 > K a3 Polyprotic Acids Week 10 CHEM 1310 - Sections L and M 20 Polyprotic Acids Sulfuric acid (H 2 SO 4 ) is unique K a1 = 100 STRONG ACID K a2 = 1.2 x 10-2 WEAK ACID H 2 SO 4 (aq) H + (aq) + HSO 4 (aq) K a1 1.0x10 2 HSO 4 (aq) H + (aq) + SO 4 2 (aq) K a2 1.2x10-2 K K a1 a2 ª 10 4 Week 10 CHEM 1310 - Sections L and M 21 7