Oxides Reacting with Water
Structure, bonding & electronegativity of the Period 3 elements table
Period 3 element | Na | Mg | Al | Si | P | S |
Period 3 oxide | Na2O | MgO | Al2O3 | SiO2 | P4O10 | SO2 SO3 |
Relative melting point | High | High | Very high | Very high | Low | Low |
Chemical bonding | Ionic | Ionic | Ionic (with a degree of covalent) | Covalent | Covalent | Covalent |
Structure | Giant ionic | Giant ionic | Giant ionic | Giant covalent | Simple molecular | Simple molecular |
Element | Na | Mg | Al | Si | P | S |
Electronegativity | 0.9 | 1.2 | 1.5 | 1.8 | 2.1 | 2.5 |
- The oxides of Na and Mg which show purely ionic bonding produce alkaline solutions with water as their oxide ions (O2-) become hydroxide ions (OH-):
O2- (aq) + H2O (l) → 2OH- (aq)
- The oxides of P and S which show purely covalent bonding produce acidic solutions with water because when these oxides react with water, they form an acid which donates H+ ions to water
- Eg. SO3 reacts with water as follows:
SO3 (g) + H2O (l) → H2SO4 (aq)
- The H2SO4 is an acid which will donate a H+ to water:
H2SO4 (aq) + H2O (l) → H3O+ (aq) + HSO4- (aq)
- Al and Si are insoluble and when they react with hot, concentrated alkaline solution they act as a base and form a salt
- This behaviour is very typical of a covalently bonded oxide
- Al can also react with acidic solutions to form a salt and water
- This behaviour is very typical of an ionic bonded metal oxide
- This behaviour of Al proves that the chemical bonding in aluminium oxide is not purely ionic nor covalent: therefore it exhibits amphoteric character
Reaction of Period 3 oxides with water table
Oxide | Chemical equation | pH | Comments |
Na2O | Na2O (s) + H2O (l) → 2NaOH (aq) | 14 (strongly alkaline) |
- |
MgO | MgO (s) + H2O (l) → Mg(OH)2 (aq) | 10 (weakly alkaline) |
- |
Al2O3 | No reaction | - | Al2O3 is insoluble in water |
SiO2 | No reaction | - | SiO2 is insoluble in water |
P4O10 | P4O10 (s) + 6H2O (l) → 4H3PO4 (aq) | 2 (strongly acidic) |
Vigorous / violent reaction |
SO2 SO3 |
SO2 (g) + H2O (l) → H2SO3 (aq) SO3 (g) + H2O (l) → H2SO4 (aq) |
1 (strongly acidic) |
- |
Behaviour of the Period 3 Oxides with Water
- Metal oxides (to the left of the periodic table):
- Sodium oxide, Na2O, and magnesium oxide, MgO, are made up of ions
- They contain an oxide ion, O2-, which is a strong base and will readily produce hydroxide ions through reaction with water
- This is why the solutions formed are strongly alkaline
- Sodium oxide forms a more alkaline solution than magnesium oxide because it is far more soluble in water
- Oxides in the middle of the periodic table
- Although ionic, aluminium oxide does not react with water because the oxide ions are held too strongly in the ionic lattice
- This means the ions cannot be separated
- Silicon dioxide is a giant covalent molecule - it is the main component of sand
- It has millions of strong covalent bonds, so it does not react with water
- Non-metal oxides (to the right of the periodic table):
- Oxides of phosphorus and sulfur are simple covalent molecules
- They will react with water to produce acidic solutions
Examiner Tip
Key thing to remember: The metal oxides form alkaline solutions in water, the oxides in the middle do not react and the non-metal oxides form acidic solutions.