ACIDS AND
BASES
A guide for A level students
KNOCKHARDY PUBLISHING
2008
SPECIFICATIONS
Acid & Bases
INTRODUCTION
This Powerpoint show is one of several produced to help students understand
selected topics at AS and A2 level Chemistry. It is based on the requirements of
the AQA and OCR specifications but is suitable for other examination boards.
Individual students may use the material at home for revision purposes or it
may be used for classroom teaching if an interactive white board is available.
Accompanying notes on this, and the full range of AS and A2 topics, are
available from the KNOCKHARDY SCIENCE WEBSITE at...
www.knockhardy.org.uk/sci.htm
Navigation is achieved by...
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Acid & Bases
CONTENTS
• Brønsted-Lowry theory of acids and bases
• Lewis theory of acids and bases
• Strong acids and bases
• Weak acids
• Weak bases
• Hydrogen ion concentration and pH
• Ionic product of water Kw
• Relation between pH and pOH
• Introduction to buffer solutions
• Check list
Acid & Bases
Before you start it would be helpful to…
• know the simple properties of acids, bases and alkalis
ACIDS AND BASES
BRØNSTED-LOWRY THEORY
ACID
proton donor
HCl ——> H+(aq) + Cl¯(aq)
BASE
proton acceptor
NH3 (aq) + H+(aq)
——>
NH4+(aq)
ACIDS AND BASES
BRØNSTED-LOWRY THEORY
ACID
proton donor
HCl ——> H+(aq) + Cl¯(aq)
BASE
proton acceptor
NH3 (aq) + H+(aq)
Conjugate systems
Acids are related to bases
ACID
Bases are related to acids
BASE
——>
PROTON
+
PROTON
NH4+(aq)
+
CONJUGATE BASE
CONJUGATE ACID
ACIDS AND BASES
BRØNSTED-LOWRY THEORY
ACID
proton donor
HCl ——> H+(aq) + Cl¯(aq)
BASE
proton acceptor
NH3 (aq) + H+(aq)
Conjugate systems
Acids are related to bases
ACID
Bases are related to acids
BASE
——>
NH4+(aq)
PROTON
+
PROTON
+
CONJUGATE BASE
CONJUGATE ACID
For an acid to behave as an acid, it must have a base present to accept a proton...
HA
acid
example
+
B
base
CH3COO¯ + H2O
base
acid
BH+
+
A¯
conjugate conjugate
acid
base
CH3COOH
acid
+
OH¯
base
ACIDS AND BASES
LEWIS THEORY
ACID
lone pair acceptor
BF3
H+
BASE
lone pair donor
NH3
H2O
LONE PAIR DONOR
LONE PAIR DONOR
LONE PAIR
ACCEPTOR
LONE PAIR ACCEPTOR
AlCl3
STRONG ACIDS AND BASES
STRONG
ACIDS
e.g.
completely dissociate (split up) into ions in aqueous solution
HCl ——> H+(aq) + Cl¯(aq)
MONOPROTIC
1 replaceable H
DIPROTIC
2 replaceable H’s
HNO3 ——> H+(aq) + NO3¯(aq)
H2SO4 ——> 2H+(aq) + SO42-(aq)
STRONG ACIDS AND BASES
STRONG
ACIDS
e.g.
completely dissociate (split up) into ions in aqueous solution
HCl ——> H+(aq) + Cl¯(aq)
MONOPROTIC
1 replaceable H
DIPROTIC
2 replaceable H’s
HNO3 ——> H+(aq) + NO3¯(aq)
H2SO4 ——> 2H+(aq) + SO42-(aq)
STRONG
BASES
e.g.
completely dissociate into ions in aqueous solution
NaOH
——> Na+(aq) + OH¯(aq)
WEAK ACIDS
Weak acids
partially dissociate into ions in aqueous solution
e.g. ethanoic acid
When a weak acid dissolves in
water an equilibrium is set up
CH3COOH(aq)
HA(aq) + H2O(l)
CH3COO¯(aq)
HA(aq)
H+(aq)
A¯(aq) + H3O+(aq)
The water stabilises the ions
To make calculations easier
the dissociation can be written...
