The trend in the bonding and structure of the Period 3 oxides can be explained in terms of the increasing electronegativity of the elements as we move across the period, resulting in a decrease in the difference in electronegativity between each element and oxygen.
Consequently, the giant ionic lattices seen in sodium, magnesium and aluminium oxides give way to the giant covalent network of silicon dioxide and the simple molecular structures of the various oxides of phosphorus, sulphur and chlorine. When the electronegativities of the elements in a bond are roughly equal, electrons are more likely to be shared = covalent rather than ionic bond.
Chemistry of the s-block oxides
Sodium oxide and magnesium oxide are bases, reacting with water to give metal hydroxides.
Na2O(s) + H2O(l) ⇾ 2NaOH(aq)
MgO(s) + H2O(l) ⇾ Mg(OH)2(aq)
Sodium hydroxide is a significantly stronger alkali than magnesium hydroxide simply because magnesium hydroxide is only sparingly soluble.
Magnesium hydroxide will decompose on heating to give the oxide.
Mg(OH)2(s) ⇾ MgO(s) + H2O(g)
As they are bases, both oxides will react with an acid to give the metal salt and water.
MgO(s) + 2HCl(aq) ⇾ MgCl2(aq) + H2O(l)
Chemistry of the p-block oxides
The p-block elements are all capable of forming compounds with variable oxidation states and this is seen in the oxides of phosphorus, sulphur and chlorine (the maximum possible oxidation state involves all the 3s and 3p valence electrons in bonding).
- aluminium oxide, Al2O3
Aluminium oxide is insoluble in water but it is amphoteric, reacting with both acids and alkalis.
Al2O3(s) + 6H3O+(aq) + 3H2O(l) ⇾ 2 [Al(H2O)6]3+(aq)
Al2O3(s) + 2OH–(aq) + 3H2O(l) ⇾ 2 [Al(OH)4]–(aq)
- silicon dioxide, SiO2
Silicon dioxide is also insoluble in water but it behaves as an acid (like all other non-metal oxides in Period 3), reacting with bases to form silicate anions, of which there are many!
e.g. reaction with hot concentrated sodium hydroxide
SiO2(s) + 2NaOH(aq) ⇾ Na2SiO3(aq) + H2O(l)
e.g. reaction with sodium carbonate at 1500°C
SiO2(s) + 2Na2CO3(s) ⇾ Na4SiO4(aq) + 2CO2(g)
It will however react with hydrofluoric acid to form silicon tetrafluoride, SiF4, or hexafluorosilicic acid, H2SiF6 if the acid is in excess.
SiO2(s) + 4HF(aq) ⇾ SiF4(g) + 2H2O(l)
SiO2(s) + 6HF(aq) ⇾ H2SiF6(aq) + 2H2O(l)
- phosphorus oxide, P4O6 and P4O10
Both oxides of phosphorus are acidic, dissolving in water to give an acid.
P4O10(s) + 6H2O(l) ⇾ 4H3PO4(aq) phosphoric acid
P4O6(s) + 6H2O(l) ⇾ 4H3PO3(aq) phosphonic acid
Phosphoric acid is tribasic, capable of losing all three protons / H+ to form a range of oxoanions (be aware that phosphorus forms many oxoacids and oxoanions, not just the ones mentioned here!). We can see this in the reaction of phosphoric acid with aqueous sodium hydroxide.
- H3PO4(aq) + NaOH(aq) ⇾ NaH2PO4(aq) + H2O(l) H2PO4– is the dihydrogenphosphate ion
- NaH2PO4(aq + NaOH(aq) ⇾ Na2HPO4(aq) + H2O(l) HPO42- is the hydrogenphosphate ion
- Na2HPO4(aq + NaOH(aq) ⇾ Na3PO4(aq) + H2O(l) PO43- is the phosphate ion
Phosphonic acid, H3PO3, is dibasic which is apparent when we consider the structure of the acid.
The reaction with an alkali produces firstly the hydrogen phosphite ion, HPO2OH–, and secondly the phosphite ion, HPO32-.
H3PO3(aq) + NaOH(aq) ⇾ NaHPO2OH(aq) + H2O(l)
NaHPO2OH(aq) + NaOH(aq) ⇾ Na2HPO3(aq) + H2O(l)
- sulphur dioxide, SO2, and sulphur trioxide, SO3
The two oxides of sulphur react with water to give acidic solutions. Firstly, sulphur dioxide:
SO2(aq) + 2H2O(l) ⇌ HSO3–(aq) + H3O+(aq)
HSO3–(aq) + H2O(l) ⇌ SO32-(aq) + H3O+(aq)
We can think of this mixture of aqueous HSO3– and SO32- as sulphurous acid, H2SO3. Essentially this is what is happening when we react aqueous sulphur dioxide with an alkali such as sodium hydroxide.
SO2(aq) + NaOH(aq) ⇾ NaHSO3(aq)
NaHSO3(aq) + NaOH(aq) ⇾ Na2SO3(aq) + H2O(l)
Sulphur trioxide reacts with water to form sulphuric acid which is a stronger acid than sulphurous with a lower pH.
SO3(g) + H2O(l) ⇾ H2SO4(aq)
- oxides of chlorine
All the main oxides of chlorine (Cl2O, Cl2O4, Cl2O6, Cl2O7) are acidic, but it is as oxoacids that they are commonly used e.g. hypochlorous acid, HOCl. You can find everything you need to know about them here 😊.
Practice questions
Just be aware that exam questions tend to be a simple memory test, you really do need to learn everything above, including equations for all the reactions ☹️.
- Write balanced equations for the following reactions.
(a) aluminium oxide and aqueous potassium hydroxide
(b) gallium(III) oxide and hydrochloric acid
(c) an aqueous solution of sulphur dioxide and sodium carbonate
(d) phosphorus(III) oxide and aqueous sodium hydroxide
(e) phosphorus(V) oxide and aqueous potassium hydroxide
- Arsenic(III) oxide is reacts with hydrochloric acid to form arsenic(III) chloride and with sodium hydroxide to form sodium arsenite. The arsenite anion is an oxyanion of arsenic(III) with a 3- charge.
(a) What term describes a substance such as arsenic(III) oxide that reacts in these ways?
(b) Write balanced equations to show each reaction.
Answers
- (a) Al2O3(s) + 2KOH(aq) + 3H2O(l) ⇾ 2 K[Al(OH)4](aq)
(b) Ga2O3(s) + 6HCl(aq) ⇾ 2GaCl3(aq) + 3H2O(l)
or …. Ga2O3(s) + 6H3O+(aq) + 6Cl–(aq) + 3H2O(l) ⇾ 2 [Ga(H2O)6]Cl3(aq)
(c) H2SO3(aq) + Na2CO3(aq) ⇾ Na2SO3(aq) + H2O(l) + CO2(g)
(d) P4O6(s) + 2NaOH(aq) ⇾ Na2HPO3(aq) + 2H2O(l)
(e) P4O10(s) + 3KOH(aq) ⇾ K3PO4(aq) + 3H2O(l)
- (a) Arsenic(III) oxide is amphoteric if it reacts with both acids and bases.
(b) As2O3 + 6HCl ⇾ 2AsCl3 + 3H2O
As2O3 + 6NaOH ⇾ 2 Na3AsO3 + 3H2O
(the arsenite oxyanion must be AsO33- according to the details given in the question)
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