| If you haven't reviewed
and Ion Hydrolysis, please
do so now before continuing.
AUTOIONIZATION OF WATER
Before we study acids and bases, let's first look at the autoionization
In this reaction, two water molecules react forming the hydronium
ion and the hydroxide ion. The equilibrium expression for this reaction
In pure water the hydronium ion concentration and
the hydroxide ion concentration are equal to 1.00 E-7 and the water
is said to be neutral.
ACIDS AND EQUILIBRIUM
An acid is any compound which can donate a proton. It reacts
with water in the equation below. If the acid is a strong acid
(ionizes 100%), the equilibrium lies
entirely to the right. However, if the acid is a weak acid
(ionizes < 100%), the equilibrium
is dependent on the acid equilibrium constant, Ka. An acid
equilibrium expression can only be written for weak acids.
BASES AND EQUILIBRIUM
A base is any compound which can accept a proton. It reacts with
water in the equation below. If the base is a strong base
(ionizes 100%), the equilibrium lies
entirely to the right. However, if the base is a weak base
(ionizes <100%), the equilibrium
is dependent on the base equilibrium constant, Kb. A base
equilibrium expression can only be written for weak bases.
CONJUGATE ACID-BASE PAIRS
A pair of compounds or ions that differ by the presence of one
proton (H+) is called a conjugate acid-base pair. For instance
in the base hydrolysis above, BH+ and B are a conjugate acid-base
pair. In addition, the water and the hydroxide ion are also a conjugate
acid-base pair. Therefore, in every reaction that involves and acid
or a base, there is a conjugate acid-base pair.
The acid equilibrium constant and the base equilibrium constant
for a conjugate acid and a conjugate base can be related.
Therefore, if you know the kA for a weak acid (HA),
you can calculate the Kb for its conjugate base (A-).
THE pH SCALE
The pH scale allows scientists to represent acid and
base concentrations in small numbers. pH represents the hydronium
ion concentration and pOH represents the hydroxide ion concentration.
They are related by the equations below.
In a buffer solution, the pH is resistance to change.
This means that the addition of an acid or a base does not affect
the hydronium ion concentration. Human blood is an example of a
buffer solution. A buffer is usually prepared from roughly equal
quantities of a conjugate acid-base pair.
- a weak acid and its conjugate base
- a weak base and its conjugate acid
The pH of a buffer solution can be calculated using the Henderson-Hasselbalch
This is equation will be useful during a titration
when trying to find the pH.
TITRATIONS AND ACID/BASE
A titration is an example of an acid/base equilibrium.
In an titration, an acid and a base react to form a salt and water.
Here are some important definitions.
- equivalence point - when the moles of acid or base
added equals the moles of acid or base which are present
- midpoint - when the moles of acid or base added is
1/2 the moles of acid or base which are present
- indicator - a dye used to indicate when the reaction
is complete. It changes color at the equivalence point.
There are five types of acid/base titrations which can occur:
- Strong Acid with Strong Base
- Strong Base with Strong Acid
- Weak Acid with Strong Base
- Weak Base with Strong Acid
- Weak Acid with Weak Base
Now, for our purposes, we will only study the first four titrations.
For a thorough study of all types of titrations, please visit your
In a each titration, there are 5 important regions
- At time = 0, before the titration begins
- Before the equivalence point
- At the midpoint
- At the equivalence point
- After the equivalence point
The table below outlines the four types of titrations and the five
important regions of each titration. The table tells you what type
of equation to use at which point in a titration to calculate the
pH of the titration.
- E.P - Equivalence Point
- SA, SB, WA, WB - Strong Acid, Strong Base, Weak Acid, Weak
- ICE Table - Initial, Change and Equilibrium Table - Please
refer to General Equilibrium
- SRFC Table - Starting Moles, Reacting Moles, Final Moles,
Concentration Table. This table is similar to an ICE table.
To learn how to do an SRFC table, follow the titration
- This table is only a study guide. It may not be used for any
Example Problem: Calculate the pH at t = 0,
at 5.00 mL, at 10.00 mL, at the equivalence point and at 40.00 ml
in a titration of 25.0 mL of 0.120 M formic acid with 0.105 M NaOH.(kA
of formic acid = 1.8 E-4)
- First, identify the reaction: WA with a SB
- At t = 0, must do an ICE table with the WA
- At 5.00 ml: this is before the EP, so do SRFC, then use Henderson-Hasselbalch
- At 10.00 ml, also before EP, do SRFC, then use Henderson-Hasselbalch
- At the equivalence point, do SRFC, then do ICE table with the
conjugate base of the weak acid
- At 40.00 ml: this is after the EP so do SRFC and use the excess
strong base to calculate the pH