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Preparation of Salts
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Is this assault?
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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)
ZnSO4Zn2+ comes from
the base, Zn(OH)2
SO42- comes from
sulfuric acid, H2SO4
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Preparing Salts in the Laboratory
Two factors to consider
Is the salt soluble in water?
Are the starting materials soluble in water?
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Solubi
lity
Ta
Soluble Salts Insoluble Salts
All Sodium
All Potassium
All Ammonium
All Nitrates
All Chlorides Lead(II)
Silver
All Sulfates Barium
Calcium
Lead
Potassium
SodiumAmmonium
All Carbonates
Potassium
Sodium
Ammonium
Hydroxides and Oxides
except
except
except
except
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Methods of preparing salts
Reaction with acids
Acid + metal
Acid + base
Acid + carbonate
3) Precipitation
Is the salt to be prepared soluble in
water?Yes No
Yes No
Is the starting material
for making the salt
soluble in water?
Titration method
Acid + alkali solution
Acid + carbonate solution
Reaction with acids
Acid + excess metal *
Acid + excess insoluble base
Acid + insoluble carbonate
Salt solution Salt crystals
(dry with filter paper)
1. Concentrating
2. Crystallizing
3. Filtering
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Preparation of insoluble salts
All insoluble salts can be prepared byprecipitation.
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Making PbSO4Preparation, Separation and Purification
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Methods of preparing salts
Reaction with acids
Acid + metal
Acid + base
Acid + carbonate
3) Precipitation
Is the salt to be prepared soluble in
water?Yes No
Yes No
Is the starting material
for making the salt
soluble in water?
Titration method
Acid + alkali solution
Acid + carbonate solution
Reaction with acids with insoluble substances
Acid + excess metal *
Acid + excess insoluble base
Acid + insoluble carbonate
Salt solution Salt crystals
(dry with filter paper)
1. Concentrating
2. Crystallizing
3. Filtering
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Preparation of soluble salts
2. The substance
(MCB) must also beinsoluble in water.
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.
Reason:
This allows the excess(unreacted) substance
to be filtered from the
salt solution produced.
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Preparing a soluble salt:
Reacting acid with insoluble metal
E.g. Prepare zinc sulfate
1. Think of the relevant starting materials.
Zinc + Sulfuric acid Zinc Sulfate + Hydrogen
Zn(s) + H2SO4(aq) ZnSO4(aq) + H2(g)
ZnSO4Zn2+ comes from
the metal, ZnSO42- comes from
sulfuric acid, H2SO4
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Making ZnSO4Preparation, Separation and Purification
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Making ZnSO4Preparation, Separation and Purification
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Reacting acid with insoluble metal
This method is suitable for moderately
reactive metals such as magnesium,
aluminium, zinc, iron and lead.
It is not suitable for reactive metals such as
potassium, sodium and calcium. They react
violently with acids.
It is not suitable for unreactive metals such as copper
and silver. These metals do not react with dilute acids.
PotassiumSodiumCalcium
Magnesium*Aluminium*Zinc*Iron
Lead
CopperSilver
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Methods of preparing salts
Reaction with acids
Acid + metal
Acid + base
Acid + carbonate
3) Precipitation
Is the salt to be prepared soluble in
water?Yes No
Yes No
Is the starting material
for making the salt
soluble in water?
Titration method
Acid + alkali solution
Acid + carbonate solution
Reaction with acids with insoluble substances
Acid + excess metal *
Acid + excess insoluble base
Acid + insoluble carbonate
Salt solution Salt crystals
(dry with filter paper)
1. Concentrating
2. Crystallizing
3. Filtering
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Preparing a soluble salt:
Reacting acid with insoluble base
E.g. Prepare copper sulfate
1. Think of the relevant starting materials.
Copper(II) oxide + Sulfuric acid Copper (II) Sulfate + Water
CuO(s) + H2SO4(aq) CuSO4(aq) + H2O(l)
CuSO4Cu2+ comes from
the insoluble base
SO42- comes from
sulfuric acid, H2SO4
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Making CuSO4
To ensure that the
sulfuric acid is
completely used up
(Why?)To ensure that the
unreacted CuO does
not contaminate the
salt formedBy heating to dryness,CuSO4 will lose its
water of crystallisation.
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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.
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Methods of preparing salts
Reaction with acids
Acid + metal
Acid + base
Acid + carbonate
3) Precipitation
Is the salt to be prepared soluble in
water?Yes No
Yes No
Is the starting material
for making the salt
soluble in water?
Titration method
Acid + alkali solution
Acid + carbonate solution
Reaction with acids with insoluble substances
Acid + excess metal *
Acid + excess insoluble base
Acid + insoluble carbonate
Salt solution Salt crystals
(dry with filter paper)
1. Concentrating
2. Crystallizing
3. Filtering
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Think!
In the preparation of soluble salt, whathappens if the solid reactant is soluble
in water?
How can we tell if the acid has completelyneutralised?
We use the method TITRATION!Use of indicator help identify that the acid has
been completely used up.
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Titration
Suitable for preparing sodium, potassium,ammonium salts. (Why?)
Find your end point with the help of asuitable indicator then repeat experimentwithout indicator to get a pure salt sample.
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So, what have you learnt?