+
A¯(aq) +
H+(aq)
WEAK ACIDS
Weak acids
partially dissociate into ions in aqueous solution
e.g. ethanoic acid
CH3COOH(aq)
When a weak acid dissolves in
water an equilibrium is set up
CH3COO¯(aq)
+
H+(aq)
A¯(aq) + H3O+(aq)
HA(aq) + H2O(l)
The water stabilises the ions
To make calculations easier
the dissociation can be written...
The weaker the acid
HA(aq)
A¯(aq) +
H+(aq)
the less it dissociates
the more the equilibrium lies to the left.
The relative strengths of acids can be expressed as Ka or pKa values
The dissociation constant for the weak acid HA is
Ka =
[H+(aq)] [A¯(aq)]
[HA(aq)]
mol dm-3
WEAK BASES
Partially react with water to give ions in aqueous solution
e.g. ammonia
When a weak base dissolves in water an equilibrium is set up
NH3 (aq)
+ H2O (l)
NH4+ (aq) + OH¯ (aq)
as in the case of acids it is more simply written
NH3 (aq) +
H+ (aq)
NH4+ (aq)
WEAK BASES
Partially react with water to give ions in aqueous solution
e.g. ammonia
When a weak base dissolves in water an equilibrium is set up
NH3 (aq)
+ H2O (l)
NH4+ (aq) + OH¯ (aq)
as in the case of acids it is more simply written
NH3 (aq) +
The weaker the base
H+ (aq)
NH4+ (aq)
the less it dissociates
the more the equilibrium lies to the left
The relative strengths of bases can be expressed as Kb or pKb values.
Hydrogen ion concentration [H+(aq)]
Introduction
hydrogen ion concentration determines the acidity of a solution
hydroxide ion concentration determines the alkalinity
for strong acids and bases the concentration of ions is very much
larger than their weaker counterparts which only partially dissociate.
Hydrogen ion concentration [H+(aq)]
pH
pOH
hydrogen ion concentration can be converted to pH
pH = - log10 [H+(aq)]
to convert pH into hydrogen ion concentration
[H+(aq)] = antilog (-pH)
An equivalent calculation for bases converts
the hydroxide ion concentration to pOH
pOH = - log10 [OH¯(aq)]
in both the above, [ ] represents the concentration in mol dm-3
[H+]
100 10-1 10-2 10-3 10-4 10-5 10-6 10-7 10-8 10-9 10-10 10-11 10-12 10-13 10-14
OH¯
10-14 10-13 10-12 10-11 10-10 10-9 10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1 10-0
pH
0
1
2
STRONGLY
ACIDIC
3
4
5
WEAKLY
ACIDIC
6
7
8
NEUTRAL
9
10
WEAKLY
ALKALINE
11
12
13
14
STRONGLY
ALKALINE
Ionic product of water - Kw
Despite being covalent, water conducts electricity to a very small extent.
This is due to the slight ionisation ... H2O(l) + H2O(l)
or, more simply
H2O(l)
H3O+(aq) + OH¯(aq)
H+(aq) + OH¯(aq)
Ionic product of water - Kw
Despite being covalent, water conducts electricity to a very small extent.
H3O+(aq) + OH¯(aq)
This is due to the slight ionisation ... H2O(l) + H2O(l)
or, more simply
Applying the equilibrium law
to the second equation gives
[ ] is the equilibrium concentration in mol dm-3
H+(aq) + OH¯(aq)
H2O(l)
Kc
=
[H+(aq)] [OH¯(aq)]
[H2O(l)]
Ionic product of water - Kw
Despite being covalent, water conducts electricity to a very small extent.
H3O+(aq) + OH¯(aq)
This is due to the slight ionisation ... H2O(l) + H2O(l)
or, more simply
Applying the equilibrium law
to the second equation gives
H+(aq) + OH¯(aq)
H2O(l)
Kc
[ ] is the equilibrium concentration in mol dm-3
=
[H+(aq)] [OH¯(aq)]
[H2O(l)]
As the dissociation is small, the water concentration is very large compared with the
dissociated ions and any changes to its value are insignificant; its concentration can
be regarded as constant.
This “constant” is combined with
(Kc) to get a new constant (Kw).
Kw = [H+(aq)] [OH¯(aq)] mol2 dm-6
=
1 x 10-14 mol2 dm-6 (at 25°C)
Because the constant is based on an equilibrium, Kw VARIES WITH TEMPERATURE
Ionic product of water - Kw
The value of Kw varies with temperature because it is based on an equilibrium.
