Explaining trends in the oxides of Period 3

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) ⇾ MgCl_2(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.

Al₂O_3(s) + 6H₃O⁺_(aq) + 3H₂O_(l) ⇾ 2 [Al(H₂O)₆]³⁺_(aq)

Al₂O_3(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

SiO_2(s) + 2NaOH_(aq) ⇾ Na₂SiO_3(aq) + H₂O_(l)

e.g. reaction with sodium carbonate at 1500°C

SiO_2(s) + 2Na₂CO_3(s) ⇾ Na₄SiO_4(aq) + 2CO_2(g)

It will however react with hydrofluoric acid to form silicon tetrafluoride, SiF₄, or hexafluorosilicic acid, H₂SiF₆ if the acid is in excess.

SiO_2(s) + 4HF_(aq) ⇾ SiF_4(g) + 2H₂O_(l)

SiO_2(s) + 6HF_(aq) ⇾ H₂SiF_6(aq) + 2H₂O_(l)

• phosphorus oxide, P₄O₆ and P₄O₁₀

Both oxides of phosphorus are acidic, dissolving in water to give an acid.

P₄O_10(s) + 6H₂O_(l) ⇾ 4H₃PO_4(aq) phosphoric acid

P₄O_6(s) + 6H₂O_(l) ⇾ 4H₃PO_3(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.

1. H₃PO_4(aq) + NaOH_(aq) ⇾ NaH₂PO_4(aq) + H₂O_(l) H₂PO₄^– is the dihydrogenphosphate ion
2. NaH₂PO_4(aq + NaOH_(aq) ⇾ Na₂HPO_4(aq) + H₂O_(l) HPO₄^2- is the hydrogenphosphate ion
3. Na₂HPO_4(aq + NaOH_(aq) ⇾ Na₃PO_4(aq) + H₂O_(l) PO₄^3- 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, HPO₃^2-.

H₃PO_3(aq) + NaOH_(aq) ⇾ NaHPO₂OH_(aq) + H₂O_(l)

NaHPO₂OH_(aq) + NaOH(aq) ⇾ Na₂HPO_3(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:

SO_2(aq) + 2H₂O_(l) ⇌ HSO₃^–_(aq) + H₃O⁺_(aq)

HSO₃^–_(aq) + H₂O_(l) ⇌ SO₃^2-_(aq) + H₃O⁺_(aq)

We can think of this mixture of aqueous HSO₃^– and SO₃^2- as sulphurous acid, H₂SO₃. Essentially this is what is happening when we react aqueous sulphur dioxide with an alkali such as sodium hydroxide.

SO_2(aq) + NaOH_(aq) ⇾ NaHSO_3(aq)

NaHSO_3(aq) + NaOH_(aq) ⇾ Na₂SO_3(aq) + H₂O_(l)

Sulphur trioxide reacts with water to form sulphuric acid which is a stronger acid than sulphurous with a lower pH.

SO_3(g) + H₂O_(l) ⇾ H₂SO_4(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 ☹️.

1. 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

2. 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

1. (a) Al₂O_3(s) + 2KOH_(aq) + 3H₂O_(l) ⇾ 2 K[Al(OH)₄]_(aq)

(b) Ga₂O_3(s) + 6HCl_(aq) ⇾ 2GaCl_3(aq) + 3H₂O_(l)

or …. Ga₂O_3(s) + 6H₃O⁺_(aq) + 6Cl^–_(aq) + 3H₂O_(l) ⇾ 2 [Ga(H₂O)₆]Cl_3(aq)

(c) H₂SO_3(aq) + Na₂CO_3(aq) ⇾ Na₂SO_3(aq) + H₂O_(l) + CO_2(g)

(d) P₄O_6(s) + 2NaOH_(aq) ⇾ Na₂HPO_3(aq) + 2H₂O_(l)

(e) P₄O_10(s) + 3KOH_(aq) ⇾ K₃PO_4(aq) + 3H₂O_(l)

2. (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 AsO₃^3- according to the details given in the question)