ACIDS & BASES
Acids and Bases reactions occur in everyday life
and are essential for understanding our world.
How does pH value affect our environment?
Why is it important to monitor and maintain the pH of the water
in aquariums, soil and our blood?
What exactly is pH? How is it measured?
Milk of magnesia is a medicine
that usually relieves
uncomfortable gastrointestinal
symptoms within 30 minutes
and constipation within six
hours.
Why is the milk of magnesia
an antacid?
Keywords
Acidity
 Basicity (Monoprotic, diprotic, triprotic)
 Bronsted-Lowry Theory
- Proton donor/acceptor
- Acid-base Conjugate pair
- Amphiprotic
 Lewis Theory
- Lone pair electrons
- Dative/Coordinate bond

Recall Questions
What is an acid?
What is a base/alkali?
•A substance which produces hydrogen ions (protons)
when dissolves in water.
•A base refers to substances like metal oxides and metal
hydroxides.
•A substance which reacts with acid to form salt and water only
•An alkali is a soluble base which in solution produces hydroxide
ions.
•Most bases are insoluble in water. 3 soluble bases are
NaO/NaOH,KO/KOH,CaO/Ca(OH)2
Recall Questions
What causes acidity?
•It is the hydrogen ions that give an acid its acidic properties
when they dissolve in water and dissociate into ions.
E.g. HCl gas is a covalent compound. When dissolves in water,
it forms HCl acid which dissolciate to form ions.
Recall Questions
What is basicity(proticity)?
Basicity
• refers to the no.of H atoms in one molecule of acid that acn be
repleced by a metal.
•refers to the no. of H+ that can be replaced by one molecule
of that acid.
E.g. HCl (monobasic),H2SO4(dibasci),H3PO4(tribasic)
Bronsted-Lowry theory
An acid is defined as a molecule or ion that acts as a
proton donor (H+).
A base is defned as a molecule or ion that acts as a
proton acceptor (H+).
HCl(g) + H2O(l)
H3O+(aq) + Cl-(aq)
Acids that have single proton to donate –
monoprotic (monobasic).
E.g. HCl(aq), HNO3(aq), HNO2(aq)
 Acids that have 2 protons to donate – diprotic
E.g. H2SO4(aq), H2SO3(aq), H2CO3(aq)
 H3PO4(aq) is triprotic.

Hydrogen chloride gas dissolved in water
HCl(g) + H2O(l)
H3O+(aq) + Cl-(aq)
The equation can be split into
(i) HCl(aq)
Cl-(aq)
acid
conjugate base
(ii) H2O(l) + H+(aq)
base
+
H+(aq)
H3O+(aq)
conjugate acid
Acid-base conjugate pair
CH3COOH(l) + H2O(l)
acid
base
donates H+
NH3(g) + H2O(l)
H3O+ (aq) + CH3OOO-(aq)
conjugate
conjugate
acid
base
+
donates H
NH4+(aq) + OH-(aq)
Water is sometimes described as amphiprotic because
it can accept or donate a proton.
Competition between
acid/base and its conjugate
(i) HCl(g) + H2O(l)
acid
base
(ii) CH3COOH(l) + H2O(l)
acid
(i)
(ii)
base
H3O+(aq) + Cl-(aq)
conjugate acid
conjugate base
H3O+ (aq) + CH3OOO-(aq)
conjugate acid
conjugate base
Water is a much stronger base than chloride ion and has a stronger
tendency to accept proton.The equilibrium shifts more to the right.
Ethanote ion is a much stronger base than water molecule. The equilbrium
shifts to the left.
Gas-phase acid-base reaction
HCl(g) + NH3(g)
NH4Cl(s)
The Bonsted-Lowry model can be extended to gasphase acid-base reaction.
 It involves the transfer
of hydrogen ion from
hydrogen chloride to
ammonia.

(i) HCl(g) + H2O(l)
acid
base
conjugate acid
(ii) CH3COOH(l) + H2O(l)
acid
H3O+(aq) + Cl-(aq)
base
conjugate base
H3O+ (aq) + CH3OOO-(aq)
conjugate acid
conjugate base

Strong acids produce relatively weak conjugate bases in aqueous solutions.

