Page 88 - Instant notes

C2
FUNDAMENTALS OF ACIDS AND
BASES

Key Notes
A Brønsted acid is a proton donor, whilst a Brønsted base is a
proton acceptor. An acid-base reaction involves the exchange of a
proton between an acid and a base. An acid reacts with water to
+
produce the hydronium ion, H 3 O , and a conjugate base. A base
−
reacts with water to produce the hydroxide ion, OH , and a
conjugate acid.
Water can act as both an acid and a base. Pure water contains
both hydronium and hydroxide ions, and their activities are
linked by the expression:

where K w is the autoprotolysis constant for water. This equation
does not just apply to pure water, but relates the activities of
hydronium and hydroxide ions in all aqueous solutions.
The pH scale is used to define the acidity of an aqueous solution,
where . When pH=7, the solution is
neutral. When pH>7 the solution has an excess of hydroxide ions
and is basic, whereas when pH<7 the solution has an excess of
hydronium ions and is acidic. An increase in pH corresponds to a
decrease in the activity (and concentration) of hydronium ions
and an increase in the activity (and concentration) of hydroxide
ions.
Related topics Fundamentals of equilibria Thermodynamics of ions in
(C1) solution (E2)
Ions in aqueous solution
(E1)

Brønsted-Lowry theory

+
+
In Brønsted-Lowry theory, an acid is a proton (H ) donor and a base is a proton (H )
+
acceptor. Examples of acids are HCl, CH 3COOH, H 3O and H 2O. Examples of bases are
−
−
NH 3, CH 3COO , H 2O and OH . H 2O can therefore act as either an acid or a base. A