Apply common oxide–water equations and recognise standard oxyacid formulae.
Method and worked examples
Many metal oxides are basic. Sodium and potassium oxides form hydroxides with water: Na2O+H2O→2NaOH and K2O+H2O→2KOH. Calcium oxide gives Ca(OH)₂. Many non-metal oxides are acidic: CO2 forms H2CO3 in the usual simplified equilibrium, SO2 is represented as forming H2SO3,SO3 forms H2SO4, and P4O10 forms H3PO4.
Read atoms directly from structural formulae. CO2 is O=C=O. Common introductory representations include O=S=O for SO2,O=S(=O)=O for SO3, HO–S(=O)–OH for H2SO3, HO–S(=O)₂–OH for H2SO4 and O=P(OH)₃ for H3PO4. Sulfur and phosphorus can be shown with expanded octets in these representations. Structural formulae describe connectivity; balance each reaction without altering a reactant’s formula.
Example 1
Balance P4O10 reacting with water to form phosphoric acid.
P4O10+6H2O→4H3PO4. Both sides contain P4H12O16.
Example 2
Count atoms and ionisable O–H groups in HO–S(=O)₂–OH.
It contains H2SO4 and two O–H groups. This is sulfuric acid, a diprotic acid with the two donations occurring in successive steps.
Watch out:Many metal oxides form basic hydroxides; many non-metal oxides form acids.
Oxide equations and oxyacid structures | Year 11 Chemistry | Ascent Arcade