Bronsted/Lowry Theory of Acids and Bases

Bronsted/Lowry Theory of Acids and
• 1923 Danish chemist Bronsted + English
chemist Lowry independently proposed new
definitions for an acid and a base
• These simply state that:
• A base is a proton donor
• An acid is a protons aceptor
Consider This
• When hydrogen chloride is added to water the
following reaction occurs
Because this
donates a
protono it is
a B/L acid
this accepts
a proton it
is a B/L base
H3O+ + Cl-
Consider This
• When ammonia gas dissolves in water the
following reaction occurs
a proton
B/L base
a proton
B/L acid
NH4 + + OH-
• Water can act as either an acid or a base and
for this reason is known as amphoteric or
Bronsted Lowry theory does not only
refer to reactions where water is the
• In the following reaction between Hydrogen
chloride and Ammonia
HCl + NH3
NH4+ + Cl-
Advantages of the Bronsted/Lowry
Theory over Arrhenius’ Theory
1. Arrhenius’ theory was confined to aqueous
2. Bronsted/Lowry’s theory broadens the range
of species that can be defined as acids and
3. Substances that are not classified as acids or
bases under Arrhenius’ theory are classified
as acids or bases in the Bronsted/Lowry
• An acid will only donate a proton when there
is something there to accept it
• A base will only accept a proton if there is
something there to donate it
Conjugate acid/base pairs
• Certain reactions are reversible and can
happen in both directions
• We use the symbol
to show this
Consider This
• If the following reaction is written as
CH3COO- + H3O+
Ethanoic acid
This means the reaction
And the reaction
CH3COO- + H3O+
Can take place
CH3COO- + H3O+
Identify the B/L acid in the following
CH3COO- + H3O+
• CH3COO- + H3O+
• An acid changes to a conjugate base when it
donates a proton
• A base changes to a conjugate acid when it
accepts a proton
• CH3COO- is the conjugate base of CH3COOH
• CH3COOH is the conjugate acid of CH3COO-
Every base has a conjugate acid and every acid has
a conjugate base
Since CH3COO- and CH3COOH only differ by a
proton we refer to them as a conjugate acid base
• Why can H2O and H3O+ be called a conjugate
acid base pair?
• NB Study examples 12.1 and 12.2 on pages
140 and 141 of your text book
• When acids and bases react with each other in
the right proportions they cancel each other
• When this happens both the acid and the base
lose their characteristic properties
• The solution formed is neutral and has no
effect on litmus paper
• This type of reaction is known as
• Neutralisation is the reaction between an acid
and a base to form salt and water
Reaction between HCl and NaOH
HCl + NaOH
H2O + NaCl
Acid + Base
Water + Salt
• The word salt is a general term used to
describe the substance formed when the
hydrogen in an acid is replaced by a metal or
an ammonium ion
Understanding what happens
• HCl + NaOH
H2O + NaCl
• In this equation the following ions are all present in
H+ + Cl- + Na+ + OHNa+ + Cl- + H2O
The spectator ions are crossed out as what is important is
the reaction where the H+ (Or more correctly
Hydronium ions )and OH- ions react to form water
H + + OHH2O
Everyday examples of Neutralisation
1. Medicine – Hydrochloric acid in stomachs
helps digestion. Over eating and stress can
produce too much acid. Antacids such as Alka
Seltzer, Bisodol and Milk of Magnesia may be
used to try to neutralise excess acid
2. Agriculture – If soil is too acidic crop yields
can be low, therefore farmers often spread
lime (CaO) on soil to neutralise the acidity
3. Environmental Protection- In areas affected
by acid rain limestone is often added to lakes
to neutralise acidity
• Toothpaste is slightly basic to neutralise acids that
cause tooth decay
• Baking soda may be used to neutralise the acid of
• Vinegar is used to neutralise the alkaline sting of
• Shampoo is slightly basic and can cause scales on
hair to stick out, conditioner is slightly acidic so
neutralises the base flattening the scales and
making hair more shiny and manageable