4. Arrhenius theory of acid
Arrhenius was a Sweedish
chemist
Put forward a theory of
acids in the 1880’s
Stated that:
An acid is a substance that dissociates
in water to form H+ ions.
5. Arrhenius theory of acid
For example: when HCl is added to
water:
HCl
H+ + ClIn general:
HA
H+
+
A-
6. Acids
HCl and HNO3 are monobasic acids as they donate
one H+ ion.
HNO3
H+
+ NO3-
H2SO4 is a dibasic acid as it donates two H+ ions.
H2SO4
2H+
+ SO42H3PO4 is a tribasic acid as it donates three H+ ions.
H3PO4
3H+
+ PO43-
7.
A strong acid is one which dissociates fully
in water
Example: HCl, H2SO4, HNO3
HCl + H2O
H3O+ + Cl-
A weak acid is one which does not fully
dissociate in water
Example: CH3COOH (ethanoic acid)
CH3COOH + H2O
H3O+ + CH3COO-
10. Arrhenius theory of bases
Arrhenius defined a base as:
A substance that dissociates in water to
produce OH- ions.
For example: when NaOH is added to water:
NaOH
Na+ + OHIn general:
XOH
X+
+
OH-
11.
A strong base is one which dissociates
fully in water
Example: NaOH
A weak base is one which does not
fully dissociate in water
Example: Mg(OH)2
13. Limitations of Arrhenius theory
1.
2.
3.
The acids and bases must be in aqueous
solutions (i.e. water). This prevents the use of
other solvents benzene.
Not all acid – base reactions are in solution, e.g.
ammonia gas and hydrogen chloride gas produce
ammonium chloride.
According to Arrhenius, the salt produced should
not be acidic or basic. This is not always the
case, for example in the above reaction
ammonium chloride is slightly acidic
14. Hydronium Ion
Arrhenius thought that an acid gives off H+ ions
in solution.
H+ ions are protons and can not exist
independently.
When the acid dissociates, the H+ ions react
with water molecules:
H + + H 2O
H 3O+
The H3O+ ion is called the hydronium ion.
This is another limitation of the Arrhenius
theory.
15. Brønsted-Lowry Theory
In 1923, Johannes Brønsted (a Danish
chemist) and Thomas Lowry (an English
chemist) proposed new definitions of acids
and bases.
Brønsted
Lowry
16. Brønsted-Lowry Theory
Brønsted and Lowry had worked
independently of each other but they both
arrived at the same definitions:
An acid is a substance that donates protons
(hydrogen ions).
A base is a substance that accepts protons.
17. Acid = Proton Donor
Donates a Proton
HCl
+
H2O
H3O+
+
Cl-
Accepts a Proton
The HCl donates a proton and so is an acid
The H2O, in this case, accepts a proton and so is a
base
Remember: Proton = H+
19. Base = Proton Acceptor
Accepts a proton
NH3 + H2O
NH4+ + OHDonates a proton
The NH3 accepts a proton and so is a base.
The H2O, in this case, donates a proton and
so is an acid.
20. Amphoteric
As can be seen from the previous two
examples, water is capable of acting
as both and acid and a base.
Any substance that can act as both an
acid and a base is said to be
amphoteric.
21. Acid – Base Reaction
Acid – Donates Protons
HCl
+
NH3
Cl- + NH4+
Base – Accepts Protons
22. Neutralisation
The reaction between an acid and
a base to produce a salt and water
A salt is formed when the
hydrogen of an acid is replaced
by a metal (or ammonium ion)
23. Neutralisation
Acid + Base
Salt + Water
HCl + NaOH
NaCl + H2O
but since the acid and base dissociate in water
we can write:
H+ + Cl- + Na+ + OHNa+ + Cl- + H2O
we can cancel the Na+ and Cl- on both sides
leaving:
H+ + OHH2O
24. Everyday Examples of Neutralisation
Indigestion remedies are
bases that neutralise excess
stomach acid
Lime is a base that
neutralises acid in soil
Toothpaste is a base
that neutralises acid in
the mouth
25. Wasp stings are basic
Nettle, bee and ant stings
are acidic
They can be neutralised
with vinegar or lemon
juice
They can be neutralised
with baking soda
26. Conjugate Acid-Base Pairs
Acids and bases exist in pairs called
conjugate acid-base pairs.
Every time an acid donates/loses a
proton, it becomes its conjugate base.
Example:
CH3COOH + H2O
Acid
CH3COO- + H3O+
Conjugate Base
27. Likewise:
When a base accepts a proton, it
becomes its conjugate acid.
Example:
NH3 + H2O
Base
NH4+ + OH-
Conjugate
Acid
