preparation of salts.pdf

7/25/2019 Preparation of Salts.pdf

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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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Reaction with acids

Acid + metal

Acid + base

Acid + carbonate

3) Precipitation


water?Yes No

Yes No


for making the salt

soluble in water?

Titration method



Reaction with acids with insoluble substances






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



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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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Reaction with acids

Acid + metal

Acid + base

Acid + carbonate

3) Precipitation


water?Yes No

Yes No


for making the salt

soluble in water?

Titration method









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



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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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Reaction with acids

Acid + metal

Acid + base

Acid + carbonate

3) Precipitation


water?Yes No

Yes No


for making the salt

soluble in water?

Titration method









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?

preparation of salts.pdf

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