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Acid Dissociation…
Work together in groups. How
many definitions do you know
for acids?
Acid Dissociation…
The AS definition of an acid is:
“A molecule that dissociates
to release a proton when
dissolved in water”
It is a PROTON DONATOR
Acid Dissociation…
There are a number of
compounds that you need to
instantly recognise as acids:
HCl
H2SO4
HNO3
Anything ending -COOH
Alkali Dissociation…
The AS definition of an alkali is:
“A molecule that dissociates
to release a hydroxide ion
when dissolved in water”
It is a PROTON ACCEPTOR
Alkali Dissociation…
There are a number of
compounds that you need to
instantly recognise as alkalis:
NH3
Metal oxides - MO
Metal hydroxides - MOH
Amines - NH2
Dissociation of water…
This is one of the few times where we
treat water as an ionic compound.
H 2O
OH- + H+
In the presence of an acid:
H 2O + H +
H3O+
Dissociation of water…
Water behaves as both an acid and an
alkali, depending on the pH of a
compound added to it.
It is described as being AMPHOTERIC.
Amino acids are also amphoteric as
they contain both
- NH2
- COOH
Strong vs Concentrated…
Terminology is likely to get you in
trouble in A level chemistry.
Up until now strong and concentrated
have been interchangeable. As have
weak and dilute.
In your groups define all 4 terms.
Strong vs Concentrated…
Concentrated
“A high number of particles in a given
volume of a fluid”
Dilute
“A low number of particles in a given
volume of a fluid”
Strong vs Concentrated…
Strong
“A compound that FULLY DISSOCIATES
into its metal and non-metal parts”
Weak
“A compound that PARTIALLY
DISSOCIATES into its metal and nonmetal parts”
Strong vs Concentrated…
Full dissociation means that ALL the
particles dissociate as shown by:
HA 
+
H
+
A
Partially dissociation means that only
some of the particles dissociate as
shown by:
HA
H+ +
A-
Titrations…
Titrations are used to find the concentration
of an unknown solution by reacting with a
solution of known concentration.
It uses the ratio of H+ in an acid to the nonmetal component of an alkali.
More simply put:
We need balanced equations!
Titrations…
In order to perform a titration we need to be
able to identify the point of neutralisation. To
do this we use INDICATORS.
The common indicators are:
• Methyl Orange
• Phenolphthalein
Titrations…
Methyl Orange:
• Yellow in alkali
• Red/Orange in acid
The point of neutralisation is the
FIRST PERMANENT COLOUR
CHANGE
Titrations…
Phenolphthalein:
• Pink in alkali
• Colourless in acid
The point of neutralisation is the
FIRST PERMANENT COLOUR
CHANGE
Titrations…
Neither of these indicators change
colour at pH7. So the choice of when
we use it depends on where its “end
point” pH lies
Titrations…
Strong acid/Strong alkali
The end point for
both indicators lie
within the
“equivalence
point” of the
reaction.
Both are therefore
suitable.
Titrations…
Strong acid/Weak alkali
The end point for
only methyl
orange lies within
the “equivalence
point” of the
reaction.
Therefore only
methyl orange is
suitable.
Titrations…
Weak acid/Strong alkali
Only the end point
for
phenolphthalein
lies within the
“equivalence
point” of the
reaction.
Therefore only
phenolphthalein is
suitable.
Titrations…
Weak acid/Weak alkali
Neither indicator’s
end point lies
within the
“equivalence
point” of the
reaction.
Therefore neither
indicator is
suitable.
Titrations…
3
13.5cm
of 0.01M hydrochloric
acid is required to neutralise
25cm3 of sodium carbonate.
What is the concentration of the
alkali?
Titrations…
1. Write out the balanced equation
2HCl + Na2CO3
2NaCl + CO2 + H2O
2. Calculate the moles of acid
n(HCl)
= 0.01 * 0.0135
=1.35 * 10-4
3. Use the equation to calculate the
moles of alkali
n(Na2CO3) = (1.35 * 10-4)/2
=6.75 * 10-5
4. Calculate the concentration of the
alkali
c(Na2CO3) = 6.75 * 10-5 / 0.025
= 2.7 * 10-3 M
Titrations…
In stretch and challenge question
you will be expected to convert
certain units. These are:
• 1ppm
= 1mgdm-3
• moldm-3 = gdm-3 / Mr