Weak acids produce relatvely strong conjugate bases in aqueous solutions.
Common acids & conjugate bases
in order of strengths
Lewis theory


A Lewis acid is defined as a substance that can
accept a pair of electrons from another atom to
form a dative (coordinate) covalent bond.
A Lewis base is defined as a substance that can
donate a pair of electrons to another atom to form
a dative (coordinate) covalent bond.
B:
H+  +BH
Examples

Reaction between ammonia and proton

H3N:
H+  +NH4
Reaction between a water molecule and proton
H2O:
H+  H3O+
Lewis bonding
In complex ions formed by transition metals
The 6 water molecules, each donate a lone pair electrons from
oxygen of their water molecules to the empty 3d orbitals of
iron.
What does each water molecule and iron(III) ion act
as in the reaction above?
Dative (Coordinate) bond


A dative covalent bond is always formed in a Lewis
acid-base reaction.
For a substance to act as a base, it must have space
to accept the lone pair of electrons.
Strong and weak acids and bases
Strong acid
 When strong acid dissolves,
virtually all acid molecules
react with the water to
produce hydronium ions In
general for a strong acid
HA
HA + H2O(l)  H3O+(aq) + A-(aq)
0%
100%
Examples : HCl, H2SO4,HNO3, HClO4
or
HA  H+(aq) + A-(aq)
0% 100%
Strong and weak acids and bases
Weak acid
 When a weak acid dissolves
in water, only a small % of its
molecules (typically 1%) react
with water molecules to
release hydrogen or
hydronium ions. The
equilibrium lies on the lefthand side of the equation.
HA + H2O(l)
99%
H3O+(aq) + A-(aq)
1%
or
HA
H+(aq) + A-(aq)
99% 1%
Examples : CH3COOH, aqueous carbon dioxide
Distinguish between strong and weak acids
0.1 mol dm-3 HCl(aq)
0.1 mol dm-3 CH3COOH (aq)
[H+(aq)]
0.1 mol dm-3
- 0.0013 mol dm-3
pH
1.00
2.87
Electrical conductivity
high
low
Relative rate of reaction with
magnesium
fast
slow
Relative rate of reaction with
calcium carbonate
fast
slow
Base on the information above, how do we distinguish betwee strong
and weak acids of the same concentration (e.g. HCl and
CH3COOH)?
How to distinguish between strong and weak
acids?




A weak acid has a lower concentration of hydrogen ions and hence
a higher pH than a stronger acid of the same concentration.
A weak acid, because of its lower concentration of hydrogen ions,
will have much poorer electrical conductor than a stronger acid of
the same concentration.
Weak acids react more slowly with reactive metals, metal oxides,
metal carbonates and metal hydrogencarbonates than strong acids
of the same concentration.
Strong and weak acids can also be distnguished by measuring and
comparing their enthalpies of neutralisation.
What is the difference between the strength (strong and weak)
and the concentrated (concentrated or dilute)?
Strong and weak acids and bases
Strong base
 A strong base undergoes almost 100% dissociation/ionisation
when in dilute aqueous solution.
BOH B+(aq) + OH-(aq)
0%
100%
Examples : NaOH, KOH, Ba(OH)2
Strong and weak acids and bases
Weak base
 All bases are weak except the hydroxides of groups 1 and 2.
 Weak bases are composed of molecules that react with water
molecules to release hydoxide ions. In general for a weak
molecular base, BOH

BOH + (aq)
B+(aq) + OH-(aq)
The equiibrium lies on the left side of the equation.
Examples : aqueous ammonia, ethylamine, caffeine, bases
of nuclei acids
The pH indicator




scale that measures the strength of an acid and
alkali.
pH of a substance is measured when it is dissolved
in water.
pH stands for “power of hydrogen”
[H+] = 1 x 10-n moldm-3 ( n = pH number)
The pH Scale
pH probe and meter
An accurate method of measuring pH value.
A pH probe is dipped into the solution being tested
and the pH value is then read directly from the
meter.
pH Calculation


pH is a measure of the concentration of H+ ions in a
solution.
pH = -log10[H+(aq)]
Example:
If the concentration of H+ is 2.50 x 10-3 moldm-3 , what is
the pH?
pH = -log (2.50 x 10-3)
= 2.60
Example:
Calculate the concentration of H+ of a solution that
has a pH = 3.2.
-log[H+] = 3.2
log [H+] = -3.2
[H+] = 6.31 x 10-4
Example:
(a) What is the pH pf 10cm3 of 0.1 moldm-3 HCl?
pH = -log (0.1) = 1
(b) If 90cm3 of water is added to the acid, what happens to the
pH?
Total volume = 100cm3
In 10cm3 solution, concentration of H+ is 0.1 moldm-3
In 100cm3 solution, concentration of H+ is 0.01 moldm-3
pH = -log (0.01) = 2
(c) If the solution from (b) is diluted by a factor of 105 , what is the
approximate pH?
The pH will increase by 4 to 6.