ACID BASE
EQUILIBRIA
Reference:
Jacaranda Chapter 10
Heinemann Chapter 17
Reaction of H2O & H2O
O Water will react with itself via a process
called self-ionisation.
Equilibrium Expression
O The equilibrium expression for the self-
ionisation of water is given the symbol KW.
O In this expression, 𝐻2 𝑂 can be considered a
constant.
Ionisation Constant
O The expression for KW can be simplified to
O In pure water at 25°C, the value of KW is 10−14
O This relation also holds true for all aqueous solutions.
Ionisation Constant
O The value of 𝐻3 𝑂+ in pure water is 10−7
O The value of 𝑂𝐻 − in pure water is also
10−7
O At 25°C
Acidic Solutions
O In acidic solutions, 𝐻3 𝑂 + is formed by the
reactions of an acid with water, as well as
the self-ionisation of water.
O This means the 𝐻3 𝑂+ will be greater than 10−7
Basic Solutions
O In basic solutions, 𝑂𝐻 − is formed by the
reactions of a BASE with water, as well as
the self-ionisation of water.
O This means the 𝑂𝐻 − will be greater than 10−7
Acidic and Basic Solutions
O Regardless of the type of solution, the
following expression doesn’t change.
O This means that:
The pH Scale
O The concentration of 𝐻3 𝑂 + can range from
100 𝑡𝑜 10−14 .
O This equates to a range from 1 to a number
very very very small.
O There must be a better way to describe
concentrations.
O Hence, the pH Scale was developed.
The pH Scale
O The pH formula is:
O Lets test a few values to see why it is better.
The pH Scale
O If the concentration of 𝐻3 𝑂 + = 10−8 , then
this is equivalent to saying that the,
𝐻3 𝑂+ = 0.000000010
O Would it not be better to use the pH scale?
Lets see…..
O If the concentration of 𝐻3 𝑂 + = 10−8 , then
this is equivalent to saying that the,
O 𝑝𝐻 = − log10 𝐻3 𝑂+ = − log10 10−8 = 8
The pH Scale
O It is preferable to describe the solution using
pH = 8, as opposed to 𝐻3 𝑂+
O In this example, the solution is considered
basic as the pH = 8.
The pH Scale – Neutral
Solution
O If 𝐻3 𝑂 + = 10−7 , then the
𝑝𝐻 = − log10 𝐻3 𝑂+ = − log10 10−7 = 7
O A pH of 7 indicates that a solution is neutral.
The pH Scale – Acidic
Solutions
O If 𝐻3 𝑂 + > 10−7 , e.g. 10−3
𝑝𝐻 = − log10 𝐻3 𝑂+ = − log10 10−3 = 3
O A pH < 7 indicates that a solution is acidic.
The pH Scale – Basic Solutions
O If 𝐻3 𝑂 + < 10−7 , e.g. 10−11
𝑝𝐻 = − log10 𝐻3 𝑂+ = − log10 10−11 = 11
O A pH > 7 indicates that a solution is basic.
The effect of temperature on
𝐾𝑊 and pH.
O The pH of pure water is not always 7. This is
only true at 25°C.
O This reaction is endothermic.
O As temperature increases, more products
are made.
The effect of temperature on
𝐾𝑊 and pH.
O Using,
O More products means the numerator
increases and therefore Kw will increase.
The effect of temperature on
𝐾𝑊 and pH.
O As the equilibrium shifts to the right and
more products are made, the amount of
𝐻3 𝑂+ and 𝑂𝐻 − made is still equal.
O Therefore it is still a neutral solution even if
Kw and the pH changes.
O However, since the 𝐻3 𝑂+ is increasing, the
pH must be decreasing.
The effect of temperature on
𝐾𝑊 and pH.
O Hence at 50°C, the pH of neutral water is
less than 7.
O And at 10°C, the pH of neutral water is
greater than 7.
Questions
O Complete Questions Q1-5 on pg 232 from
Study On Textbook
O Supplementary Work Q1-3 on pg 288 of
Heinemann Textbook
Weak Acids
O Consider the following equation describing
the reaction of an acid with water.
O Applying the equilibrium law we obtain,
Weak Acids
O The expression can be rearranged to,
O Ka is referred to as the acidity constant or
dissociation constant.
Weak Acids
Weak Bases
O Consider the following equation describing
the reaction of a base with water.
(l)
O Applying the equilibrium law we obtain,
Weak Bases
O The expression can be rearranged to,
O Kb is referred to as the basicity constant or
dissociation constant.
Weak Acids and Bases
Polyprotic Acids
• Sulphuric Acid is polyprotic. An equilibrium expression can
be written for both equations.
• The Ka for the second equation can be referred to as the
second acidity constant for H2SO4
Strength vs. Concentration
O Strength is a measure of how much an acid
or base ionises in water.
O Concentration refers to how much of a
particular substance you have.
Buffer Solutions
O A buffer solution, or buffer, contains a
solution of an acid and one of its salts.
O The salt provides the conjugate base.
O The purpose of a buffer solution is to
maintain a constant pH.
Buffer Solutions
O Consider the following example.
Buffer Solutions - Acids
O If a strong acid is added, then initially more
𝐻3 𝑂+ is made. This will cause equation 1 to
shift to the left, hence producing more HA.
O The net effect is that a strong acid is being
converted to a weak acid, and hence the pH
is not altered as much as it otherwise would
have.
Buffer Solutions - Bases
O If a strong base is added, then initially more
𝑂𝐻 − is made. This will cause equation 2 to
shift to the left, hence producing more 𝐴− .
O The net effect is that a strong base is being
converted to a weak base, and hence the pH
is not altered as much as it otherwise would
have.
Questions
O Complete Questions Q6-8 on pg 238 from
Study On Textbook
O Supplementary Work Q4-6 on pg 292 of
Heinemann Textbook