Acid-Base Equilibria
The Bronsted-Lowry theory states that:
 An acid is a proton donor (H⁺ ion)
 A base is a proton acceptor(H⁺ ion)
Water acts as both base and acid. One H₂O acts as a
base and gains H⁺ to become H₃O⁺; the other H₂O acts
as an acid and loses H⁺ to become OH⁻.
Consider the reaction of hydrochloric acid and water.
HCl₍aq₎ + H₂O₍l₎ ⇆ H₃O⁺₍aq₎ + Cl⁻₍aq₎
acid
base
conjugate acid
conjugate acid conjugate base
:forward reaction:
acid
conjugate base forward rxn
base
backward rxn
In the above reaction the HCl is the base because it
donates a proton to H₂O.
The water is the base because it accepts the proton
from the HCl.

:backward reaction;
The H₃O⁺ is the acid because it donates a proton to the
Cl⁻ ion.
The Cl⁻ is a base because it accepts the proton from
the H₃O⁺.
When the acid, HCl, donates a proton it forms the base
Cl⁻. When the base, Cl⁻,accepts a proton, it reforms
the acid, HCl. Thus HCl and Cl⁻ are referred to as a
conjugate acid-base pair.
ACIDS
Acids are classified into two categories:
• Strong acids
• Weak acids
A strong acid ionizes completely in solutions while a weak acid
partially ionizes in solutions.

BASES
Bases are also classified as strong and weak bases.
Strong bases ionize completely in water while weak bases do not
ionize completely in water.




The relative strengths of acids can be compared by
measuring the concentration of hydrogen ions.
The pH scale is used to measure acidity and range
form 0 to 14.
It is the negative logarithm to base 10 of the
concentration of hydrogen ion in a solution.
pH=-log₁₀[H⁺]

The concentration of the hydroxide ion can be
measured similarly using a pOH scale
pOH=-log₁₀[OH⁻₍aq₎]
CALCULATING THE pH OF A STRONG ACID
Strong acids are fully dissociated in aqueous solution
and the concentration of the H⁺ ions can be found
using stoichiometry.

CALCULATING THE pH OF A STRONG BASE
To calculate the pH a strong base determine the
hydroxide ion concentration in mole per dm⁻³ , then
determine the pOH using pOH=-log₁₀[OH⁻].
From here pH can be determined by using:
pH=14-pOH
G.C.E Past Papers Paper 1;2001#2
a)
Outline the Bronsted-Lowry theory of acids and bases.
b)
The following is a list of compounds that react with or
dissolve in water:
sodium chloride, hydrogen chloride, silicon
tetrachloride, ammonia, methanol.
Water can react as either an acid or a base. Choose a
compound from the above list with which water acts
as:
(i)
(ii)
a Bronsted base,
a Bronsted acid,
Construct a balanced equation for each reaction.