A levels chemistry: Acid-Base Equilibria

what is a strong acid

undergoes complete dissociation in aqueous solution to produce H+

• eg [H+] = initial [HCl]

• degree of dissociation, α=1

what is a weak acid

undergoes partial dissociation in aqueous solution to produce H+

• eg [H+] < initial [CH3COOH]

• degree of dissociation, 0 < α < 1

what is a strong base

undergoes complete dissociation in aqueous solution to produce OH-

• eg [OH-] = initial [NaOH]

• degree of dissociation, α=1

what is a weak

undergoes partial dissociation in aqueous solution to produce OH-

• eg [OH-] < initial [NH3]

• degree of dissociation, 0 < α < 1

what is an arrhenius acid

an acid releases H+ ions in aqueous solution

eg HCl (aq) —> H+ + Cl-

what is an arrhenius base

a base releases OH- ions in aqueous solution

characteristics of proton, H+

• high charge density

• attracts any molecule with unshared electrons such as H2O

  • in aqueous solution, a water molecule forms a dative covalent bond to the H+ ion to produce the hydronium ion

what is a bronsted-lowry acid and base

acid: a proton donor

base: a proton acceptor

• an acid-base reaction involves the transfer of a proton from the acid to the base

• an acid is only an acid in the presence of a base, and a base is only a base in the presence of an acid

what does it mean by Water is amphiprotic

Water is amphiprotic

• can either donate or accept a proton

• thus can act as both an acid and a base

• HCl(aq) + H2O (l) —> Cl- (aq) + H3O+ (aq)

  • HCl is the acid, H2O is the base

• CO3 2- (aq) + H2O (l) —> HCO3- (aq) + OH- (aq)

• CO3 2- is the base, H2O is the acid

what is a lewis acid

an electron pair acceptor

• accept a pair of electrons from a base to form a dative covalent bond

what is a lewis base

an electron pair donor

• donate a pair of electrons to an acid to form a dative covalent bond

what is an acid and its conjugate base

• in each conjugate pair, the acid and base differ from each other by a proton, H+

Conjugate acid-base pairs

• HA and A-

• HB+ and B

In the forward reaction

• HA is an acid as it donates a proton to B

• B is a base as it accepts a proton from HA

In the backward reaction

• HB+ is an acid as it donates a proton to A-

• A- is a base since it accepts a proton from HB+

what is pH of solution defined as

pH = -lg[H+]

how to find [H+] from pH

[H⁺] = 10^−pH

relation between [H+] and pH

the higher the [H+] in a solution, the lower the pH of the solution

methods of measuring pH of a solution

1. using universal indicator paper or solution for an approximate pH value

2. using a pH meter for an accurate pH determination

what is pOH

pOH = -lg[OH-]

how to find [OH-] from pOH

[OH^-] = 10^-pOH

relationship between [OH-] and pOH

the higher the [OH-] in a solution, the lower the pOH of the solution

what is the ionic product of water

Kw = [OH-][H+]

• units: mol2 dm-6

• the exact value of Kw depends on the temperature

• at 25°C, Kw = [H+][OH-] = 1.0 x 10⁻¹⁴

  • since Kw is a constant, the concentrations of H+ and OH- are limited by this value

what is the pH and pOH of a neutral aqueous solution

For a neutral aqueous solution, [H+] = [OH-]

Kw = [H+][OH-] = 1.0 x 10⁻¹⁴

[H+]² = 1.0 x 10⁻¹⁴

[H+] = √1.0 x 10⁻¹⁴ = 1.0 x 10⁻⁷ mol dm⁻³

pH = -lg[H+] = -lg (1.0 × 10⁻⁷) = 7

[OH-] = [H+] = 1.0 x 10⁻⁷ mol dm⁻³

pOH = -lg[OH-] = -lg (1.0 × 10⁻⁷) = 7

how does the value of Kw vary with temperature

the value of Kw increases with temperature

• this is because the self-ionisation of water is an endothermic process

• As temperature is increased, the equilibrium position of the above reaction shifts to the right to absorb the heat added. This is in accordance with LCP

• with the forward endothermic reaction favoured as temperature increases, there will be higher concentrations of H+ and OH- ions

what is pKw

pKw = -lg Kw

express pKw in terms of pH and pOH

pKw = pH + pOH

At 25°C, pKw = pH + pOH = 14