Acid-base titrations 4.3 exercise 4

Simulation: http://users.skynet.be/eddy/titratie.swf Strong acid/ strong base Leave out

strong acid (HCl) v. strong base (NaOH) pH curves Types There are four types of acid-base titration; each has a characteristic curve. strong acid (HCl) v. strong base (NaOH) weak acid (CH3COOH) v. strong alkali (NaOH) strong acid (HCl) v. weak base (NH3) weak acid (CH3COOH) v. weak base (NH3) In the following examples, alkali (0.1M) is added to 25cm3 of acid (0.1M) End points need not be “neutral‘ due to the phenomenon of salt hydrolysis

Equivalence point: the point at which sufficient base has been added to just neutralise the acid (or vice-versa).

strong acid (HCl) v. strong base (NaOH)

strong acid (HCl) v. strong base (NaOH) pH 1 at the start due to 0.1M HCl (strong monoprotic acid)

strong acid (HCl) v. strong base (NaOH) Very little pH change during the initial 20cm3 pH 1 at the start due to 0.1M HCl (strong monoprotic acid)

strong acid (HCl) v. strong base (NaOH) Very sharp change in pH over the addition of less than half a drop of NaOH Very little pH change during the initial 20cm3 pH 1 at the start due to 0.1M HCl (strong monoprotic acid)

strong acid (HCl) v. strong base (NaOH) Curve levels off at pH 13 due to excess 0.1M NaOH (a strong alkali) Very sharp change in pH over the addition of less than half a drop of NaOH Very little pH change during the initial 20cm3 pH 1 at the start due to 0.1M HCl (strong monoprotic acid)

strong acid (HCl) v. weak base (NH3) Curve levels off at pH 10 due to excess 0.1M NH3 (a weak alkali) Sharp change in pH over the addition of less than half a drop of NH3 Very little pH change during the initial 20cm3 pH 1 at the start due to 0.1M HCl

weak acid (CH3COOH) v. strong base (NaOH) Curve levels off at pH 13 due to excess 0.1M NaOH (a strong alkali) Sharp change in pH over the addition of less than half a drop of NaOH Steady pH change pH 4 due to 0.1M CH3COOH (weak monoprotic acid)

weak acid (CH3COOH) v. weak base (NH3) Curve levels off at pH 10 due to excess 0.1M NH3 (a weak alkali) NO SHARP CHANGE IN pH Steady pH change pH 4 due to 0.1M CH3COOH (weak monoprotic acid) Types

4.3 exercise 4

Other pH curves - acid v. carbonate Sodium carbonate reacts with hydrochloric acid in two steps... Step 1 Na2CO3 + HCl ——> NaHCO3 + NaCl Step 2 NaHCO3 + HCl ——> NaCl + H2O + CO2 Overall Na2CO3 + 2HCl ——> 2NaCl + H2O + CO2

Other pH curves - acid v. carbonate There are two sharp pH changes Sodium carbonate reacts with hydrochloric acid in two steps... Step 1 Na2CO3 + HCl ——> NaHCO3 + NaCl Step 2 NaHCO3 + HCl ——> NaCl + H2O + CO2 Overall Na2CO3 + 2HCl ——> 2NaCl + H2O + CO2 There are two sharp pH changes The second addition of HCl is exactly the same as the first because the number of moles of HCl which react with the NaHCO3 is the same as that reacting with the Na2CO3. 17.50cm3 35.00cm3

Other pH curves - polyprotic acids (H3PO4) Phosphoric acid is triprotic; it reacts with sodium hydroxide in three steps... Step 1 H3PO4 + NaOH ——> NaH2PO4 + H2O Step 2 NaH2PO4 + NaOH ——> Na2HPO4 + H2O Step 3 Na2HPO4 + NaOH ——> Na3PO4 + H2O

Other pH curves - polyprotic acids (H3PO4) Phosphoric acid is triprotic; it reacts with sodium hydroxide in three steps... Step 1 H3PO4 + NaOH ——> NaH2PO4 + H2O Step 2 NaH2PO4 + NaOH ——> Na2HPO4 + H2O Step 3 Na2HPO4 + NaOH ——> Na3PO4 + H2O There are three sharp pH changes Each successive addition of NaOH is the same as equal number of moles are involved.

Other pH curves - polyprotic acids (H3PO4) Phosphoric acid is triprotic; it reacts with sodium hydroxide in three steps... Step 1 H3PO4 + NaOH ——> NaH2PO4 + H2O Step 2 NaH2PO4 + NaOH ——> Na2HPO4 + H2O Step 3 Na2HPO4 + NaOH ——> Na3PO4 + H2O pH of H3PO4 = 1.5

Other pH curves - polyprotic acids (H3PO4) Phosphoric acid is triprotic; it reacts with sodium hydroxide in three steps... Step 1 H3PO4 + NaOH ——> NaH2PO4 + H2O Step 2 NaH2PO4 + NaOH ——> Na2HPO4 + H2O Step 3 Na2HPO4 + NaOH ——> Na3PO4 + H2O pH of NaH2PO4 = 4.4 pH of H3PO4 = 1.5

Other pH curves - polyprotic acids (H3PO4) Phosphoric acid is triprotic; it reacts with sodium hydroxide in three steps... Step 1 H3PO4 + NaOH ——> NaH2PO4 + H2O Step 2 NaH2PO4 + NaOH ——> Na2HPO4 + H2O Step 3 Na2HPO4 + NaOH ——> Na3PO4 + H2O pH of Na2HPO4 = 9.6 pH of NaH2PO4 = 4.4 pH of H3PO4 = 1.5

Other pH curves - polyprotic acids (H3PO4) Phosphoric acid is triprotic; it reacts with sodium hydroxide in three steps... Step 1 H3PO4 + NaOH ——> NaH2PO4 + H2O Step 2 NaH2PO4 + NaOH ——> Na2HPO4 + H2O Step 3 Na2HPO4 + NaOH ——> Na3PO4 + H2O pH of Na3PO4 = 12 pH of Na2HPO4 = 9.6 pH of NaH2PO4 = 4.4 pH of H3PO4 = 1.5

Page 50, Question 4a), 4bi, 4bii