The halogens refer to chlorine, bromine and iodine.
1) Colour and volatility
Chlorine is a dense green gas.
Bromine is a brown volatile liquid.
Iodine is a shiny black solid
Down the group, the number of electrons in the halogen molecule increases, hence the strength of van der Waals forces between the halogen molecules increases. This explains why chlorine exists as a gas, bromine exists as a liquid and iodine exists as a solid.
2) Relative reactivity of halogens as oxidizing agents
Chlorine is the strongest oxidizing agent followed by bromine and iodine. Recall that an oxidizing agent causes oxidation and it itself is reduced, i.e. an oxidizing agent has a tendency to gain electrons.
The electrode potential values become less positive down the group, indicating that down the group the halogens has a reduced tendency to gain electrons, hence the oxidizing power of halogen decreases.
3) Reaction of halogens with hydrogen
Chlorine reacts explosively in sunlight with hydrogen. It also reacts slowly in the dark with hydrogen.
Bromine reacts with hydrogen only at high temperatures
Iodine reacts with hydrogen to form an equilibrium mixture of hydrogen, iodine and HI.
4) Relative thermal stability of hydrides (hydrogen halides)
Thermal stability of hydrogen halides decreases down the group.
Down the group, the atomic radii of the halogen increases and the bond dissociation enthalpy of hydrogen halides decreases.
Bond energy of H--Cl : 431 kJ/mol
Bond energy of H--Br: 366 kJ/mol
Bond energy of H--I: 299 kJ/mol
Note: In the exam, students may be asked to quote bond energy values from the Data booklet. You do not have to memorize these bond energies. they are in the data booklet.
5) Reaction of halide ions with aqueous silver ions followed by aqueous ammonia
Chloride ions form a white precipitate with silver ions. White ppt is soluble in aqueous NH3
Bromide ions form a pale yellow ppt with silver ions. Yellow ppt is sparingly soluble in aqueous ammonia.
Iodide ions form a deep yellow ppt with silver ions. Deep yellow ppt is insoluble in aqueous ammonia.
Silver ions is usually supplied as silver nitrate. This is a common test for halides. Aqueous ammonia is added to differentiate between silver chloride and silveer bromide as it is very hard to differentiate between the white and pale yellow ppt.
Aqueous ammonia has to be added slowly as the pale yellow silver bromide is soluble in excess aqueous ammonia. Aqueous ammonia forms a complex with silver ions, shifting the equlibrium to the left, and increases the solubility of AgCl. AgCl is most soluble followed by AgBr and AgI, thus the addition of ammonia can fully solubilise AgCl. AgBr is only soluble in concentrated ammonia while ammonia has no effect on the solubility of AgI.
6) Reaction of halide ions with concentrated sulphuric acid
Concentrated sulphuric acid can act as a proton donor, i.e. it supplies H+ ions which reacts with the halide ion to form hydrides (HCl, HBr and HI). The hydrides appear as steamy white fumes
Concentrated sulphuric acid can also act as an oxidizing agent.
Recall that iodine is the weakest oxidizing agent, hence it is the most easily oxidized. Concentrated sulphuric acid can oxidize iodide ions to iodine, hence you will see purple vapour
Concentrated sulphuric acid can also oxidize bromide ions to bromine gas.
Concentrated sulphuric acid cannot oxidize chloride ions to chlorine gas.
7) Reaction of chlorine with NaOH
Chlorine reacts with cold NaOH to form NaCl and NaClO (sodium hypochlorate)
Note that in this reaction one atom of chlorine is undergoing oxidation and the other atom of chlorine is undergoing reduction.
Chlorine is oxidized. The oxidation number of chlorine increases from 0 in Chlorine gas to +1 in NaClO. Chlorine ia also reduced. The oxidation number of chlorine decreases from 0 in Chlorine gas to -1 in NaCl.
Chlorine reacts with hot NaOH to form NaCl and NaClO3 (sodium chlorate)
In this reaction one atom of chlorine is undergoing oxidation and the other atom of chlorine is undergoing reduction.
Chlorine is oxidized. The oxidation number of chlorine increases from 0 in Chlorine gas to +5 in NaClO3. Chlorine ia also reduced. The oxidation number of chlorine decreases from 0 in Chlorine gas to -1 in NaCl.
End of group VII notes
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