Chemistry-Ionic Chemistry

Ionic Chemistry
Sodium hydroxide solution and aqueous ammonia

When sodium hydroxide solution and aqueous ammonia are added to solutions of salts, the cation(s) combine with the hydroxide ions from the alkali to form a precipitate, insoluble hydroxide. In a few cases, for example, zinc hydroxide, lead(II) hydroxide, aluminium hydroxide and copper(II) hydroxide, the precipitate formed is soluble in excess of the alkali due to formation of a complex compound.

Anions such as sulphate and chloride in solution react with barium ions and silver ions respectively to form precipitates. The above tests confirm the presence of the named anions in a salt.

Below is a summary of the observations made when aqueous sodium hydroxide and ammonia solution are added to cations in solution:

Aqueous sodium hydroxide and ammonia solution when added to cations in solution:

Cations	Aqueous sodium hydroxide	Aqueous ammonia
Ca2+	A white precipitate was formed insoluble in excess.	A white precipitate was formed very slowly.
Zn2+	A white precipitate was formed, soluble in excess forming a colourless solution.	A white precipitate was formed, soluble in excess forming a colourless solution.
Pb2+	A white precipitate was formed, soluble in excess forming a colourless solution.	A white precipitate was formed insoluble in excess.
Al3+	A white precipitate was formed, soluble in excess forming a colourless solution.	A white precipitate was formed insoluble in excess.
Cu2+	A blue precipitate was formed insoluble in excess.	A pale blue precipitate was formed, soluble in excess forming a deep blue solution.
Fe2+	A dirty green precipitate was formed insoluble in excess.	A dirty green precipitate was formed insoluble in excess.
Fe3+	A red-brown precipitate was formed insoluble in excess.	A red-brown precipitate was formed insoluble in excess.
NH4+	No precipitate was formed but on warming the mixture a gas with a choking irritating smell is detected which turns red litmus blue.	Not applicable

Equations for the reactions taking place when sodium hydroxide or ammonia solution is added to various cations in solution:

Ca²⁺ (aq) + 2OH^-(aq) Ca(OH)₂(s)

Zn²⁺ (aq) + 2OH^-(aq) Zn(OH)₂(s)

Pb²⁺ (aq) + 2OH^-(aq) Pb(OH)₂(s)

Al³⁺ (aq) + 3OH-(aq) Al(OH)₃(s)

Cu²⁺ (aq) + 2OH^-(aq) Cu(OH)₂(s)

Fe²⁺ (aq) + 2OH^-(aq) Fe(OH)₂(s)

Fe³⁺ (aq) + 3OH^-(aq) Fe(OH)(s)

NH₄⁺ (aq) + OH^-(aq) NH₃ (g) + H₂O (l)

Hydroxides of zinc, lead(II) and aluminium are soluble in excess sodium hydroxide because of their acidic nature. They form complex compounds with the alkali. The equations for the reactions are:

Zn(OH)₂(s) + 2OH^- (aq) [Zn(OH)₄]^2-(aq) zincate ion

Pb(OH)₂ (s)  + 2OH^- (aq) [Pb(OH)₄]^2-(aq) plumbate ion

Al(OH)₃ (s)  + OH^- (aq) [Al(OH)₄]^-(aq) aluminate ion

Zinc hydroxide and copper(II) hydroxide are soluble in aqueous ammonia, due to formation of complex ions, tetra ammine zinc(II) ion and tetra ammine copper(II) ion respectively. The formation of the complex compound is illustrated by the following equations:

Zn(OH)₂ (s)  + 4NH₃ (aq) [Zn(NH₃)₄]²⁺(aq) tetra ammine zinc(II) ion

Cu(OH)₂ (s)  + 4NH₃ (aq) [Cu(NH₃)₄]²⁺(aq) tetra ammine copper(II) ion

MAKING OF SOLUTIONS

2M Sodium hydroxide solution: is made by weighing out 80g of the pellets, dissolving them in a small amount of water and making up the solution to the 1 litre mark.

2M Ammonia solution: is made by measuring out 135cm3 of conc. ammonia and diluting it with water to the 1 litre mark.

2M Nitric acid: is made by measuring out 127cm3 of conc. nitric acid and diluting it with water to the 1 litre mark.

2M Hydrochloric acid: is made by measuring out 167cm3 of conc. hydrochloric acid and diluting it with water to the 1 litre mark.

2M Sulphuric acid: is made by measuring out 111cm3 of conc. sulphuric acid and adding it to some water in a 1 litre measuring cylinder. Now dilute this with water to the mark.

Salt solutions: 2% - 5% salt solutions are adequate for use. They are prepared by weighing out (2xRFM/100) g of the salt, dissolving it in water and making up the salt solution to 100cm3. Barium nitrate, lead(II) nitrate, potassium iodide, silver nitrate and iron(II) sulphate are prepared by following the above procedure.

NB: 1) RFM refers to Relative Formula Mass. 2) When diluting concentrated acids, add the acid to water and NOT water to the acid because the reaction the between two is highly exothermic.