• All the elements of the first group react with water liberating hydrogen and become hydroxides. Example : Na reacts rapidly with water liberating hydrogen.
2Na(s) + 2H2O(l) → 2NaOH(aq) + H2 (g)
• When a small piece of K is added into water it reacts while burning. As K reacts with water more rapidly than Na, it can be concluded that the rate of reaction with water increases down the group.
• A reaction is not seen when a clean piece of Mg is added into water. When the water with Mg is warmed, it is seen to react slowly.
Mg(s) + 2H2O(l) → Mg(OH)2 (s) + H2 (g)
• As the reactivity shown by Mg with water is lower compared to Na, it can be said that the metals of group II compared to metals of group I show a lower reactivity.Be does not react with water. Ca, Sr, and Ba react with water liberating hydrogen and forming the hydroxides.
Ca(s) + 2H2O(l) → Ca(OH)2 (aq) + H2 (g)
• Be and Mg react with steam to form the oxides.
Be(s) + H2O(g) → BeO(s) + H2 (g)
Mg(s) + H2O(g) → MgO(s) + H2 (g)
There are several reactions of group I metals with air .
4Na(s) + O2 (g) → 2Na2O(s)
2Na(s) + Excess O2(g) → Na2O2(s)
2Na(s) + 2H2O(g) → 2NaOH(s) + H2 (g)
K, Rb and Cs readily react with O2 forming superoxides.
Excess K(s) + O2 (g) → KO2 (s)
When heated in air only Li of Group I reacts with nitrogen.
6Li(s) + N2 (g) → 2Li3N(s)
When a clean piece of Mg ribbon and a small cut piece of Na are exposed to air Na tarnishes faster than Mg. Hence it is clear that the reactivity of Mg is lower than Na. Accordingly it can be said that relative to metals of group I, the reactivity of group II metals with air is lower.
Metals of the Group II when heated in air burn forming oxides and nitrides.
2Mg(s) + O2 (g) → 2MgO(s)
3Mg(s) + N2 (g) → Mg3N2 (s)
For Be to react it should be heated to a very high temperature.
As the metals of the group I react with acids liberating large quantity of heat, an explosion takes place. Therefore it should not be tested.
2Na(s) + dil.H2SO4(aq) → Na2SO4(aq) + H2(g)
Group II metals reacts with dilute acids to liberate H2 rapidly
Mg(s) + dil.H2SO4(aq) → MgSO4(aq) + H2(g)
Group II metals can be oxidised by concentrated acids.
Mg(s) + 2 conc.H2SO4(aq) → MgSO4(aq) + SO2(g) + H2O(l)
Mg(s) + 4 conc.HNO3(aq) → Mg(NO3)2(aq) + 2NO2(g) + 2H2O(l)
Lithium and Group II metals form metal nitrides by reacting with N2 gas or N2 presents in air.
6Li(s) + N2(g) → 2Li3N(s)
3Mg(s) + N2(g) → Mg3N2(s)
s Block elements form metal hydrides by reacting with H2 gas.
2Li(s) + H2(g) → 2LiH(s)
2Na(s) + H2(g) → 2NaH(s)
Mg(s) + H2(g) → MgH2(s)
• Because s block elements easily remove their electrons and form cations they are considered as good reducing agents.
• While atomic radius increases down the group nuclear attraction decreases. Consequently, reducing ability of the elements also increases.
