Reactions with Water
Na reacts with water to form the hydroxide, NaOH
Mg reacts with steam to form the oxide, MgO
Trend in the Reaction of the Elements
The reactions of the elements become less vigorous from Na to Si.
The reactivity then increases from Si onwards.
Reaction of the Elements with Oxygen
All the elements: Na, Mg, Al , Si, P and S react with oxygen to form the oxide.
Below are the formulae of the products:
Trend in Melting Points
The melting points of the period 3 oxides generally decrease as you across the period. This is due to the bonding and structure of the compounds:
Na2O: ionic lattice
MgO: ionic lattice
Al2O3: mostly ionic bonding
SiO2: giant covalent
P4O10: simple covalent
SO2 / SO3: simple covalent
Ionic lattices have high melting points because the electrostatic forces between oppositely charges ions are very strong and require a lot of energy to break.
Simple covalent molecules have relatively low melting points because between the molecules there are van der Waals forces, which are relatively weak, therefore require little energy to break.
Reactions of the Oxides with Water
Na2O reacts with water to form the hydroxide, NaOH
MgO also reacts with water to form the hydroxide, Mg(OH)2
Al2O3 does not react with water
SiO2 does not react with water
P4O10 reacts with water to form phosphoric acid, H3PO4
SO2 reacts with water to form sulfurous acid, H2SO3
SO3 reacts with water to form sulfuric acid, H2SO4
As you can see the reactivity of the period 3 oxides with water generally decreases up to aluminium oxide and silicon dioxide which both do not react with water. However, phosphorus (V) oxide, sulfur dioxide and sulfur trioxide react to form the corresponding acids.
Change in pH across the Period
Na2O Hydroxide Ionic
MgO Hydroxide Ionic
Al2O3 Does not react Ionic
SiO2 Does not react Covalent
P4O10 Acidic Covalent
SO2 / SO3 Acidic Covalent
As you can see the oxides form solutions which are generally get more acidic as you go from left to right, therefore the acidity increases and the pH decreases.
The reason for this is due to the bonding. Ionic compounds generally form basic solutions, whereas covalent compounds normally form acidic compounds.
Reactions between the Oxides and Acids and Bases
Both NaOH and Mg(OH)2 react with acids to form a salt and water
Al(OH)3 is amphoteric, so reacts with both acids and bases:
Al(OH)3 + 3HCl = AlCl3 + 3H2O
Al(OH)3 + NaOH = Na[Al(OH)4]
Si(OH)2 reacts with bases to form the ion, Si(OH)3-
Si(OH)2 + NaOH = Na[Si(OH)3]
P4O10 reacts with a base to form a salt and water:
P4O10 + 12NaOH = 4Na3PO4 + 6H2O
SO2 reacts with a base to form a salt (sulfite) and water, SO3 reacts with a base to form a salt (sulfate) and water:
SO3 + 2NaOH = Na2SO4 + H2O