Write the equation for magnesium with cold water.

Mg + 2H₂O → Mg(OH)₂ + H₂

What is formed when magnesium reacts with steam?

Magnesium oxide and hydrogen: Mg + H₂O → MgO + H₂.

Why is magnesium hydroxide solution only weakly alkaline?

Mg(OH)₂ is only sparingly soluble, giving a low OH⁻ concentration (about pH 10).

Why is sodium more reactive than magnesium?

It loses one electron, not two, and has a lower ionisation energy.

What is seen when sodium burns in oxygen?

A yellow flame and a white solid, Na₂O.

Why does aluminium foil appear unreactive in air?

A thin, unreactive layer of Al₂O₃ stops oxygen reaching the metal.

What is formed when sulfur burns in oxygen?

SO₂, a colourless, choking gas, with a blue flame.

How is SO₃ formed from SO₂?

2SO₂ + O₂ ⇌ 2SO₃ with a V₂O₅ catalyst at about 450 °C.

Reactions of Period 3 elements with water and oxygenAQA A-Level Chemistry: Flashcards

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Write the equation for sodium with water.

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Write the equation for sodium with water.: 2Na + 2H₂O → 2NaOH + H₂
Write the equation for magnesium with cold water.: Mg + 2H₂O → Mg(OH)₂ + H₂
What is formed when magnesium reacts with steam?: Magnesium oxide and hydrogen: Mg + H₂O → MgO + H₂.
Why is magnesium hydroxide solution only weakly alkaline?: Mg(OH)₂ is only sparingly soluble, giving a low OH⁻ concentration (about pH 10).
Why is sodium more reactive than magnesium?: It loses one electron, not two, and has a lower ionisation energy.
What is seen when sodium burns in oxygen?: A yellow flame and a white solid, Na₂O.
Write the equation for magnesium burning in oxygen.: 2Mg + O₂ → 2MgO
Why does aluminium foil appear unreactive in air?: A thin, unreactive layer of Al₂O₃ stops oxygen reaching the metal.
Write the equation for aluminium with oxygen.: 4Al + 3O₂ → 2Al₂O₃
Write the equation for silicon with oxygen.: Si + O₂ → SiO₂
Write the equation for phosphorus burning in excess oxygen.: P₄ + 5O₂ → P₄O₁₀
What is formed when sulfur burns in oxygen?: SO₂, a colourless, choking gas, with a blue flame.
How is SO₃ formed from SO₂?: 2SO₂ + O₂ ⇌ 2SO₃ with a V₂O₅ catalyst at about 450 °C.
What is the oxidation state of phosphorus in P₄O₁₀?: +5

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

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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).