Acid-Base Equilibria I - pH, Kw and Ka
January 2002 Number 25
1
ChemFactsheet
To succeed in this topic you need to be able to:
••
••
•understand the writing of Kc expressions and their units (covered
in Factsheet No.21);
••
••
•use your calculator to convert numbers into ‘logarithms to the
base 10 (log10) using the lg key;
••
••
•turn log10 values back into numbers using the inverse of log10 i.e.
the 10x key (or second function above the lg key).
After working through this Factsheet you will be able to:
••
••
•use the A2 level definitions of acids and bases (Br∅∅
∅∅
∅nsted-Lowry
theory) and identify conjugate acid/base pairs;
••
••
•define pH and perform calculations using its mathematical
expression;
••
••
•understand how to use the ionic product of water, Kw, to calculate
the pH of a base;
••
••
•understand the difference between the terms weak, strong,
concentrated and dilute as applied to acids and bases;
••
••
•write Ka expressions for weak acids and perform calculations
using these expressions.
Generally, the conjugate base of a weak acid will show weak basic character,
whilst the conjugate base of a strong acid will not show basic character.
Usually there are two acid/base pairs:
e.g. HCl + NH 3 →NH4++ Cl−
acid 1 base 2 acid 2 base 1
e.g. H2SO4+H
2O→H3O++ HSO4−
acid 1 base 2 acid 2 base 1
This is ‘chase the H+’ because it is not seen as a separate entity in the
equation although it is being donated and accepted.
The 1 and 2 used have no significance except to identify which acid goes
with which base.
For practice on conjugate acid-base pairs go to question 1.
Exam Hint
:-
Questions tend to be based on:
• learning the definitions for acids/bases, pH, pOH, K
w
and K
a
• performing calculations on them
The calculations all involve using the lg and 10
x
keys on the calculator
and being able to input ‘powers to the 10’ e.g. 1.2 x 10
-3
, 5 x 10
-6
- this
usually involves using the EXP key - eg 1.3 EXP -2 = 1.3 x 10
-2
(be
careful where you put the minus!)
* Many marks are lost in examination questions on this topic by
candidates who cannot use their calculators for these purposes. you
must practice this until you are competent!*
Br∅∅
∅∅
∅nsted-Lowry Theory
At GCSE level you were taught that acids needed water to be present to
show their acidic properties, and that they produced H+ ions. The Br∅∅
∅∅
∅nsted-
Lowry Theory takes this one step further:
Acids are species that donate protons/H+
Bases are species that accept protons/H+
Monobasic acids (e.g. HCl, HNO3) donate one H+, dibasic acids (e.g. H2SO4)
donate two H+ and tribasic acids (e.g. H3PO4) donate three H+
Acids and bases are linked by the H+ as this example shows:-
HCl → H+ + Cl−
acid conjugate base
The HCl and Cl- are a conjugate acid/base pair.
Note that this new definition of bases includes species such as Cl− that
would not have been regarded as bases at GCSE, and do not show basic
character.
Using acid and base terms
Strong - acid or base which undergoes 100% dissociation as shown by
→
e.g. HNO3 → H+ + NO3−
NaOH → Na+ + OH−
N.B. At A2 level there are only
four strong acids: HCl, HNO3, H2SO4and H3PO4
two strong bases: NaOH and KOH
Weak - acid or base that only partially dissociates so it has the equilibrium
sign ž
e.g. CH3COOH ž CH3COO− + H+
NH4OH ž NH4+ + OH−
N.B. At A2 level organic acids e.g. methonoic, ethonoic, proponoic, etc
are weak, but NH3(aq) / NH4OH is the only weak base.
Concentrated/dilute
These terms refer to the ratio of moles to volume i.e. mol dm-3 of the
solutions of acid and bases,
i.e. 2 mol dm-3 (2M) would be considered ‘dilute’ and 6 mol dm-3 (6M)
‘concentrated’.
There is no ‘cut-off’ value between ‘dilute’ and ‘concentrated’ as
descriptions of acids and alkalis – and it will not give you any problems.
So it is possible to have a concentrated weak acid and a dilute strong
acid.
