( . to . ) are used to prepare dihydrogen gas in the laboratory. Here, the reactivity of metals is reflected in the rate of hydrogen gas evolution, which is the slowest for the least active metal Fe, and the fastest for the most reactive metal, Mg.
Very less active metals, which may occur in the native state such as silver (Ag), and gold (Au) do not react even with hydrochloric acid. In section ( . . ) we have already discussed that the metals – zinc (Zn), copper (Cu) and silver (Ag) through tendency to lose electrons show their reducing activity in the order Zn> Cu>Ag.
Like metals, activity series also exists for the halogens. The power of these elements as oxidising agents decreases as we move down from fluorine to iodine in group of the periodic table. This implies that fluorine is so reactive that it can replace chloride, bromide and iodide ions in solution. In fact, fluorine is so reactive that it attacks water and displaces the oxygen of water : + – + – 2H O (l) + 2F (g) → 4HF(aq) + O (g) ( .
) It is for this reason that the displacement reactions of chlorine, bromine and iodine using fluorine are not generally carried out in aqueous solution. On the other hand, chlorine can displace bromide and iodide ions in an aqueous solution as shown below: + – + – Cl (g) + 2KBr (aq) → KCl (aq) + Br (l) ( . ) + – + – Cl (g) + 2KI (aq) → KCl (aq) + I (s) ( . ) As Br and I are coloured and dissolve in CCl , can easily be identified from the colour of the solution.
The above reactions can be written in ionic form as: – – Cl (g) + 2Br – (aq) → 2Cl – (aq) + Br (l) ( .41a) – – Cl (g) + 2I – (aq) → 2Cl – (aq) + I (s) ( .42b) Reactions ( . ) and ( . ) form the basis