Periodicity Set 2 Flashcards
How does the oxidation number change across period 3
It generally increases although P, S and Cl can form several oxidation states
What is the trend in melting points of period 3 oxides?
It increases to from NaO to MgO as the ionic bond is stronger because the charge density of Mg2+ is higher than Na+ (higher charge, smaller ion) so the attraction between oppositely charged ions increases. After MgO the melting point decreases. Al2O3 has some covalent character. SiO2 is a giant macromolecule with strong covalent bonds to be broken in melting. P4O10 & SO2 are simple covalent molecules with weaker intermolecular forces to be broken (the strength of which depends on their surface area)
Trend in acid-base character of period 3 oxides?
Na2O dissolves in water to produce NaOH. OH ions cause pH 11. MgO only sparingly soluble, fewer OH ions when dissolved so pH 1O. Al2O3 insolube so no OH ions pH7. SiO2 insoluble; pH7. P4O10 forms H3PO4 in water PH1. SO2 and SO3 form acidic solutions pH 1-2
Equation for reaction of P4O10 with water
P4O10 + 6 H2O –> 4 H3PO4
Equation for reaction of SO2 with water
SO2 + H2O –> H2SO3 (pH 2-3)
Equation for reaction of SO3 with water
SO3 + H2O –> H2SO4 (pH 0-1)
General trend in acid-base character of oxides
Metals usually form basic oxides and non-metals form acidic oxides
Reaction of NaO / MgO with acids and bases
No reaction with bases. Forms salt and water with acids
Reaction of Al2O3 with acids and bases
Amphoteric. Reacts with acid to form salt and water. Reacts with base eg NaOH –> NaAl(OH)4
Reaction of SiO2 with acids and bases
Acts as a weak acid SiO2 + NaOH –> Na2SiO3 + H2O
Reaction of P4O10 with acids and bases
Acts as an acid P4O10 + 12 NaOH –> 4 Na3PO4 + 6 H2O
Reaction of SO2 with acids and bases
Acts as an acid SO2 + NaOH –> NaHSO3. Then NaHSO3 + NaOH –> Na2SO3 + H2O
