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Periodicty
22.1 Reactions of period 3 elements: Phosphors and Sulphur oxides are covalent molecules
have small van der waal forces so have a low melting
All reactions of period 3 are redox, they start as elements point.
with an oxidation state of 0 and are either reduced or
oxidised. Reactions with water, oxides will react to be basic,
acidic or some are insoluble.
Reactions with water- sodium, magnesium and chlorine Basic oxides are sodium and magnesium.
are the only elements that react with cold water. Na2O(s) + H2O ——-> 2NaOH(aq) pH = 14
Sodium reacts vigorously, fizzing and melting because of Sodium has a lattice made of O2- ions which is a
the heat energy released. Sodium is oxidised to form a strong base and reacts readily to make OH- to make a
strong basic solution. strong basic solution.
2Na(s) + 2H2O ———-> 2NaOH(aq) + H2(g)
MgO(s) + H2O(l) ———> Mg(OH)s(s) pH = 9
Magnesium reacts slowly at room temperature with only a Magnesium has same O2- ion as sodium but is less
few bubbles. Magnesium is oxidised to produce a less basic basic because Mg(OH)2 is less soluble
solution.
Mg(s) + 2H2O(l) ——-> Mg(OH)2(aq) + H2(g) Insoluble oxides are aluminium and silicon
Reacts much faster with steam Aluminium is insoluble because the ionic bonding is
Mg(s) + H2O(g) ——-> MgO(s) + H2(g) too strong to be separated (due to covalent character).
Water won’t be attracted to the macromolecular
Reactions with oxygen all period 3 elements react apart structure of SiO2.
from Argon in an exotheric reaction.
Sodium burns brightly with a yellow flame. Acidic oxides are phosphorus and sulphur.
2Na(s) + 0.5 O2(g) ———> Na2O(s) P4O10(s) + 6H2O(l) ———> 4H3PO4(aq)
Magnesium burns with a bright white flame. SO2(g) + H2O(l) ———> H2SO3(aq)
2Mg(s) + O2(g) ——-> 2MgO(s) So3(g) +H2O(l) ——-> H2SO4(aq)
Aluminium always has a coating of oxide, even when
scratched it reacts rapidly.
Silicon will form an oxidised when heated. 22.3 the acidic/basic nature of period 3 oxides:
Si(s) + O2(g) ——> SiO2(s)
Phosphorus has allotropes which react differently with Magnesium and sodium oxides react with acid to give
oxygen (red and white) salt and water only.
White ignites and gives off white smoke of Phosphorus Na2O + H2SO4(aq) ———-> Na2SO4(aq) + H2O(l)
pentoxide. Red must be heated before it reacts. MgO(s) + 2HCl(aq) ———> MgCl2(aq) + H2O(l)
4P(s) + 5O2(g) ———> P4O10(s)
In limited oxygen P2O3 is formed also. Aluminium oxide is amphoteric, reacts with both acid
Sulfur is heated and lowered into a jar of oxygen, burns and bases.
with a blue flame. Al2O3(s)+2NaOH(aq)+3H2O(l)->2NaAl(OH)4(aq)
S(s) + O2(g) ———> SO2(g) AL2O3(s) + 6HCl(aq) ——> 2AlCl3(aq) + 3H2O(l)
Allotropes: the same element with the atoms arranged Silicon dioxide acts as a base with a strong bases
differently. SiO2(s) + 2NaOH(aq) —-> Na2SiO3(aq) + H2O(l)
Phosphorous pentoxide reacts in stages as phosphoric
22.2 The oxides of elements in period 3:
acid.
H3PO4(aq) + NaOH(aq) —> NaH2PO3(aq) +H2O(l)
Metal oxides form giant lattice structures, with a high
NaH2PO4(aq)+NaOH(aq)—>Na2HPO4(aq)+H2O
electrostatic attraction across the compound it gives a high
Na2HPO4(aq) + NaOH(aq) —> Na3PO4(aq) + H2O
melting point.
Al2O3 has covalent character as the highly charged small
Sulphur dioxide reacts in two stages.
Al3+ ion polarises the O2- to give some covalent bonding
SO2(aq) + NaOH(aq) —> NaHSO3(aq)
in the structure.
NaHSO3(aq) + NaOh(aq) ——> Na2SO3(aq) + H2O
Non metal oxides
SiO2 has a macromolecular structure, the large Mr means
there’s more van der waal forces with a higher electrostatic
attraction and so has a higher melting point.
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