The Oxides of Elements in Period 3 Flashcards | A-Level Chemistry AQA

Compare the melting points of sodium oxide and magnesium oxide, and explain the difference.

Magnesium oxide (MgO) has a higher melting point than sodium oxide (Na₂O).

The Mg²⁺ ions are smaller and have a higher positive charge compared to Na⁺ ions. This results in stronger electrostatic forces attracting the Mg²⁺ ions to the oxide ions (O^2-). Consequently, more energy is needed to break these stronger ionic bonds in MgO, leading to a higher melting point compared to Na₂O.

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Compare the melting points of phosphorus(V) oxide and sulfur trioxide, and explain the difference.

Phosphorus(V) oxide (P₄O₁₀) has a higher melting point than sulfur trioxide (SO₃).

P₄O₁₀ molecules are larger and contain more electrons than SO₃ molecules. This results in stronger induced dipole-dipole forces between the P₄O₁₀ molecules. Consequently, more energy is required to overcome these stronger intermolecular forces and separate the molecules in P₄O₁₀, leading to a higher melting point compared to SO₃.

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Why does the Al-O bond in aluminium oxide have some covalent character?

The small size and large positive charge of the aluminium (Al³⁺) ion distorts the electron cloud of the oxide (O^2-) ion.

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Name the period 3 oxides that are insoluble in water.

Aluminium oxide and silicon dioxide

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Describe, and give the equation for, the reaction of phosphorus(V) oxide with water.

Phosphorus(V) oxide (P₄O₁₀) dissolves in water to form a strongly acidic solution of phosphoric(V) acid.

P₄O_10(s) + 6H₂O_(l) ➔ 4H₃PO_4(aq)

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What is the general trend in the pH of the solutions formed by the oxides of period 3 elements as you move across the period?

As you move across period 3, the pH of the solutions formed by the oxides decreases.

The solutions go from alkaline (Na₂O and MgO) to acidic (P₄O₁₀. SO₂ and SO₃).

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Give the equation for the reaction of sulfur trioxide and water.

SO_3(g) + H₂O_(l) ➔ H₂SO_4(aq)

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Describe the bonding and structure of the oxides of period 3 from sodium to sulfur.

Period 3 oxide	Na₂O	MgO	Al₂O₃	SiO₂	P₄O₁₀	SO₂	SO₃
Bonding	Ionic	Ionic	Ionic (with covalent character)	Covalent	Covalent	Covalent	Covalent
Structure	Giant ionic	Giant ionic	Giant ionic	Giant covalent	Simple molecular	Simple molecular	Simple molecular

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What type of solution is formed when sodium oxide reacts with water? Explain your answer with an equation.

A strongly alkaline solution of sodium hydroxide is formed:

Na₂O_(s) + H₂O_(l) ➔ 2NaOH_(aq)

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Explain the difference in pH of the solutions formed when magensium oxide and sodium oxide react with water.

Both reactions produce alkaline solutions as the metal hydroxide is formed.

NaOH is soluble and produces a solution with a pH of ≈14.

Mg(OH)₂ is only sparingly soluble so produces a solution with a pH of ≈10.

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What type of solution is formed when phosphorus pentoxide reacts with water? Explain your answer with an equation.

An acidic solution of phosphoric(V) acid is formed:

P₄O_10(s) 6H₂O_(l) ➔ 4H₃PO_4(aq)

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What type of solution is formed when sulfur dioxide reacts with water? Explain your answer with an equation.

An acidic solution of sulfurous acid is formed:

SO_2(g) + H₂O_(l) ➔ H₂SO_3(aq)

Sulfurous acid is also called sulfuric(IV) acid.

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What type of solution is formed when sulfur trioxide reacts with water? Explain your answer with an equation.

An acidic solution of sulfuric acid is formed:

SO_3(g) + H₂O_(l) ➔ H₂SO_4(aq)

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What type of oxides (metal or non-metal) form alkaline solutions in water?

Metal oxides

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What type of oxides (metal or non-metal) form acidic solutions in water?

Non-metal oxides

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Give the equation for the reaction of sulfur dioxide and water.

SO_2(g) + H₂O_(l) ➔ H₂SO_3(aq)

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