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Brown's Chemistry
Theoretical-Physical
Advanced Level
Chemistry - Equilibria - Chemical Equilibrium Revision Notes PART 6.3
6.3 Buffer solutions - definition, formulation and
action
What is a buffer? How do buffers work?
Buffers are defined and their actions explained with appropriate
examples such as ethanoic acid/sodium ethanoate and ammonia/ammonium
chloride mixtures.
GCSE/IGCSE
reversible reactions-equilibrium notes
*
GCSE/IGCSE notes on acids and bases
Equilibria
Part 6 sub-index: 6.1 Salt hydrolysis * 6.2
Acid-base indicator theory, pH curves and
titrations * 6.3 Buffers - definition, formulation
and action * 6.4 Buffer calculations * 6.5
Case studies
of buffer function
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,
solubility product and ion-exchange *
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
*
Part 8 Phase equilibria-vapour
pressure, boiling point and intermolecular forces
6.3
Buffer solutions - definition, formulation and action
-
6.3.1 A buffer is a
solution that minimises pH change on the addition of small amounts
of acid or alkali.
-
Buffers and
their chemical reactions must obey Le Chatelier's Equilibrium
Concentration Principle, and act in a way to remove H+
and OH- ions. BUT, they cannot theoretically
prevent the pH being lowered/raised by the addition of acid/alkali,
however small the change.
-
Note that any buffer will eventually be 'used up' if
large quantities of acid or alkali are added to the solution.
-
6.3.2 Typical
buffers and their action.
-
Buffering
action example 6.3.2a
-
A mixture of
a weak acid and the salt of the weak acid with a strong base.
-
Organic acids
like methanoic, ethanoic, propanoic, citric, benzenedicarboxylic
etc. are frequently used in buffer mixtures i.e. those with the
carboxylic acid functional group -COOH
-
The salts are
usually those of the strong base-alkalis sodium and potassium
hydroxide.
-
e.g. ethanoic
acid CH3COOH and sodium ethanoate CH3COO-Na+
gives buffers in the range pH 3.7-5.6
-
CH3COOH
and CH3COO- constitute a conjugate
acid-base pair.
-
In solution
most of the weak acid is NOT ionised and the relatively high
concentration of the CH3COO- ion actually
inhibits ionisation.
-
The salt is
fully ionised in solution to give a relatively high concentration of
the ethanoate ions.
-
How to choose the best
weak acid and its corresponding salt is explained in
section 6.4.1
-
Buffering
action example 6.3.2b
-
A mixture of
a weak base and the salt of the weak base with a strong acid.
-
e.g. ammonia NH3
and ammonium chloride NH4+Cl-
-
NH4+
and NH3 constitute a conjugate acid-base pair.
-
In solution most
of the ammonia is NOT ionised (and even suppressed by the ammonium
ions from the salt).
-
The salt is
fully ionised in solution giving relatively high concentrations of
the ammonium ion.
-
-
-
6.3.3 Preparing
buffer solutions.
-
Quite often
several solutions of salts, weak acids/bases are prepared and then
mixed in different ratios to provide buffers of a wide pH range.
-
Sometimes a
single salt will do to give a single accurate pH value for calibrating a pH
meter. (see Case study 6.5.1)

Equilibria
Part 6 sub-index: 6.1 Salt hydrolysis * 6.2
Acid-base indicator theory, pH curves and
titrations * 6.3 Buffers - definition, formulation
and action * 6.4 Buffer calculations * 6.5
Case studies
of buffer function A level Revision notes for GCE Advanced
Subsidiary Level AS Advanced Level A2 IB
Revise AQA GCE Chemistry OCR GCE Chemistry Edexcel GCE Chemistry Salters
Chemistry CIE Chemistry, WJEC GCE AS A2 Chemistry, CCEA/CEA GCE AS A2 Chemistry revising courses for pre-university students
(equal to US grade 11 and grade 12 and AP Honours/honors level courses)

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