Chapter 29 – Acids, Bases and pH
Section A – introduction to acids and bases
Properties of ACIDS
Properties of BASES
Have a pH of _______________
Have a pH of _______________
Can have a ____________ taste
Can have a ____________ feel
Turn blue litmus _____________
Turn red litmus _____________
Examples of household bases:
Examples of household acids:
Neutralisation reactions
Acid + Base
Examples from everyday life:


A basic wasp sting is neutralised by
Acidic soil is neutralised by
Section B – The Arrhenius theory of acids and bases
Arrhenius acid
A strong acid
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A weak acid
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Chapter 29 – Acids, Bases and pH
Arrhenius base
A weak base
A strong base
Section C – Bronsted-Lowry theory of acids and bases
An acid is a ___________ donor, and a base is a __________ acceptor.
Section D – Reversible reactions and Conjugate base pairs
A reversible reaction is one which happens in ______________ directions, this is indicated by the
arrow shown below:
NH3 +H30+
NH4+ H20
In the forward direction ____________ is the acid and it turns into ____________________ after it
donates the proton. The base is _________________ and it turns into ______________ after it
accepts the proton.
In the reverse reaction____________ is the acid and it turns into ____________________ after it
donates the proton. The base is _________________ and it turns into ______________ after it
accepts the proton __________________.
Conjugate acid- base pair
*This means the Bronsted acid turns into the conjugate base, and the Bronsted base turns into the
conjugate base.
This example contains two conjugate base pairs:
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Chapter 29 – Acids, Bases and pH
Section E – Self ionisation of water
Water can dissociate itself (_____________) in the following way:
H+ + OH-
H20
Kw = [OH-][H+]
The ionic product of water :
(Square Brackets mean concentration in moles per litre)
*A note about Kw and temperature - At room temperature 25OC Kw = 1 x 10-14 but this will increase
as temperature increases because as temperature increases more ______________ happen
between water molecules causing more ions to be generated.
in water:
Kw = [OH-][H+]
= [OH-] [H+] at 25OC
[H+] = [OH-], because for
every one water
molecule that dissociates
_______H+ ion is made
and ______ OH- ion is
made every one water
molecule that dissociates
_______H+ ion is made
and ______ OH- ion is
made
Kw = [H+][H+]
Kw = [H+]2
1 x 10-14 =[H+]2
= [H+]
=
[H+]
at 25OC
******Also
1 x 10-7 = [OH-] at 25OC
 Any neutral solution – like pure water will have [H+] = [OH-],
 If extra H+ ions are added into a neutral solution then [H+] > [OH-], and the solution will be
_______
 If extra OH- ions are added or generated in a neutral solution then [H+] < [OH-] and the solution
will be ______________.
Section F – The pH scale
pH=
the square brackets mean concentration in moles per litre
**Using this formula provides a way to discuss the acidity of a solution without having to express the
concentration of [H+] in mol/L which can often involve very small numbers
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Chapter 29 – Acids, Bases and pH
Example 1 - A solution has a concentration of 7x10-7 moles of H+ per litre. What is the pH?
pH= - log10 [H+],
Answer:
The pH scale
This is a scale from _______________which tell you how acidic or basic a solution is.
The pH of a solution can be measured by using:
1)___________________________________
2) ________________________________________
The pH scale can be very useful but also has its limitations: it can only be used at a temperature of
__________ and the substance you are measuring must be in a __________ ______________
solution.
*An interesting example is the case of water at temperature over 25OC – we already mentioned that
the Kw (__________________________) of water increases with increasing temperatures.
At
100 OC is Kw = 52.3 x 10-14
[H+] = √52.3 x 10-14
[H+] =
pH =
So you might assume that the water is now _______________, but it is not because even though the
concentration of [H+] is higher than normal we still know that in pure water [H+] = [OH-] and this
means the solution is still neutral!
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Chapter 29 – Acids, Bases and pH
Section G – Calculating pH values
Strong acids
Strong acids _______________dissociate in water to produce H+ ions.
Example 1
Calculate the pH of a 0.5 M HF solution
Answer
H+ + F-
HF
Example 2
Find the pH of a 0.4M solution of sulphuric acid
Answer
Strong bases
Strong bases _______________dissociate in water to produce ____ ions.
Calculating the pH of a strong base:
pOH= - log10 [OH-]
pH =
Example 1 - Calculate the pH of 0.2 M KOH solution
Answer :

