Period 3 - Reactions with water
- Sodium is more reactive than magnesium as it takes less energy to lose one electron instead of two.
- Sodium reacts vigorously with cold water, forming a silvery molten ball on the surface that fizzes and produces H2 gas.
2Na(s) + 2H2O(l) --> 2NaOH(aq) + H2(g) (strongly alkaline solution formed, ph 12-14)
- Magnesium reacts very slowly with cold water. No reaction is visible, but it forms a weakly alkaline solution which shows that a reaction has occured:
Mg(s) + H2O(l) --> Mg(OH)2(aq) + H2(g) ph 9-10
- Magnesium reacts much faster when heated and reacted with steam:
Mg(s) + H2O(g) --> MgO(s) + H2(g)
Period 3 - Reactions with oxygen
General equation is element + oxygen --> oxide
- 2Na(s) + 1/2 O2(g) --> Na2O(s)
- Mg(s) + 1/2 O2(g) --> MgO(s)
- 2Al(s) + 1 1/2 O2(g) --> Al2O3(s)
- Si(s) + O2(g) --> SiO2(s)
- P4(s) + 2O2(g) --> P4O10(s)
- S(s) + O2(g) --> SO2(g)
Period 3 - Element, oxide & flame test summary
Element Oxide Reaction in air Flame
Na Na2O Vigorous Yellow
Mg MgO Vigorous Brilliant white
Al Al2O3 Slow N/A
Si SiO2 Slow N/A
P P4O10 Spontaneously combusts Brilliant white
S SO2 Burns steadily Blue
Period 3 Oxides - Boiling/Melting points
(Note: SO3 should read SO2 and be just below 0)
The difference in boiling/melting points is all to do with the differences in structure and bonding.
- Na2O, MgO and Al2O3 all have high melting points as they form giant ionic lattices.
- SiO2 has a higher melting point than the other non-metal oxides as it forms a macromolecular structure.
- P4O10 and SO2 form simple covalent molecules.
Period 3 Oxides- Melting trends in detail
- The giant ionic lattices (of Na2O, MgO and Al2O3) have strong electrostatic attraction between the ions which results in a high amount of energy being needed to break the bonds and melt them.
- The giant macromolecular structure (of SiO2) has strong covalent bonds holding the structure together, so lots of energy is needed to overcome these bonds and melt them.
- Simple molecular structures (of P4O10 and SO2) are held together by weak van der Waals and dipole-dipole forces which requires little energy to overcome.
- MgO has a higher melting point than Na2O as it forms a 2+ ion which attracts oxygen's 2- ion more strongly than sodium's 1+ ion.
- Al2O3 has a lower melting point than expected as the 3+ ions distort the oxygen's electron cloud - making it partially covalent (not a perfect ionic model).
- P4O10 has a higher melting point than SO2 as it is a larger molecule and therefore it's van der Waals forces are stronger - increasing it's melting point in comparision to SO2.
Period 3 Oxides - Physical trends summary
Oxide Melting point (K) Bonding Structure
Na2O 1548 Ionic Giant Ionic
MgO 3125 Ionic Giant Ionic
Al2O3 2345 Ionic/Covalent Giant Ionic
SiO2 1883 Covalent Macromolecular
P4O10 573 Covalent Molecular
SO3 290 Covalent Molecular
SO2 200 Covalent Molecular
Period 3 Oxides - Reactions with water
Na2O(s) + H2O(l) --> 2Na+(aq) + 2OH-(aq) pH ~14
MgO(s) + H2O (l) --> Mg(OH)2(s) <----> Mg2+(aq) + 2OH-(aq) pH ~9 Magnesium oxide is sparingly soluble in water.
Al2O3 and SiO2 are insoluble in water.
P4O10(s) + H2O(l) --> 4H3PO4(aq) Violent reaction. This ionises so the solution is acidic. H3PO4(aq) <----> H+(aq) + H2PO4-(aq) pH ~ 1
SO2(g) + H2O(l) --> H2SO3(aq) This partially dissociates which causes the acidity of the solution. H2SO3(aq) <----> H+(aq) + HSO3-(aq) pH~2
SO3(g) + H2O(l) --> H2SO4(aq) --> H+(aq) + HSO4-(aq) Violent reaction. pH ~1
Period 3 Oxides - Reactions with water summary
Oxide Bonding Ions present Approx. pH (after H2O reaction)
Na2O Ionic Na+(aq), OH-(aq) 13-14
MgO Ionic Mg2+(aq), OH-(aq) 10
Al2O3 Ionic/Covalent insoluble, no reaction 7
SiO2 Covalent insoluble, no reaction 7
P4O10 Covalent H+(aq) + H2PO4-(aq) 0-1
SO2 Covalent H+(aq) + HSO3-(aq) 2-3
SO3 Covalent H+(aq) + HSO4-(aq) 0-1
Period 3 Oxides - Reasons for water reactions
- Na2O contains the O 2- ion, which is a very strong base and so readily reacts with water to produce hydroxide ions - a strongly alkaline solution.
- MgO also contain the oxide ion but it produces a less alkaline solution than Na2O because it is less soluble than Na2O.
- Al2O3 is ionic but the bonding is too strong to be separated, partly due to the additional covalent bonding it has.
- SiO2 is a giant macromolecule and water will not affect it's structure.
- P4O10, SO2 and SO3 are covalent molecules and react with water to form acid solutions.
The general trend is alkalis --> acids as we go across the period.
Period 3 Oxides - Acids & Bases (1)
Na2O & MgO:
- React with acids to give a salt and water only.
- Na2O(s) + H2SO4(aq) --> Na2SO4(aq) + H2O(l)
- MgO(s) + 2HCl(aq) --> MgCl2(aq) + H2O(l)
- Amphoteric (reacts with both acids and alkalis).
- Al2O3(aq) + 6HCl(aq) --> Al2Cl6(aq) + 3H2O(l)
- Al2O3(aq) + NaOH(aq) --> 2NaAl(OH)4(aq) (NaOH must be hot and concentrated. Sodium aluminate is formed).
- Will act as a weak acid with strong bases.
- SiO2(s) + 2NaOH(aq) --> Na2SiO3(aq) + H2O(l) (NaOH must be hot and concentrated.Sodium silicate - a colourless solution - is formed).
Period 3 Oxides - Acid & bases (2)
- Reaction of phosphoric acid (H3PO4) with an alkali, as P4O10 forms H3PO4 when reacted with water.
- Reacts in three stages as each H atom reacts with an OH group and is replaced by a Na ion.
- H3PO4(aq) + NaOH(aq) --> NaH2PO4 (aq) + H2O(l)
- NaH2PO4 (aq) + NaOH(aq) --> Na2HPO4(aq) + H2O(l)
- Na2HPO4 (aq) + NaOH(aq) --> Na3PO4(aq) + H2O(l)
- Overall: 3NaOH(aq) + H3PO4(aq) --> Na3PO4(aq) + 3H2O(l)
- Adding NaOH to an aqueous SO2 first forms hydrogensulfate(IV) then sodium sulfate (IV).
- SO2(aq) + NaOH(aq) --> NaHSO3(aq)
- NaHSO3(aq) + NaOH(aq) --> Na2SO3(aq) + H2O(l)