3 Reaction of period 3 oxides with water

The reactions of the oxides of the elements Na
S with water know the change in pH of the resulting solutions across the Period. Metal ionic oxides tend to react with water to form hydroxides which are alkaline Na2O (s) + H2O (l)
2Na+ (aq) + 2OH- (aq) pH 13 (This is a vigorous exothermic reaction) The ionic oxides are basic as the oxide ions accept protons to become hydroxide ions in this reaction (acting as a bronsted lowry base) MgO (s) + H2O (l)
Mg(OH)2 (s) pH 9 Mg(OH)2 is only slightly soluble in water as its lattice is stronger so fewer free OHions are produced and so lower pH Al2O3 and SiO2 do not dissolve in water because of the high strength of the Al2O3 ionic lattice and the SiO2 macromolecular structure, so they give a neutral pH 7 The non-metal, simple molecular, covalent, oxides react with water to give acids P4O10 (s) + 6 H2O (l)
4 H3PO4 (aq) pH 0 (this is a vigorous exothermic reaction) SO2 (g) + H2O (l)
H2SO3 (aq) pH 3 (weak acid) SO3 (g) + H2O (l)
H2SO4 (aq) pH 0 Learn the equations ! The trend is the ionic metal oxides show basic behaviour and the non-metal covalent oxides show acidic behaviour. The slightly intermediate nature of the bonding in Aluminium oxide is reflected in its amphoteric behaviour: it can act as both a base and an acid MgO is better than NaOH for treating acid in rivers and the stomach as it is only sparingly soluble and weakly alkaline so using an excess would not make the water excessively alkaline. N Goalby chemrevise.org SO2 + H2O
H+ + HSO3 – SO3 + H2O
H+ + HSO4 – Equations showing formation of ions in solution