Preparation of Salts
Is this assault?
What are salts?
In a acid-base reaction, a salt is formed when
a metallic ion or an ammonium ion replaces
one or more hydrogen ions in an acid.
E.g.
Zinc hydroxide + sulfuric acid zinc sulfate + water
Zn(OH)2 (s) + H2SO4 (aq) ZnSO4 (aq) + H20 (l)
Zn2+ comes from
the base, Zn(OH)2
ZnSO4
SO42- comes from
sulfuric acid, H2SO4
Preparing Salts in the Laboratory
Two factors to consider
Is the salt soluble in water?
Are the starting materials soluble in water?
Solubility Ta
Soluble Salts
Insoluble Salts
All Sodium
All Potassium
All Ammonium
All Nitrates
All Chlorides
except
Lead(II)
Silver
except
Barium
Calcium
Lead
All Sulfates
Potassium
Sodium
Ammonium
except
All Carbonates
Potassium
Sodium
Ammonium
except
Hydroxides and Oxides
Methods of preparing salts
Is the salt to be prepared soluble in
water?
Yes
Reaction with acids
Acid + metal
Acid + base
Acid + carbonate
Yes
No
3) Precipitation
Is the starting material
for making the salt
soluble in water?
Titration method
Acid + alkali solution
Acid + carbonate solution
Salt solution
No
Reaction with acids
Acid + excess metal *
Acid + excess insoluble base
Acid + insoluble carbonate
1.
2.
3.
Concentrating
Crystallizing
Filtering
Salt crystals
(dry with filter paper)
Preparation of insoluble salts
All insoluble salts can be prepared by
precipitation.
Precipitation
Aqueous solutions of two suitable soluble
salts are mixed.
Eg. Prepare lead(II) sulfate
Pb2+
comes from
soluble lead(II) salt
PbSO4
SO42- comes from
sulfuric acid, H2SO4 or
any soluble sulfate
The insoluble salt appears as a precipitate and
can be obtained by filtration.
Making PbSO4
Preparation, Separation and Purification
Methods of preparing salts
Is the salt to be prepared soluble in
water?
Yes
Reaction with acids
Acid + metal
Acid + base
Acid + carbonate
Yes
No
3) Precipitation
Is the starting material
for making the salt
soluble in water?
Titration method
Acid + alkali solution
Acid + carbonate solution
No
Reaction with acids with insoluble substances
Acid + excess metal *
Acid + excess insoluble base
1. Concentrating
Acid + insoluble carbonate
2.
3.
Salt solution
Crystallizing
Filtering
Salt crystals
(dry with filter paper)
Preparation of soluble salts
1. We react the acid
with an excess of the
substance (MCB).
Reason:
This ensures that all the
acid is used up.
The reaction is complete.
2. The substance
(MCB) must also be
insoluble in water.
Reason:
This allows the excess
(unreacted) substance
to be filtered from the
salt solution produced.
Preparing a soluble salt:
Reacting acid with insoluble metal
E.g. Prepare zinc sulfate
1. Think of the relevant starting materials.
Zn2+ comes from
the metal, Zn
ZnSO4
SO42- comes from
sulfuric acid, H2SO4
Zinc + Sulfuric acid Zinc Sulfate + Hydrogen
Zn(s) + H2SO4(aq) ZnSO4(aq) + H2(g)
Making ZnSO4
To ensure that the sulfuric acid
is completely used up (Why?)
Salts
combine
with water
molecules
to form
crystals
By heating to dryness, ZnSO4 will lose
its water of crystallisation
To ensure that
the unreacted
zinc does not
contaminate the
salt formed
Making ZnSO4
Preparation, Separation and Purification
Making ZnSO4
Preparation, Separation and Purification
Reacting acid with insoluble metal
Potassium
Sodium
Calcium
It is not suitable for reactive metals such as
potassium, sodium and calcium. They react
violently with acids.
Magnesium*
Aluminium* This method is suitable for moderately
reactive metals such as magnesium,
Zinc*
Iron
aluminium, zinc, iron and lead.
Lead
Copper
Silver
It is not suitable for unreactive metals such as copper
and silver. These metals do not react with dilute acids.
Methods of preparing salts
Is the salt to be prepared soluble in
water?
Yes
Reaction with acids
Acid + metal
Acid + base
Acid + carbonate
Yes
No
3) Precipitation
Is the starting material
for making the salt
soluble in water?
Titration method
Acid + alkali solution
Acid + carbonate solution
No
Reaction with acids with insoluble substances
Acid + excess metal *
Acid + excess insoluble base
1. Concentrating
Acid + insoluble carbonate
2.
3.
Salt solution
Crystallizing
Filtering
Salt crystals
(dry with filter paper)
Preparing a soluble salt:
Reacting acid with insoluble base
E.g. Prepare copper sulfate
1. Think of the relevant starting materials.
Cu2+ comes from
SO42- comes from
the insoluble base
4 sulfuric acid, H2SO4
CuSO
Copper(II) oxide + Sulfuric acid Copper (II) Sulfate + Water
CuO(s) + H2SO4(aq) CuSO4(aq) + H2O(l)
Making CuSO4
To ensure that the
sulfuric acid is
completely used up
(Why?)
By heating to dryness,
CuSO4 will lose its
water of crystallisation.
To ensure that the
unreacted CuO does
not contaminate the
salt formed
Think!
In the making of copper(II) sulfate, why is
copper not used to react with sulfuric acid?
This method is used to prepare metal salts
whose metal is not too reactive.
Methods of preparing salts
Is the salt to be prepared soluble in
water?
Yes
Reaction with acids
Acid + metal
Acid + base
Acid + carbonate
Yes
No
3) Precipitation
Is the starting material
for making the salt
soluble in water?
Titration method
Acid + alkali solution
Acid + carbonate solution
No
Reaction with acids with insoluble substances
Acid + excess metal *
Acid + excess insoluble base
1. Concentrating
Acid + insoluble carbonate
2.
3.
Salt solution
Crystallizing
Filtering
Salt crystals
(dry with filter paper)
Think!
In the preparation of soluble salt, what
happens if the solid reactant is soluble
in water?
How can we tell if the acid has completely
neutralised?
We use the method TITRATION!
Use of indicator help identify that the acid has
been completely used up.
Titration
Suitable for preparing sodium, potassium,
ammonium salts. (Why?)
Find your end point with the help of a
suitable indicator then repeat experiment
without indicator to get a pure salt sample.
So, what have you learnt?
