Reactions of Period 3 elements with water and oxygenAQA A-Level Chemistry: Revision notes
Section 1
Sodium and magnesium with water
Sodium reacts vigorously with cold water: it fizzes, melts into a ball (the reaction is exothermic), moves across the surface and a strongly alkaline solution forms (about pH 14).
2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g)
Magnesium reacts very slowly with cold water, with few bubbles, forming a weakly alkaline solution because Mg(OH)₂ is only sparingly soluble (about pH 10).
Mg(s) + 2H₂O(l) → Mg(OH)₂(s) + H₂(g)
Magnesium reacts vigorously with steam, burning with a bright white flame to leave white magnesium oxide:
Mg(s) + H₂O(g) → MgO(s) + H₂(g)
Sodium forms NaOH, not Na₂O, with water. Balance 2Na + 2H₂O → 2NaOH + H₂ carefully.
Section 2
Why sodium is more reactive than magnesium
The metals react by losing electrons. Sodium loses one electron to form Na⁺; magnesium must lose two to form Mg²⁺.
Sodium has a larger atomic radius and a smaller nuclear charge than magnesium, so its outer electron is further from the nucleus and less strongly held. Less energy is needed to ionise sodium, so it reacts more readily.
Reactivity of the metals decreases from Na to Mg to Al across the period.
Section 3
Reactions with oxygen: the metals
- Sodium: burns with a yellow flame to form a white solid. 4Na + O₂ → 2Na₂O
- Magnesium: burns with a bright white flame to form white MgO. 2Mg + O₂ → 2MgO
- Aluminium: foil is protected by a thin layer of Al₂O₃, so it seems unreactive at room temperature. Powder burns brightly when heated. 4Al + 3O₂ → 2Al₂O₃
The oxides of these metals are ionic, formed by transfer of electrons from metal to oxygen.
Aluminium is a reactive metal. It appears unreactive only because of its oxide layer.
Section 4
Reactions with oxygen: the non-metals
- Silicon: reacts when heated strongly. Si + O₂ → SiO₂
- Phosphorus: burns with a white flame, forming white fumes of P₄O₁₀. P₄ + 5O₂ → P₄O₁₀ (phosphorus +5)
- Sulfur: burns with a blue flame to give choking SO₂ gas. S + O₂ → SO₂ (sulfur +4)
SO₃ forms only when SO₂ is oxidised further: 2SO₂ + O₂ ⇌ 2SO₃. This reversible reaction needs a vanadium(V) oxide catalyst at about 450 °C.
The oxides of silicon, phosphorus and sulfur are covalent.
Section 5
Trends across the period
Going from Na to P the oxidation state of the element in its oxide rises: Na₂O (+1), MgO (+2), Al₂O₃ (+3), SiO₂ (+4), P₄O₁₀ (+5), as more outer-shell electrons are involved. Sulfur gives SO₂ (+4) directly and SO₃ (+6) only by further oxidation.
The oxides change from ionic (Na, Mg, Al) to covalent (Si, P, S) as the elements become non-metals.
Must Know
- 2Na + 2H₂O → 2NaOH + H₂ (vigorous); Mg + 2H₂O → Mg(OH)₂ + H₂ (slow); Mg + steam → MgO + H₂
- Na loses one electron, Mg two, so Na is more reactive
- Oxides: Na₂O, MgO, Al₂O₃, SiO₂, P₄O₁₀, SO₂, SO₃
- Al is protected by its oxide layer
- SO₃ needs a V₂O₅ catalyst and about 450 °C
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Reactions of Period 3 elements with water and oxygen
- A teacher demonstrates the reactions of small pieces of sodium and of magnesium ribbon with cold water, in separate troughs, adding universal indicator to each.Explain why sodium reacts more vigorously with water than magnesium does.2 marks
- A student heats small samples of magnesium ribbon, aluminium powder, silicon and sulfur in separate combustion spoons in oxygen, in a fume cupboard.Write equations for the reactions of aluminium and of silicon with oxygen.2 marks
- A student compares how readily the Period 3 elements react with oxygen. She notes that sodium and magnesium burn vigorously, that phosphorus burns to give white fumes, and that aluminium foil stays unreactive in air at room temperature even though aluminium is a reactive metal.Write an equation for the burning of phosphorus in excess oxygen, state the oxidation state of phosphorus in the product and state what is seen.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).