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Brown's Advanced A Level Chemistry Revision Notes
Theoretical–Physical
Advanced Level
Chemistry – Equilibria – Chemical Equilibrium Revision Notes PART 6.1
6.1 Salt Hydrolysis, acidity and alkalinity of salt solutions
What is salt hydrolysis? Why can
salt solutions can be either neutral, alkaline or acidic?
Chemical Equilibrium Notes Parts 5 & 6 Index
(I shouldn't use it, but
M = old fashioned shorthand for mol dm–3 !)
6.1
Salt Hydrolysis, acidity and
alkalinity of salt solutions
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Despite being
taught at lower academic levels that salts e.g. sodium chloride,
dissolve in water to form neutral solutions of pH 7.
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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.
-
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.
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6.1.1
Examples of
acidic salt solutions: pH <7
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6.1.2
Examples of
nearly neutral salt solutions: pH approx. 7
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6.1.3
Examples of
alkaline salt solutions: pH>7
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6.1.3a: Salts of a
weak acid and a strong base e.g. sodium ethanoate
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The hydrated
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.
-
6.1.3b: Potassium
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.
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6.1.3c: Sodium carbonate
is the 'salt' of the strong base sodium hydroxide and the very weak
'carbonic acid'
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Again the
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.
-
CO32–(aq)
+ H2O(l) HCO3–(aq)
+ OH–(aq)
-
–
Chemical Equilibrium Notes Parts 5 & 6 Index
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Chemical Equilibrium Notes Parts 5 & 6 Index |