2.7.2d: describe and carry out the following reactions: (i) potassium halides with concentrated sulfuric acid, halogens and silver nitrate solution (ii) silver halides with sunlight and their solubilities in aqueous ammonia solution (iii) hydrogen halides with ammonia and with water (to produce acids)

POTASSIUM HALIDES + CONCENTRATED SULPHURIC ACID

Potassium chloride

• turns damp blue litmus paper red (acidic gas = HCl)
• ammonia held on a glass rod above the mouth of the test tube, white fumes given off = test for HCl
• eg. KCl(s) + H2SO4(aq) → KHSO4(s) + HCl(g)
• 
• steamy fumes observed at the mouth of the test tube = hydrogen chloride dissolving in the moisture in the air

Potassium bromide

• turns damp blue litmus paper red (acidic gas = SO2)

• brown vapour
• strip of filter paper dipped in potassium dichromate, turns from orange to green = test for SO2
• 

Potassium iodide

• turns damp blue litmus paper red (acidic gas = HI)
• purple vapour
• yellow or black solid formed
• strip of filter paper dipped in lead ethanoate (II) turns from colourless to black = test for H2S
• 

When concentrated sulfuric acid is added to solid sodium chloride, no chlorine is produced. The reason for this is:

• the chloride ion is a weaker reducing agent than the bromide ion

POTASSIUM HALIDES + HALOGENS

	KCl	KBr	KI
chlorine	no change	pale green to red-brown	pale green to brown
bromine	no change	no change	red-brown to brown
iodine	no change	no change	no change

POTASSIUM HALIDES + SILVER NITRATE SOLUTION

Potassium chloride: white precipitate

Potassium bromide: cream precipitate

• Ag+(aq) + Br-(aq) → AgBr(s)

Potassium iodide: pale yellow precipitate

SILVER HALIDES + SUNLIGHT

Darken in sunlight as the halogen vaporises, leaving the silver behind

SILVER HALIDES + AQUEOUS AMMONIA SOLUTION

Silver chloride: dissolves in dilute ammonia solution

Silver bromide: dissolves in concentrated ammonia solution

Silver iodide: insoluble in dilute and concentrated ammonia solution

HYDROGEN HALIDES + AMMONIA OR WATER

The hydrogen halides react with water and ammonia to make acids

• the hydrogen halides are very soluble in water
• a proton is donated from the hydrogen halide to one of the lone pairs on a water molecule, forming a dative covalent bond
• H2O + HCl → H3O+ + Cl-
• HCl, HBr and HI are all strong acids
• eg. NH4Cl

HI(g) + NH3(g) → NH4I(s)