Acid-base equilibria and buffers 1

1. Acid-Base Equilibria and Buffer Solutions REVISION

2. Assessment criteria

• understand acidity/alkalinity in aqueous solution and that, in this medium, the
approaches of Arrhenius and Bronsted-Lowry are equivalent
• recognise that in aqueous solution acids are proton donors and bases are proton
acceptors
• - understand pH as -log10[H+], and be able to convert pH to concentration and the
reverse
• be able to calculate the pH of a strong acid
• know that water is weakly dissociated
• know the ionic product of water, Kw and that Kw = [H+] [OH-]
• understand that weak acids and bases dissociate slightly in water.
• be able to write, and perform calculations with Ka
• understand how to obtain and work with pH curves for the various acid/base
combinations
• understand indicators as having a characteristic pH at their endpoint and be able to use
this to choose an appropriate indicator for a titration

3. ACID-BASE THEORIES

The Arrhenius Theory
Acids produce H+
Bases produce OH-
The Bronsted-Lowry Theory
Acids are proton donors
Bases are proton acceptors
Lewis Theory
Acids are electron pair acceptor
Bases are electron pair donor
HCl(aq) → H+(aq) + Cl–(aq)
Lewis acids have an
empty orbital
NaOH(s) → Na+(aq) + OH–(aq)
and Lewis bases have a
lone pair of electrons
1. (a) Do you remember the 6
strong acids?
(b) Name one dibasic acid.
2. Identify the Brownsted-Lowry
acid and base in this reaction.
H2SO4 + HNO3 HSO4– + H2NO3+
3. Identify the Lewis acid in the
reaction below.

4. Strong and weak acid

Strong acids and strong bases
Dissociate / ionize / break down completely in
water
Weak acids and weak bases
Ionize / dissociate partially when dissolved in water
HCl(aq) → H+(aq) + Cl–(aq)
pH= -log10[H+]
NaOH(aq) → Na+(aq) + OH–(aq)
pOH= -log10[OH-]
pH = 14-pOH
What is the pH of 0.01 molar HCl?
0.1 molar H2SO4?
0.1 molar KOH?
2.0
0.699
13.0
The bigger the Ka the stronger the acid.
What is the pH of 0.01 mol dm-3 HNO2 of Ka
5.6x10-4?
2.63

5. Self / Autoionisation of water

Equation
H2O(l) ⇋ H+(aq) + OH–(aq)
or
2H2O(l) ⇋ H3O+(aq) + OH–(aq)
Ionic product of H20 / Dissociation
constant
Kw=[H+][OH-]
Units?
Endothermic process so the ionic
product of water (Kw) increases with
temperature
For pure water [H+]=[OH-]=