Temperature / °C
0
20
25
30
60
Kw / 1 x 10-14 mol2 dm-6
0.11
0.68
1.0
1.47
5.6
H+ / x 10-7 mol dm-3
pH
0.33
7.48
0.82
7.08
1.0
7
1.27
6.92
2.37
6.63
What does this tell you about the equation
H2O(l)
H+(aq) + OH¯(aq) ?
Ionic product of water - Kw
The value of Kw varies with temperature because it is based on an equilibrium.
Temperature / °C
0
20
25
30
60
Kw / 1 x 10-14 mol2 dm-6
0.11
0.68
1.0
1.47
5.6
H+ / x 10-7 mol dm-3
pH
0.33
7.48
0.82
7.08
1.0
7
1.27
6.92
2.37
6.63
What does this tell you about the equation
H2O(l)
H+(aq) + OH¯(aq) ?
• Kw gets larger as the temperature increases
• this means the concentration of H+ and OH¯ ions gets greater
• this means the equilibrium has moved to the right
• if the concentration of H+ increases then the pH decreases
• pH decreases as the temperature increases
Ionic product of water - Kw
The value of Kw varies with temperature because it is based on an equilibrium.
Temperature / °C
0
20
25
30
60
Kw / 1 x 10-14 mol2 dm-6
0.11
0.68
1.0
1.47
5.6
H+ / x 10-7 mol dm-3
pH
0.33
7.48
0.82
7.08
1.0
7
1.27
6.92
2.37
6.63
What does this tell you about the equation
H2O(l)
H+(aq) + OH¯(aq) ?
• Kw gets larger as the temperature increases
• this means the concentration of H+ and OH¯ ions gets greater
• this means the equilibrium has moved to the right
• if the concentration of H+ increases then the pH decreases
• pH decreases as the temperature increases
Because the equation moves to the right as the
temperature goes up, it must be an ENDOTHERMIC process
Relationship between pH and pOH
Because H+ and OH¯ ions are produced
in equal amounts when water dissociates
their concentrations will be the same.
[H+] = [OH¯] = 1 x 10-7 mol dm-3
Kw = [H+] [OH¯] =
1 x 10-14 mol2 dm-6
take logs of both sides
log[H+] + log[OH¯] = -14
multiply by minus
- log[H+] - log[OH¯] = 14
change to pH and pOH
pH + pOH = 14
(at 25°C)
Relationship between pH and pOH
Because H+ and OH¯ ions are produced
in equal amounts when water dissociates
their concentrations will be the same.
[H+] = [OH¯] = 1 x 10-7 mol dm-3
Kw = [H+] [OH¯] =
N.B.
1 x 10-14 mol2 dm-6
take logs of both sides
log[H+] + log[OH¯] = -14
multiply by minus
- log[H+] - log[OH¯] = 14
change to pH and pOH
pH + pOH = 14
(at 25°C)
As they are based on the position of equilibrium and that varies with
temperature, the above values are only true if the temperature is 25°C (298K)
Neutral solutions may be regarded as those where [H+] = [OH¯].
Therefore a neutral solution is pH 7 only at a temperature of 25°C (298K)
Kw is constant for any aqueous solution at the stated temperature
Buffer solutions - Brief introduction
Definition
“Solutions which resist changes in pH when
small quantities of acid or alkali are added.”
Acidic Buffer (pH < 7)
made from
Alkaline Buffer (pH > 7) made from a
Uses
a weak acid + its sodium or potassium salt
ethanoic acid
sodium ethanoate
weak base
ammonia
Standardising pH meters
Buffering biological systems (eg in blood)
Maintaining the pH of shampoos
+
its chloride
ammonium chloride
REVISION CHECK
What should you be able to do?
Recall the definition of acids and bases in the Brønsted-Lowry system
Recall the definition of acids and bases in the Lewis system
Recall and explain the difference between strong and weak acids
Recall and explain the difference between strong and weak bases
Recall the definition of pH
Recall the definition of the ionic product of water
Explain how and why pH varies with temperature
Recall the relationship between pH, [H+], [OH¯], pOH and Kw
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ACIDS AND
BASES
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© 2008 JONATHAN HOPTON & KNOCKHARDY PUBLISHING