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This presentation was tailored to the GCE 'O' Level Pure Chemistry syllabus.
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This presentation was produced by the Science department of Temasek Secondary School. Redistribution or reproduction of this resource is prohibited by copyright regulations. This resource should be used for educational purposes.
SALTS……..continuation of
Acids and Alkalis
Overview of Today’s Lesson
What are Salts?
Uses of Salts
Methods of Preparing Salts
What is a Salt?Recap: Reactions of Acids
✽ Acids + Metal
✽ Acids + Carbonate
✽ Acids + Alkalis
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What is a Salt?Recap: Reactions of Acids
✽ Acids + Metal
✽ Acids + Carbonate
✽ Acids + Alkalis
Salt + H2 (g)
2HCl + Zn ZnCl2 + H2
2HCl + CaCO3 CaCl2 + CO2 + H2O
Salt + CO2 (g) + H2O (l)
Salt + H2O(l) HCl + NaOH NaCl + H2O
What is a Salt? A salt is a compound formed when the hydrogen of
an acid is replaced by a metal.
They are made when acids react with a metal, an alkali or a metal carbonate.
Mg + 2HCl → MgCl2 + H2
acid
metal
NaOH + HNO3 → NaNO3 + H2O
acid
metal
What is a Salt?
Different acids form different salts.
The salts of sulfuric acid are known as sulfates.
The salts of hydrochloric acid are known as chlorides.
The salts of nitric acid are known as nitrates.
Some Acids and their Salts
Hydrochloric acid HCl
Sodium chloride
Magnesium chloride
Copper(II) chloride
Aluminium chloride
NaCl
CuCl2
AlCl3
MgCl2
Some Acids and their Salts
Sulfuric acid H2SO4
Sodium sulfate
Copper(II) sulfate
Aluminium sulfate
Na2SO4
CuSO4
Al2(SO4 )3
Some Acids and their Salts
Nitric acid HNO3
Sodium nitrate
Potassium nitrate
Copper(II) nitrate
Aluminium nitrate
NaNO3
Cu(NO3)2
Al(NO3)3
KNO3
Importance of Salts Salts are important to our bodies. They help
us maintain the concentration of our body fluids.
We lose salt from our bodies through sweat and urine.
Salt must be replaced through food and drinks.
Uses of some salts1) Silver salts in photography eg. silver bromide (AgBr) A component on photographic film. A pale-yellow, insoluble, very sensitive to light. It turns black on exposure to light.
Uses of some salts
2) Food preservatives eg. sodium sulphite, sodium nitrite, sodium
citrate
Uses of some salts
3) Food flavourings eg. sodium chloride (salt),
monosodium glutamate (MSG)
4) Medical uses eg. calcium sulfate in plaster of Paris
Uses of some salts5) Fertilisers in agriculture eg. ammonium sulfate
6) Chemicals in industry eg. sodium chloride
Test your understanding1. Identify the acids needed to prepare the following
salts.
2. State the alkali that can react with the acid you mentioned in Question 1 to produce the three salts.
SALTS ACID NEEDED ALKALI
Potassium nitrate
Sodium sulfate
Aluminium ethanoate
Test your understanding1. Identify the acids needed to prepare the following
salts.
2. State the alkali that can react with the acid you mentioned in Question 1 to produce the three salts.
SALTS ACID NEEDED ALKALI
Potassium nitrate
Nitric acid
HNO3
Potassium hydroxide
KOH
Sodium sulfate
Sulfuric acid H2SO4
Sodium hydroxide
NaOH
Aluminium ethanoate
Ethanoic acid CH3COOH
Aluminium hydroxide
Al(OH)3
Methods of Preparing Salts
Depends on: Solubility of Salts in water;
Solubility of the chemicals used to prepare the salts.
• Acid + (Insoluble) Metal
Acid + Insoluble Carbonate
Preparation of soluble salts
Method 1
Method 2
Insoluble Metals
Insoluble metals that react with acid- magnesium, zinc, iron, aluminium
(Copper, silver and gold are insoluble metals that do not react with acid)
Solubility of Carbonates Soluble- carbonates of Group 1 elements e.g : sodium carbonate, potassium carbonate etc
Insoluble – All other carbonates e.g : calcium carbonate, copper carbonate zinc carbonate etc
PREPARATION OF SALT
Method 1
- Acid and Metal reaction
Acid + Metal → Salt + Hydrogen Salt: ZnSO4
Zn (s) + H2SO4 (aq) → ???
excess
Substances left
in the beaker?
Sulfuric
acid
Acid + Metal → Salt + Hydrogen
Salt: ZnSO4
Zn (s) + H2SO4 (aq) → ZnSO4 (aq) + H2(g)
excess
Substances left in the beaker?
Zn (s) , ZnSO4 solution
all used up escapes
How to remove the zinc from the zinc sulfate solution?
●Excess zinc can be removed by filtration.
●The zinc sulfate solution will be collected as the filtrate
Zinc sulfate solution
How to obtain the zinc sulfate salt from its solution?
Carry out evaporation of the zinc sulfate solution until it is saturated.
(not to dryness as the salt might decompose )
Allow the saturated zinc sulfate solution to cool so that the crystals of zinc sulfate will appear.
Filter to obtain the zinc sulfate crystals and dry them between sheets of filter paper.
Crystallization- involves evaporation and cooling processes.
Solution of zinc sulfate
Evaporating dish
Crystals of zinc sulfate
Reviewing the whole process of preparing zinc sulfate salt
Mixing
Filtration Evaporation & Crystallisation
Drying crystals
zinc sulfate crystals filter paper
Hot Sulfuric acid
zinc sulfate solution
zinc sulfate solution from step 2
Crystals of zinc sulfate
Is this method suitable for preparing all salts?
Recall:
Under the topic on Acids, which metals cannot be used to react with acids?
• Metals that are too reactive, eg: Na, K and Ca
• Metals that are unreactive, eg: Cu, Ag and Au
Acid + Metal → Salt + Hydrogen
Method 2
Acid and insoluble carbonate reaction
Acid + Insoluble Carbonate
CuCO3(s) + 2HCl(aq) → CuCl2(aq) + H2O (l) + CO2(g)
Substances left in the beaker?
CuCO3 (s) , CuCl2 solution
• Salt : CuCl2
escapesexcess all used up
copper(II) carbonate powder
residue:CuCO3
filter paper
filtrate:CuCl2 solution
evaporating dish
copper(II) chloride crystals
filter paper
1
4
2
5
3
6
PREPARATION OF SALTS
Crystals of copper(II) chloride
dilute HClacid
dilute HClacid
IMPORTANCE OF THE VARIOUS STEPS
STEPS WHY?
1) Warm / Heat some acid in a beaker. More metal/carbonate can dissolve in the acid & to speed up reaction.
2) Add excess metal/carbonate to the acid.
Stir until no more solid can dissolve.
To ensure that all acid is used up.
3) Filter the mixture to remove the residue and collect filtrate of salt solution.
To remove excess metal/carbonate as residue.
4) Heat the filtrate until half or one-third of the original volume.
To obtain a saturated solution.
5) Allow the solution to cool. For crystallization to take place.
6) Filter to collect the crystals.
Dry the crystals between filter paper.
Filter to separate crystals from solution.
Dry to collect dry crystals.