IB Chemistry, Topic 8
2012
Notes for Acids and Bases SL
Theories of Acids and Bases:
Acids and bases can be defined in a number of ways, based on their chemical behaviour.
In early times they were defined in terms of their macroscopic properties, such as sourness
for acids and slipperiness for bases. Arrhenius then proposed that acids were substances
which split up (ionised) to form a hydrogen ion and an anion in water. Though this is correct
to a certain extent, there are two theories which are used today to describe acids and bases.
Brønsted-Lowry Theory:
The Brønsted-Lowry Theory states that:
•
A Brønsted-Lowry acid is a hydrogen ion donor (more specifically, it reacts
with water to form hydronium ions)
•
A Brønsted-Lowry base is a hydrogen ion acceptor (more specifically, it reacts
with water to form hydroxide ions)
Therefore, when a substance donates a proton in solution or in a reaction with another
substance, it is acting as a Brønsted-Lowry acid, while when a substance accepts a proton in
solution or in a reaction with another substance, it is acting as a Brønsted-Lowry base.
Brønsted-Lowry acids: Reaction with water to form acidic solution:
Brønsted-Lowry acids ionise when mixed with water in the general form:
The hydrogen ion (H+) is a lone proton. It is very small and therefore has a large positive
charge density. Hence, it is attracted to the δ- end of the water molecule, forming a dative
bond with one of the lone pairs of electrons on the oxygen atom and hence creating the H 3O+
ion:
Therefore, overall, the reaction between a Brønsted-Lowry acid and water is of the general
form:
In this reaction, the HX molecule has donated a proton and hence acted as a Brønsted-Lowry
acid, while the H2O molecule has accepted this proton and hence acted as a Brønsted-Lowry
base.
IB Chemistry, Topic 8
2012
It is hydronium ions (H3O+) or hydrogen ions (H+) which give acids their acidic properties –
ions which are released when the substance ionises and reacts with water. Therefore, a
substance cannot truly be considered an ‘acid’ until it has been combined with water to form
an acidic solution. Consequently, acids are always aqueous.