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Advanced A level theoretical
chemistry - acid-base equilibrium
Part
5.4 Definition of a weak acid, examples explained, pH, Ka
and pKa for weak acid calculations
explained and worked through[Author
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acid–base equilibrium revision notes on
weak acid theory
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Part 5 acid-base equilibria and calculations
5.0
Basic notes and equations on acids, bases, salts,
uses of
acid–base titrations
5.1
Equilibria:
Lewis and Bronsted–Lowry acid–base theories
5.2
Self–ionisation of water and pH scale
5.3
Strong acids – examples and pH calculations
5.4
Weak acids – examples & pH, Ka and pKa calculations
(sub-index for this page)
5.5
Strong bases – examples and pH calculations
5.6
Weak bases – examples and pH, Kb and pKb calculations
5.4 Definition, examples and pH, Ka
and pKa calculations
of weak acids
Sub-index for this page on weak acids
5.4.0
Introduction
5.4.1
Definition and examples of WEAK ACIDS in terms of Bronsted-Lowry
theory
5.4.2
Expressing the weak acid-base equilibrium in terms of the conjugate
base
5.4.3
Ionic acid–base
equilibrium can be more complicated in the case of dibasic/diprotic
acids
5.4.4
Examples of weak inorganic acids
5.4.5
Comparing the chemical properties of weak and
strong acids
5.4.6
Considering the equilibrium expression for a weak
acid and Ka or pH calculations
5.4.0 Introduction
What is a weak acid?
How do we write
equilibrium expressions to show the acid dissociation–ionisation of a weak
acids? What is a weak acid's pKa?
How do we calculate the pH of a
solution of a weak acid?
How do we calculate the Ka of a weak acid?
First some mathematical reminders
(i)
pH = –log10[H+(aq)], (ii)
[H+(aq)] = 10–pH
(iii)
pKa = –log10(Ka), (iv)
Ka = 10–pKa
[H+(aq)] =
hydrogen ion concentration mol dm–3
and
Ka
= acid dissociation constant mol dm–3
5.4.1 Definition
and examples of WEAK ACIDS expressed in terms of Bronsted-Lowry
theory
-
Weak acids
are only partially ionised in water.
-
In principle the
equilibrium reaction is in terms of a Bronsted-Lowry acid-base
interaction:
-
HA(aq) + H2O(l)
H3O+(aq)
+ A–(aq)
-
or more simply:
HA(aq)
H+(aq)
+ A–(aq)
-
The low % of
ionisation gives a less acidic solution of higher pH than for strong acids, but still
pH < 7.
-
HA simply
represents the weak acid.
-
-
5.4.2
Expressing the weak acid-base equilibrium in terms of the conjugate base
-
The
equilibrium can also be expressed as the acid–base reaction of the
conjugate base with water.
-
e.g. for
ethanoic acid:
CH3COO–(aq)
+ H2O(l)
CH3COOH(aq)
+ OH–(aq)
-
CH3COO–(aq)
+ H2O(l)
CH3COOH(aq)
+ OH–(aq)
-
|
Kb =
|
[CH3COOH(aq)] [OH–(aq)] |
|
–––––––––––––––––––––––– |
|
[CH3COO–(aq)] |
-
Note that:
Ka–acid
x Kb–conj. base = Kw and pKa
+ pKb = pKw
-
check it out for yourself.
-
-
5.4.3 Ionic acid–base
equilibrium can be more complicated in the case of dibasic/diprotic
acids.
-
Two equilibria are
involved in the ionisation/dissociation processes
-
e.g.
, ethanedioic acid, more simply shown as HOOC–COOH.
-
HOOC–COOH(aq)
H+(aq)
+
HOOC–COO–(aq)
-
HOOC–COO–(aq)
H+(aq)
–OOC–COO–(aq)
-
and Ka1
> Ka2, showing, not surprisingly, the 1st proton is
released more readily than the 2nd.
-
Ka1 =
5.89 x 10–2 mol dm–3 (pKa1 = 1.23)
-
and Ka2 = 5.24 x 10–5 mol dm–3 (pKa2
= 4.28)
-
-
5.4.4 Examples of weak inorganic acids
5.4.5 Comparing the chemical properties of weak and
strong acids
Comparison of weak and strong acids in terms of
equimolar aqueous solutions.
-
Some
observations from simple experiments.
-
Due to the difference in the concentration
of H+ ions produced. e.g. say for the sake of argument,
0.1–1.0 molar solutions of hydrochloric acid (100% ionised) and
ethanoic acid (approx. 2% ionised).
-
This means the hydrochloric acid
is effectively about 50x more acidic than the ethanoic acid and
results in the following sorts of observations:
-
(1) pH of
solution and Ka/pKa
-
For equimolar
solutions the pH of HCl(aq) is much lower than for
CH3COOH(aq) (about pH 0.0–1.0 and 2.5–3.0
respectively, and remember 1 pH unit change represents a 10x [H+]
ion change in concentration.
-
The acid
dissociation/ionisation constant show very different numerical value
ranges.
-
The Ka
for strong acids is large, typically >102 to 1010
mol dm–3 and a negative pKa, typically
–2 to –10.
-
The Ka
for weak acids is small, typically 10–2 to 10–10
mol dm–3 and a positive pKa, typically 2 to 10.
-
-
-
(2) Chemical
reactivity
-
(3) Electrical
conductivity
-
(4) Differences
in enthalpy of neutralisation ΔHneutralisation
-
Their widely
differing values and simplified explanations.
-
The ΔHneut
for a strong acid and strong base (SA+SB) it is usually
about –57.1 to –57.3 kJ mol–1, because they are fully
ionised to give the H+ and OH– ions
respectively, so the ΔH value essentially corresponds to the ΔH
for the reaction ...
-
H+(aq)
+ OH–(aq) ==>
H2O(l) (ΔH = –57.1 kJ mol–1)
-
e.g. for
the SA/SB pairs: HCl/NaOH, HCl/KOH, HNO3/NaOH, HNO3/0.5Ba(OH)2,
-
The ΔHneut
for a strong acid–weak base (SA+WB) OR a weak
acid–strong base neutralisation is less exothermic than the
SA+SB above, and in some cases considerable less! e.g. reacting
pair and (ΔH/kJmol–1),
-
The ΔHneut
for a weak acid and weak base (WA+WB) neutralisation the
ΔH values are even less exothermic.
-
WA/WB: CH3COOH/NH3
(–50.2), HCN/NH3 (–5.4)
-
CH3COOH(aq)
+ NH3(aq)
CH3COO–(aq) +
NH4+(aq)
-
HCN(aq)
+ NH3(aq)
CN–(aq) + NH4+(aq)
-
Basically,
the weaker and weaker the acid or base, the less and less the
neutralisation goes to completion, hence the reaction becomes
less and less exothermic.
5.4.6 Considering the equilibrium expression for a
weak acid and Ka or pH calculations
-
(1) In principle the
full equilibrium expression for any weak acid HA is
-
|
Kc =
|
[H3O+(aq)] [A–(aq)] |
|
––––––––––––––––– |
|
[HA(aq)] [H2O(l)] |
-
(2) Weak acid
calculations – calculating the pH of a weak acid
-
See also a calculation
involving sulfuric acid
WHAT NEXT?
INDEX of ALL my chemical equilibrium
context revision notes
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
*
Part 8.
Phase equilibria–vapour
pressure, boiling point and intermolecular forces watch out for sub–indexes
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