For a strong base like KOH

To find pOH Use the formula pOH= - log10 [OH-

pH = 14 - Poh
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KOH
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K + OH-
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Chapter 29 – Acids, Bases and pH
Weak acids
A weak acid only ________________dissociates in water to produce H+ ions.
For this weak acid which we will call HA:
H+ + A-
HA
Ka is the ______________________________ It will tell you to what extent a weak acid will actually
dissociate in water.
For a weak acid : [H+] = √Ka x [Acid]
pH =
****Square brackets mean concentration in moles per litre!
Example 1 Calculate the pH of a 0.1 solution of methanoic acid given that the value of the acid
dissociation constant is 1.8 X 10-4
Answer
[H+] = √Ka x [Acid]
[H+] =
[H+] =
[H+] =
pH=
Weak bases
A weak acid only partially _________________ in water to produce OH- ions.
For this weak base like NH3
NH3+ H2O
NH4+ OH-
Kb is the ______________________________ It will tell you to what extent a weak acid will actually
dissociate in water.
pH = 14 - pOH
pOH= - log10 [OH-]
[OH-] = √Kb x [Base]
****Square brackets mean concentration in moles per litre!
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Chapter 29 – Acids, Bases and pH
Example 1
Calculate the pH of a 0.01 solution of ammonia NH3 given that the value of the base dissociation
constant is 1.8 X 10-5
Answer
[OH-] = √Kb x [Base]
[OH-] =
[OH- ] =
[OH- ] =
pOH= - log10 [OH-]
pOH=
pH = 14 – pOH
pH =
Section H – Acid Base Indicators
Acid/ Base Indicators
Since acid/ base indicators are themselves either acids or bases you should not add too much of them
to an acid/base titration as they will themselves react in the reaction and cause an inaccurate titre
result!
Litmus as an example of an indicator
In its undissociated form (HIn)
Litmus is ___________ in colour.
HIn
H+ + In-
In its dissociated form (In-) Litmus
is ___________ in colour.
If blue Litmus is mixed with an acid then then the acid will donate ____________ ions to Litmus IN –
and will turn it into HIn which is ____________ in colour
If red Litmus is mixed with a base then the Indicator HIn will donate ______________ ions to the
base and it will change to In- which is _______________ in colour.
*We will look at the dynamics of these reactions in more detail in ch. Chemical Equilibrium
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Chapter 29 – Acids, Bases and pH
Section G - Titration curves and acid- base indicators
During an acid- base titration the pH will change gradually at first but will make a s___________
change close at the ____________ of the reaction. Using an indicator can tell us when the end point
has been reached by changing colour at this exact moment.
Strong acid strong base titration
Example:
At the end point the pH jumps suddenly from ____ to
____________.
A suitable indicator must have one colour at pH 3 and a different
colour at pH10
Suitable indicators:
Strong acid weak base titrations
Example:
At the end point the pH jumps suddenly from ____ to
____________ so the indicator must have a distinct colour change
in this range
Suitable indicator:
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Chapter 29 – Acids, Bases and pH
Weak acid strong base titration
Example
At the end point the pH jumps suddenly from ____ to
____________ so the indicator must have a distinct colour change
in this range
Suitable indicator:
Weak acid weak base titration
Example
There is NO sharp or sudden pH change during this reaction so there
is NOT a suitable indicator available to detect the end point by means
of a colour change.
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Chapter 29 – Acids, Bases and pH
Self-assessment of
Green = I already know this
Orange = I am not sure – have to check it
Red = I don’t know this yet – have to start learning
Green
Orange
Red
Household acids and bases (two examples of each).
Everyday examples of neutralisation, e.g. use of lime in agriculture,
use of stomach powders for acid indigestion.
Neutralisation – formation of a salt from an acid and a base.
Arrhenius theories of acids and bases
Brønsted-Lowry theories of acids and bases
Conjugate acid-base pairs. ( Definition and you must be able to
identify these pairs in an equation)
HL ONLY Self-ionisation of water; K w; ( Definition and you must be
able to show why [H+] and [OH-] are 1 x 10 -7
pH scale – explain what it is
Use of universal indicator paper or pH meter to measure pH of a
solution
Limitations of the pH scale – usefulness confined to dilute aqueous
solutions.
Calculation of pH of strong acids and bases
HL ONLY Calculation of pH of weak acids and bases.
HL ONLY Theory of acid-base indicators.( what an indicator is, how
they work)
HL ONLY Titration curves.( for each type of acid- base titration you
must be able to draw the curve, and know the end points)
HL ONLY Choice of indicator( for each type of indicator you must
know its colours at various pHs)
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Chapter 29 – Acids, Bases and pH
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