Brown's Chemistry Advanced A Level Notes - Theoretical–Physical
Chemistry – Equilibria – Chemical Equilibrium Revision Notes PART 6
6.1 Salt Hydrolysis, acidity and alkalinity of salt solutions
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Part 6 sub–index
Salt hydrolysis (this page)
Acid–base indicator theory, pH curves and
Buffers – definition, formulation
of buffer function
What is salt hydrolysis? Why can
salt solutions can be either neutral, alkaline or acidic?
(I shouldn't use it, but
M = old fashioned shorthand for mol dm–3 !)
6.1 Salt Hydrolysis, acidity and
alkalinity of salt solutions
taught at lower academic levels that salts e.g. sodium chloride,
dissolve in water to form neutral solutions of pH 7.
In reality, and
looking at a wider variety of 'salts', the picture is much more complicated
and a 'salt' solution may be acid, neutral or alkaline
depending on the nature of the interaction of the salt ions with water
- do any of the salt ions have an acidic or basic nature - can they
act as a Bronsted-Lowry acid or base.
The reasons are
quite clear when you consider the possible Bronsted–Lowry interactions
that can take place between the ions of the salt and water.
acidic salt solutions: pH <7
nearly neutral salt solutions: pH approx. 7
alkaline salt solutions: pH>7
6.1.3a: Salts of a
weak acid and a strong base e.g. sodium ethanoate
sodium ion shows no acidic character but the ethanoate ion is a
strong conjugate base of a the weak ethanoic acid (pKa
= 4.76, Ka = 1.74 x 10–5
mol dm–3), so an acid–base
hydrolysis reaction occurs to generate hydroxide ions to raise the
pH to about pH 9.
cyanide: is the salt of the very strong base potassium hydroxide
and the very weak hydrocyanic acid (pKa = 9.31, Ka =
4.9 x 10–10 mol dm–3). The hydrated potassium ion shows
no acidic behaviour, but the cyanide ion is a strong conjugate base
of the very weak hydrocyanic acid (HCN) which interacts with water to generate
hydroxide ions. Hydrocyanic acid (pKa = 9.4) is weaker
than ethanoic acid (pKa = 4.76) , so the equilibrium is
more on the right, more OH–, and so the pH is more
alkaline, i.e. over 9.
6.1.3c: Sodium carbonate
is the 'salt' of the strong base sodium hydroxide and the very weak
hydrated sodium ion shows no acidic character but the carbonate
ion is a strong conjugate base of a the weak 'carbonic' acid, so
an acid–base hydrolysis reaction occurs to generate hydroxide ions
to raise the pH.
+ H2O(l) HCO3–(aq)
Advanced Equilibrium Chemistry Notes Part 1. Equilibrium,
Le Chatelier's Principle–rules
* Part 2. Kc and Kp equilibrium expressions and
calculations * Part 3.
Equilibria and industrial processes * Part 4
Partition between two
phases, solubility product Ksp, common ion effect,
ion–exchange systems *
Part 5. pH, weak–strong acid–base theory and
calculations * Part 6. Salt hydrolysis,
acid–base titrations–indicators, pH curves and buffers * Part 7.
Redox equilibria, half–cell electrode potentials,
electrolysis and electrochemical series
pressure, boiling point and intermolecular forces watch out for sub-indexes
to multiple sections or pages
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