3.9 Acids and Bases
Acid Definitions
Acid – A proton donor
Monoprotic Acid – Releases one H+ ion e.g. HCl, HNO3
Diprotic Acid – Releases two H+ ions e.g. H2SO4
Strong Acid – Fully dissociates in solution to release H+ ions e.g. HCl H+ + Cl-
Weak Acid – Partially dissociates in solution to form H+ ions e.g. CH3COOH ⇌ CH3COO- + H+
Calculating pH of strong acids
pH – measure of hydrogen ion concentration of a solution
pH = -log[H+]
[H+] = 10-pH
- For monoprotic acids [acid]=[H+]
- For diprotic acids 2[acid]=[H+]
Dilution of a strong acid
1. Calculate moles in original volume
2. Calculate new conc. with original moles and new volume
3. Put [H+] into pH expression
Kw and Ionic Product of Water
H2O ⇌ H+ + OH-
- When water dissociates it is a reversible reaction and establishes an equilibrium
- There are equal moles of OH- and H+
Able to use equilibrium expression:
Kw = [H+][OH-]
Units – mol2dm-6
Pure water:
[H+] = [OH-]
Kw = [H+]2
, Impure water:
e.g. an alkali
Dissociation of water is endothermic
- As temperature increases, pH decreases
- This means more H+ released and [H+] increases
- Equilibrium shifts to endothermic side as temperature increases to reduce the change
- Shifts to dissociated side so the dissociation of water is endothermic
Does water remain neutral even as pH changes?
- Yes – there is no excess of H+ or OH-
- Neutralisation – H+ + OH- H2O
Base Definitions
Base – Proton acceptor (releases OH- ions in solution)
Monobasic bases – Releases one OH- ion e.g. NaOH
Dibasic bases – Release two OH- ions in solution e.g. Ca(OH)2
Calculating pH of strong bases
- Need to find [H+] but bases only release [OH-] ions in solution
- Use the ionic product of water (Kw)
Kw = [H+][OH-]
Under standard conditions Kw = 1.0 x10-14
1. Calculate [OH-]
Monobasic [base] = [OH-]
Dibasic 2[base] = [OH-]
2. Use Kw to calculate [H+]
3. Put [H+] into pH expression
Dilution of a strong base
1. Calculate moles in original solution
2. Calculate [base] using new volume
3. Use [base] to find [OH-]
4. Use in Kw expression to find [H+]
5. Put [H+] in pH expression
Acid Definitions
Acid – A proton donor
Monoprotic Acid – Releases one H+ ion e.g. HCl, HNO3
Diprotic Acid – Releases two H+ ions e.g. H2SO4
Strong Acid – Fully dissociates in solution to release H+ ions e.g. HCl H+ + Cl-
Weak Acid – Partially dissociates in solution to form H+ ions e.g. CH3COOH ⇌ CH3COO- + H+
Calculating pH of strong acids
pH – measure of hydrogen ion concentration of a solution
pH = -log[H+]
[H+] = 10-pH
- For monoprotic acids [acid]=[H+]
- For diprotic acids 2[acid]=[H+]
Dilution of a strong acid
1. Calculate moles in original volume
2. Calculate new conc. with original moles and new volume
3. Put [H+] into pH expression
Kw and Ionic Product of Water
H2O ⇌ H+ + OH-
- When water dissociates it is a reversible reaction and establishes an equilibrium
- There are equal moles of OH- and H+
Able to use equilibrium expression:
Kw = [H+][OH-]
Units – mol2dm-6
Pure water:
[H+] = [OH-]
Kw = [H+]2
, Impure water:
e.g. an alkali
Dissociation of water is endothermic
- As temperature increases, pH decreases
- This means more H+ released and [H+] increases
- Equilibrium shifts to endothermic side as temperature increases to reduce the change
- Shifts to dissociated side so the dissociation of water is endothermic
Does water remain neutral even as pH changes?
- Yes – there is no excess of H+ or OH-
- Neutralisation – H+ + OH- H2O
Base Definitions
Base – Proton acceptor (releases OH- ions in solution)
Monobasic bases – Releases one OH- ion e.g. NaOH
Dibasic bases – Release two OH- ions in solution e.g. Ca(OH)2
Calculating pH of strong bases
- Need to find [H+] but bases only release [OH-] ions in solution
- Use the ionic product of water (Kw)
Kw = [H+][OH-]
Under standard conditions Kw = 1.0 x10-14
1. Calculate [OH-]
Monobasic [base] = [OH-]
Dibasic 2[base] = [OH-]
2. Use Kw to calculate [H+]
3. Put [H+] into pH expression
Dilution of a strong base
1. Calculate moles in original solution
2. Calculate [base] using new volume
3. Use [base] to find [OH-]
4. Use in Kw expression to find [H+]
5. Put [H+] in pH expression
