Transcript Salt Hydrolysis

Title: Lesson 10: Salt Hydrolysis
Learning Objectives:
– Understand why some salts do not form neutral solutions
– Predict whether a given salt will form an acidic, neutral or alkaline solution
– Complete an experiment to determine any trends in acidity/basicity of salt
solutions
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a)
Explain what is meant by the term buffer solution.
b)
Calculate the pH of a solution prepared by mixing 50.0
cm3 of 0.200 mol dm–3 CH3COOH(aq) and 50.0 cm3 of
0.100 mol dm–3 NaOH(aq), showing your working.
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Salt Hydrolysis
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A neutralisation reaction between an acid and a base produces a salt – an ionic
compound containing a cation from the parent base and an anion from the
parent acid.
The cation is the conjugate acid of the parent base.
The anion is the conjugate base of the parent acid.
However, even though the salts are products of a neutralisation reaction, they do
not all form neutral aqueous solutions.
Their pH in solution depends on whether and to what extent their ions, which
are conjugate acids and bases, react with water and hydrolyse it, releasing H+ or
OH- ions.
The weaker the acid or base, the stronger the conjugates and vice versa.
The relative strengths of the conjugate acid and bases in the salt determines the
hydrolysis reaction and so the pH of the salt solution.
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Acid Salt Hydrolysis
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When you dissolve sodium chloride in water it forms a neutral solution of
Na+ and Cl- ions:
NaCl(s)  Na+(aq) + Cl-(aq)
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When you dissolve ammonium chloride in water it forms a weakly acidic
solution of NH4+ and Cl- ions
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Why?....discuss
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Why acidic?
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Ammonium chloride contains the NH4+ ion
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NH4+ is the conjugate acid of the weak base NH3:
NH3 + H2O  NH4+ + OH-
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The NH4+ ion therefore is weakly acidic and will establish the following
equilibrium:
NH4+  NH3 + H+
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Thus, ammonium chloride will form a weakly acidic solution
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Basic Salt Hydrolysis
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When you dissolve sodium chloride in water it forms a neutral solution of
Na+ and Cl- ions:
NaCl  Na+(aq) + Cl-(aq)
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When you dissolve sodium ethanoate in water it forms a weakly basic
solution of Na+ and CH3COO- ions
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Why?....discuss
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Why basic?
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Sodium ethanoate contains the CH3COO- ion
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CH3COO- is the conjugate base of the weak acid CH3COOH:
CH3COOH  H+ + CH3COO-
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The CH3COO- ion therefore is weakly basic and will establish the following
equilibrium:
CH3COO- + H2O  CH3COOH + OH-
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Thus, sodium ethanoate will form a weakly basic solution
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Salt of strong acid and strong base – no hydrolysis
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Strong acid and strong base  salt with weak conjugate acid and base.
Virtually no hydrolysis of ions, the pH is close to neutral.
Salt of weak acid and strong base – anion hydrolysis
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Weak acid  strong conjugate base (A-)
Hydrolysis occurs
Release of OH- causes the pH of the solution to increase
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Salt of strong acid and weak base – cation
hydrolysis
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Weak base  strong conjugate acid (M+)
Hydrolyses water
Release of H+ causes the pH of the solution to decrease.
The above cation is a non metal, but it gets a little more complex when the
cation is a metal ion.
For a metal ion, it must have a high charge density (e.g. Al3+, Fe3+) to carry out
hydrolysis
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Salt of weak acid and weak base
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Both acid and base will form strong conjugates.
Hydrolysis carried out.
pH depends on the relative Ka and Kb values of the acids and bases involved.
Summary
pH can be deduced from the relative strengths of the parent acids and bases, as
seen below:
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Solutions
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Recap: Conjugate Acids and Bases
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A conjugate acid/base pair are two species that differ by a single proton.
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A conjugate base is a species with has one less proton
A conjugate acid is a species with one more proton
For example:
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Hydrochloric acid, HCl
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HCl is the acid, Cl- is a conjugate base
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The HCl can donate a proton…it is an acid
The Cl- could accept a proton….it is a base
Ammonia, NH3
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NH3 is the base, NH4+ is its conjugate acid
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The NH3 can accept a proton….it is a base
The NH4+ could donate a proton….it is an acid
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Rules of Thumb
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Conjugate bases:
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Conjugate acids:
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Conjugate bases of weak acids will form basic solutions
Conjugate bases of strong acids will form neutral solutions
Conjugate acids of weak bases will form acidic solutions
Conjugate acids of strong bases will form neutral solutions
The final pH of an individual salt will depend on the relative acidity of both the
conjugate acid and conjugate base is formed from
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Investigate
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Plan and conduct an
experiment to order the
following salts from most to
least acidic.You should make a
prediction first:
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Sodium ethanoate, Mr = 82.0
Sodium chloride, Mr = 58.4
Sodium carbonate, Mr = 106
Ammonium chloride, Mr = 53.5
Ammonium carbonate, Mr = 96.1
Ammonium Ethanoate, Mr = 77.1
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Plan and conduct an
experiment to look at the
effect of charge density of
the metal ion on acidity.You
will have access to:
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Sodium chloride, Mr = 58.5
Potassium chloride, Mr = 74.6
Calcium Chloride, Mr = 111
Magnesium chloride, Mr = 95.2
Aluminium chloride, Mr = 133
Iron (II) chloride, Mr = 127
Iron (III) chloride, Mr = 162
Copper (II) chloride, Mr = 134
Analysis
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You need to fully explain the results of your experiment, using equations
where necessary.
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You may wish to use data from the data booklet and other sources to help
with your explanations.
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Key Points
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Salts of a weak acid and strong base form basic solutions:
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Salts of a weak base and a strong acid form acidic solutions:
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Due to the conjugate acid of the weak base being acidic
Salts of a strong acid and strong base form neutral solutions
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Due to the conjugate base of the weak acid being basic
Neither the conjugate base of the acid or the conjugate acid of the base form acid/base
equilibria
Salts of a weak acid and weak base can be acidic, basic or neutral
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Depends on the relative basicity of the conjugate base and acidity of the conjugate acid
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