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Ionic halides are formed by most metals. The ionic character:
- Decreases across the Periodic Table. The most ionic halides are that of Group I.
- Decreases down a group. Fr in Group I forms the least ionic halides.
- Decreases from fluorides to iodides. Iodides are the least ionic. E.g. LiF is ionic but LiI is covalent.
Halides of metals in higher oxidation states tend to be covalent, e.g. SnCl2 is ionic whereas SnCl4 is covalent.
Covalent halides are formed from non-metals and metals which are weakly electronegative or in a higher oxidation state.
Si(s) + 2Cl2(g) → SiCl4(l)
2P(s) + 5 Cl2(g) → 2PCl5(l)
F2 always gives the highest possible oxidation state.
2P(s) + 5F2(g) → 2PF5(l)
Cl2 gives the higher states if in excess.
Covalent halides are either gases, volatile liquids or low melting solids. They are all readily hydrolysed by water and hence the need for anhydrous reaction conditions.
Hydrolysis occurs by a stepwise mechanism and makes use of empty 'd' orbitals on the element. The only halides which cannot be hydrolysed are those of fluorine (as this element has no available 'd' orbitals) and carbon tetrachloride.
Add dilute HNO3 followed by AgNO3 solution and, test the solubility of the precipitate in excess NH3 solution.
General ionic equation:
Ag+(aq) + X-(aq) → AgX(s)
The colour of the precipitate AgX identifies the halide present.
- AgF - White
- AgCl - White
- AgBr - Cream
- AgI - Yellow