Chap 9B - Acid-base equilibria B

Describe Kw + equation

• Aka autoionisation of water 

• Pure water ionises to a very slight extent: 

  • H2O (I) + H2O (I) ⇌ H3O+ (aq) + OH- (aq), Kw = [H3O+][OH-]

• Amount of water dissociated is negligible so [H2O] is constant -> omitted from Kw expression

• Kw: ionic product of water

  • Units: mol^2 dm^-6 

  • Constant at constant temperature 

  • At 25 degrees, Kw = 10^-14 for all aqueous solutions 

Calculate pH of water at 25 degrees 

Describe conditions for acid, basic and neutral solutions

Solution	Condition	At 25 degrees
Acidic	[H+] > [OH-]	[H+] > 10^7	[OH-] > 10^7	pH <  7
Neutral	[H+] = [OH-]	[H+] = 10^7	[OH-] = 10^7	pH = 7
Basic	[H+] < [OH-]	[H+] < 10^7	[OH-] < 10^7	pH > 7

Describe relationship between Kw and temp

• Dissociation of water is endothermic

• By Le Chatelier's Principle, as temperature increases, the position of equilibrium shifts right to favour the endothermic reaction by absorbing the added heat -> [H*], [OH] and Kw will increase

NOTE: At 50°C, pH of water is 6.63 -> water is NOT acidic -> at a higher temperature (above 25°C), the pH for neutrality is not 7 but at a lower pH. At higher temperatures, water still remains neutral, since [H+] is still equal to [OH-]

Describe relationship between Kw, Ka and Kb

Describe calculations for strong acids

• Commonly encountered strong acids include the monobasic acids (HCI04, HC/ and HNO3) (single dissociable proton) and a dibasic acid (H2SO4) (2 dissociable protons)

• [H+] in solution = initial [HCI] 

• If concentration of acid > 1.0 x 10-7 mol dm^-3 -> H+ (aq) ions from acid will suppress self-ionisation of water according to Le Chatelier's Principle -> assume H+ (aq) comes from acid ONLY 

• If the concentration of acid < 1.0 x 10^-7 mol dm^-3 -> include H+ (aq) from water too! 

• To show acid is strong: Using pH, find [H+], in strong acid [H+] = [HA]

Explain why the pH of 1.0 x 10-8 mol dm-3 nitric acid, HNO3, is 6.96 and not 8.00.

Describe calculation for weak acid

• HA (aq) + H2O (I) ⇌ H3O+ (aq) + A- (aq): At equilibrium, there is some H3O+, A- and some undissociated HA 

• NOTE: In reality, before any weak acid dissociates, [H+] = 10^-7 due to autoionisation of water. But weak acid is still stronger and dissociates to a larger extent -> H+ from weak acid suppress autoionisation of water according to Le Chatelier’s Principle -> assume H+ come from acid only 

Calculate the pH of 1 mol dm-3 aqueous ethanoic acid, CH3CO2H, given that its Ka = 1.8 × 10-5 mol dm-3

Explain when a salt undergoes hydrolysis

A salt will undergo hydrolysis (reaction with water) if: 

1. Anion is a conjugate base of the weak acid (Eg. CH3CO2-) 

2. Cation that is a conjugate acid of the weak base (Eg. NH4+) 

3. Cation has high charge density (Eg. Ai3+, Cr3+)

Describe salts formed from strong acid and base (Eg. NaCI)

• Form neutral salt solutions 

Eg. NaCI (pH = 7) 

• Na+ has a low charge density -> not polarising enough to undergo hydrolysis 

• CI- is a conjugate base -> does not undergo hydrolysis

Describe salts formed from strong acid and weak base (Eg. NH4CI)

• Produce H3O+ in water 

Eg. NH4CI (pH < 7) 

• NH4+ is a conjugate acid of weak base NH3 -> NH4+ will undergo hydrolysis in water producing H3O+ 

• NH4+ + H2O ⇌ NH3 + H3O+ 

Resulting solution is acidic as there is more H3O+ than OH-

Describe salts formed from weak acid and strong base (Eg. CH3CO2Na)

• Produce OH- ions in water 

Eg. CH3CO2Na 

• CH3CO2- is a conjugate base of weak acid CH3CO2H -> CH3CO2- will undergo hydrolysis in water producing OH- 

• CH3CO2- + H2O ⇌ CH3CO2H + OH- 

• Resulting solution is basic because there is more OH- than H3O+

Describe salts that contain small, highly charged cations (Eg. AICI3)

• Salts that contain small, highly charged cations form acidic solutions because these cations hydrolyse to produce excess H3O+ ions

• All metal ions exist in aqueous solution as hydrated cations but the acidity depends on charge density unhydrated metal ion

• Eg. Cr3+, Fe3+ 

Eg. AICi3 

• A/Cl completely ionises in solution: A/CI(s) + 6H20(I) → [A/(H2O)6]3+ (aq) + 3C/ (aq)

• As AI3+ ion has a high charge density (high charge and small radius), [Al(H2O)6]  undergoes hydrolysis to form H3O+ 

• AI3+ draws electrons from O-H bond in the water molecule towards itself, weakening the O-H bond to transfer the H+ ion to H2O to form H3O+ -> acidic solution