What do they have in common?

And finally ….. BUFFERS

Objectives 1.be able to explain qualitatively the action of acidic and basic buffers; 2.know some applications of buffer solutions; 3.be able to calculate the pH of acidic buffer solutions.

How informative is the definition below? How would you improve it? “A buffer resists changes in pH.”

Buffer ”A solution able to resist changes in pH when small amounts of acid or base are added. It is also able to maintain its pH on dilution with water.”

Acidic Buffer “Made by mixing a weak acid and one of its salts. It maintains a solution at a pH below 7.” (Can also be made by partial neutralisation of a weak acid with a strong base)

Alkaline Buffer “Made by mixing a weak base and one of its salts. It maintains a solution at a pH above 7.”

How does an acidic buffer work? CH 3 COOH (aq) ↔ CH 3 COO - (aq) + H + (aq) Acid 1 Base 2 un-dissociated acid conjugate base/anion e.g. ethanoic acid CH 3 COOH / sodium ethanoate CH 3 COO - Na + CH 3 COOH (aq) + OH - (aq) ↔ CH 3 COO - (aq) + H 2 O (l) The un-dissociated acid can remove any added OH - ions CH 3 COO - (aq) + H + (aq) ↔ CH 3 COOH (aq) The conjugate base can remove any added H+ ions

How does an acidic buffer work? CH 3 COOH (aq) ↔ CH 3 COO - (aq) + H + (aq) un-dissociated acid conjugate base/anion A large reservoir of the weak acid and its conjugate base is needed. [CH 3 COO - ][H + ] [CH 3 COOH] K a = [CH 3 COOH] [CH 3 COO - ] [H + ] =KaKa [weak acid] [salt/anion] KaKa

How does an acidic buffer work? CH 3 COOH (aq) ↔ CH 3 COO - (aq) + H + (aq) un-dissociated acid conjugate base/anion [CH 3 COOH] [CH 3 COO - ] [H + ] =KaKa [weak acid] [salt/anion] 1. What does pH depend on? 2. What happens when the [acid] = [anion]? 3. What will happen when a small amount of acid (H+ ions) is added to the buffer solution? 4. What will happen when a small amount of base (OH- ions) is added to the buffer solution? 5. Why would the pH be maintained? KaKa

How does an acidic buffer work? 1.pH depends on Ka and the ratio of [acid] and [anion] 2.pH = pKa 3.Eqm will shift to the LHS and a little more ethanoic acid will be formed 4.Eqm will shift to the RHS as the acid will mop up the OH- ions and a little more of the ethanoate ion will be formed 5.Because the acid and anion concentrations are very large and the concentrations will not be affected by much K a [CH 3 COOH] [CH 3 COO - ] [H + ] =

How does a basic buffer work? NH 3 (aq) + H+ (aq) ↔ NH 4 + (aq) The un-dissociated ammonia can remove any added H+ ions e.g. ammonia/ammonium chloride NH 4 + (aq) + OH- (aq) → NH 3 (aq) + H 2 O (aq) The conjugate acid can remove any added OH- ions NH 3 (aq) HCl (aq) ↔ NH 4 + (aq) + Cl - (aq) Base 1 Acid 2 /conjugate acid

Calculating buffer pH Calculate the pH of the buffer made by mixing 14 cm 3 of a 2.0 moldm -3 solution of ethanoic acid (K a = 1.76 x moldm -3 ) with 15 cm 3 of a 1.5 moldm -3 solution of sodium ethanoate. pH=4.66

14.0 cm cm moldm moldm -3 Strategy 1.Moles of ethanoic acid 2.Moles of ethanoate ions 3.Use Ka expression 4.Cancel out volume! 5.[H + ] concentration & pH CH 3 COOH(aq) → CH 3 COO - (aq) + H + pH = 4.66

Calculate the change in pH of the buffer solution when 10 cm 3 of a 0.10 moldm -3 solution of hydrochloric acid is added. Calculating change in buffer pH pH=4.62 Change in pH = 0.04

10 cm 3 of a 0.10 moldm -3 solution of hydrochloric acid is added. Strategy 1.Moles of acid added 2.Equilibrium shifts 3.New moles of ethanoic acid 4.New moles of ethanoate ions 5.[H + ] concentration & pH CH 3 COOH(aq) → CH 3 COO - (aq) + H + pH = 4.63

Practise …..