Periodicity Of Period 3 Page 2
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2The reactions of the oxides of the elements Na
S with water
know the change in pH of the resulting
solutions across the Period.
Metal ionic oxides tend to react with water to form hydroxides
which are alkaline
Al
O
and SiO
do not dissolve in
+
-
Na
O (s) + H
O (l)
2Na
(aq) + 2OH
(aq) pH 13 (This is a
2
3
2
2
2
water because of the high strength of
vigorous exothermic reaction)
the Al
O
ionic lattice and the SiO
The ionic oxides are basic as the oxide ions accept protons to
2
3
2
macromolecular structure, so they give
become hydroxide ions in this reaction (acting as a bronsted
a neutral pH 7
lowry base)
MgO (s) + H
O (l)
Mg(OH)
(s) pH 9
2
2
Mg(OH)
is only slightly soluble in water as its lattice is stronger
2
-
so fewer free OH
ions are produced and so lower pH
MgO is better than NaOH for treating acid in rivers and the stomach as it is only sparingly soluble and weakly
alkaline so using an excess would not make the water excessively alkaline.
Learn the equations !
The non-metal, simple molecular, covalent, oxides react with water to give acids
P
O
(s) + 6 H
O (l)
4 H
PO
(aq) pH 0 (this is a vigorous exothermic reaction)
Equations showing
4
10
2
3
4
+
-
SO
+ H
O
H
+ HSO
SO
(g) + H
O (l)
H
SO
(aq)
pH 3 (weak acid)
2
2
3
formation of ions in
2
2
2
3
+
-
SO
+ H
O
H
+ HSO
SO
(g) + H
O (l)
H
SO
(aq)
pH 0
3
2
4
solution
3
2
2
4
The trend is the ionic metal oxides show basic behaviour and the non-metal
covalent oxides show acidic behaviour.
The slightly intermediate nature of the bonding in Aluminium oxide is reflected
in its amphoteric behaviour: it can act as both a base and an acid
Acid base reactions between period 3 oxides and simple acids and bases.
The basic oxides react with acids to make salts
Rather than learning the equations by rote,
Na
O (s) + 2 HCl (aq)
2NaCl (aq) + H
O (l)
learn the pattern. Most follow the pattern acid
2
2
Na
O (s) + H
SO
(aq)
Na
SO
(aq) + H
O (l)
+ base = salt + water
2
2
4
2
4
2
3-
MgO (s) + 2 HCl (aq)
MgCl
(aq) + H
O (l)
PO
,
Know the charges on the ions e.g.
2
2
4
2-
Or ionically
SO
4
+
+
Na
O (s) + 2H
(aq)
2Na
(aq) + H
O (l)
2
2
+
2+
MgO (s) + 2 H
(aq)
Mg
(aq) + H
O (l)
2
Amphoteric Oxides
Aluminium oxide can act as both an acid and an alkali and is therefore called amphoteric
Aluminum oxide acting as a base
Be careful for
Al
O
(s)+ 3H
SO
(aq)
Al
(SO
)
(aq) + 3H
O (l)
2
3
2
4
2
4
3
2
whether the
Al
O
+ 6HCl
2AlCl
+ 3H
O
2
3
3
2
question is asking
+
3+
Or ionically Al
O
+ 6H
2Al
+ 3H
O
for an ionic
2
3
2
equation or a full
Aluminum oxide acting as a acid
one
Al
O
(s)+ 2NaOH (aq) + 3H
O (l)
2NaAl(OH)
(aq) (this equation needs learning carefully)
2
3
2
4
-
-
Al
O
(s)+ 2OH
(aq) + 3H
O (l)
2Al(OH)
(aq)
2
3
2
4
The other simple molecular acidic oxides react with
SiO
has a giant covalent structure with very
2
bases to form salts.
strong bonds. This stops SiO
dissolving or reacting
2
P
O
(s) + 12 NaOH (aq)
4Na
PO
(aq) + 6 H
O (l)
4
10
3
4
2
with water and weak solutions of alkali. It will,
P
O
+ 6 Na
O
4Na
PO
4
10
2
3
4
however, react with very concentrated NaOH
SO
(g) + 2NaOH (aq)
Na
SO
(aq) + H
O (l)
2
2
3
2
2NaOH (l) + SiO
(s)
Na
SiO
(aq) + H
O
2
2
3
2
SO
(g) + 2NaOH(aq)
Na
SO
(aq) + H
O (l)
3
2
4
2
It is still classed as an acidic oxide
Or show ionically
-
3-
P
O
(s) + 12 OH
(aq)
4PO
(aq) + 6 H
O (l)
4
10
4
2
-
2-
SO
(g) + 2OH
(aq)
SO
(aq) + H
O (l)
2
3
2
-
2-
SO
(g) + 2OH
(aq)
SO
(aq) + H
O (l)
3
4
2
2
N Goalby
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