Salts

SALTS

salt is a compound formed when hydrogen in an acid is wholly or partially replaced by a metal or ammonium ion.



Types of salts

Normal salt: This is a salt where all the hydrogen ions are replaced. eg Na2SO4

Acid salt: This is a salt that contains hydrogen ion in it. They have acidic properties. eg NaHSO4

Basic salt: They contain hydroxide ion in them. They have basic properties. eg Zn(OH)Cl

Double salt: They contain two actions or two anions. eg Na2CO3.NaHCO3.2H2O

images of salts

SOLUBILITY OF SALTS

When sodium nitrate is added to water a little at a time while stirring it reaches a point where no more dissolves and the solution is said to be saturated. Warming the solution more solid dissolves. A saturated solution is a solution in which no solute can dissolve at a given temperature.

The salts below are soluble in water.

All nitrates

All common salts of potassium, sodium and ammonium

All common sulphates except those of lead, barium and calcium. Calcium sulphate is sparingly soluble.

All common chlorides except those of silver mercury and lead. Lead (II) chloride is soluble warm Water.

The following are insoluble

All carbonates except those of sodium, potassium and ammonium.

All hydroxides apart from those of sodium, potassium and calcium. Calcium hydroxide is sparingly soluble.




PREPARATION OF SOLUBLE SALTS.

Metal – acid reaction.

Preparation of zinc sulphate

Place 50cm3 of dilute sulphuric (VI) acid in a clean beaker. To the acid add zinc powder a little at a time when effervescence stops and some solid remains unreacted. This ensures all the acid has reacted.

Zn (s) + H2SO4 (aq) → ZnSO4 (aq) + H2 (g)

Filter to remove excess unreacted zinc. Boil the filtrate until it becomes saturated cool it for crystals to form. Separate the crystals from the remaining solution and dry it by pressing it gently between filter papers.




Carbonate – acid reaction.

Preparation of lead (II) nitrate

Place 50cm3 of dilute nitric (V) acid in a clean beaker. To the acid add lead (II) carbonate powder a little ata time when effervescence stops and some solid remains unreacted. This ensures all the acid has reacted. Filter to remove excess unreacted lead (II) carbonate.

PbCO3 (s) + 2HNO3 (aq) → Pb(NO3)2 (aq) + H2O(l) + CO2 (g)

Boil the filtrate until it becomes saturated cool it for crystals to form. Separate the crystals from the remaining solution and dry them by pressing it gently between filter papers.

METAL OXIDE (INSOLUBLE BASE) – ACID REACTION.

Preparation of copper (II) sulphate

Place 50cm3 of dilute sulphuric (VI) acid in a clean beaker. To the acid add copper (II) oxide powder a little ata time when effervescence stops and some solid remains unreacted. This ensures all the acid has reacted. Filter to remove excess unreacted copper (II) oxide.

CuO (s) + H2SO4 (aq) → CuSO4 (aq) + H2O (l)

Boil the filtrate until it becomes saturated cool it for crystals to form. Separate the crystals from the remaining solution and dry them by pressing it gently between filter papers.

Alkali (soluble base) – acid reaction

Measure 2cm3 of sodium hydroxide in a clean conical flask. Add 2 drops of phenolphthalein indicator. Fill the burette with dilute hydrochloric acid. From the burette add dilute hydrochloric acid to the sodium hydroxide in the beaker drop by drop and the colour changes from pink to colourless. Close the tap and record the volume of the acid used from the burette.

Discard this solution and clean the beaker. Measure 25cm3 of the sodium hydroxide and place in the clean beaker. Measure dilute hydrochloric acid the volume obtained in the above case and mix the two solutions.

NaOH (aq) + HCl (aq) → NaCl (aq) + H2O (g)

Boil the solution until it becomes saturated cool it for crystals to form, separate the crystals from the remaining solution wash with little cold water and dry by pressing gently between filter papers.



INSOLUBLE SALTS

PRECIPITATION REACTION (DOUBLE DECOMPOSITION)

This is a reaction between two soluble salts to form an insoluble salt and a soluble salt or a reaction between a soluble salt and a dilute acid to form insoluble salt and an acid.

Preparation of lead (II) sulphate

In a clean beaker add sodium sulphate powder add water and stir to dissolve. In another clean beaker add lead (II) nitrate crystals add water and stir to dissolve. Mix the two solutions. A white precipitate of lead (II) sulphate is formed

Pb (NO3)2 (aq) + Na2SO4 (aq) → PbSO4 (s) + 2NaNO3 (aq)

Filter to obtain the residue wash and dry.

Other insoluble salts that can be prepared in a similar way include barium sulphate, lead (II) chloride, silver chloride, calcium sulphate

PREPARATION OF LEAD (II) SULPHATE FROM LEAD (II) CARBONATE.

Both lead(II)sulphate and lead(II)carbonate are insoluble in water, therefore two reactions are involved. One is preparation of a soluble salt from the carbonate and secondly precipitation reaction from the soluble salt.

Procedure

Place dilute nitric (V) acid in a clean beaker. Add lead (II) carbonate powder a little at a time while stirring until when effervescence stops and some remains unreacted. Filter to remove the excess lead (II) carbonate.

PbCO3 (s) + 2HNO3 (aq) → Pb (NO3)2 (aq) + H2O (l) + CO2 (g)

In another clean beaker add sodium sulphate powder add water and stir to dissolve. Mix the solution formed with the filtrate.

Pb (NO3)2 (aq) + Na2SO4 (aq) → PbSO4 (s) + 2NaNO3 (aq)

Filter to obtain the residue of lead (II) sulphate. Wash and dry the residue.

Assignment

Preparation of lead (II) chloride from lead (II) oxide.

Preparation of copper (II) carbonate from copper (II) oxide.

DIRECT SYNTHESIS (COMBINATION)

When a metal and a non-metal are heated together a salt is formed.



Effects of heat on salts

Nitrates

Nitrates of sodium and potassium decompose at high temperate forming a nitrite and oxygen gas.

Nitrates of heavy metals decompose when heated to metal oxide nitrogen (II) oxide and oxygen gas.

Nitrates of less reactive metals such as silver and mercury decompose forming a free metal nitrogen (II) oxide and oxygen.

Carbonates

Carbonates of sodium and potassium are thermally stable hence do not decompose when heated.

Other Carbonates decompose to a metal oxide and carbon (IV) oxide.

Ammonium carbonate decomposes forming ammonia gas water and carbon (IV) oxide gas.