Period elements and oxides (A-level)

Question 1

Describe and explain the acid–base character of the Period 3 oxides. (3)

Answer 1

-Basic: Na₂O, MgO (react with acids)

-Amphoteric: Al₂O₃ (reacts with both acids and bases)

-Acidic: SiO₂, P₄O₁₀, SO₂, SO₃ (react with bases)

Question 2

Write the equation for sodium oxide reacting with water. State the pH. (2)

Answer 2

• Na₂O + H₂O → 2NaOH
• pH ≈ 13–14 (strongly alkaline)

Question 3

Write the equation for magnesium oxide reacting with water. What is the pH of the solution? (2)

Answer 3

• MgO + H₂O → Mg(OH)₂
• pH ≈ 9–10 (sparingly soluble, weakly alkaline)

Question 4

What is the equation and pH when phosphorus(V) oxide reacts with water? (2)

Answer 4

• P₄O₁₀ + 6H₂O → 4H₃PO₄
• pH ≈ 0–2 (strongly acidic)

Question 5

Write an equation for SO₂ reacting with water and state the pH.

Answer 5

• SO₂ + H₂O → H₂SO₃
• pH ≈ 3 (weakly acidic)

Question 6

Write the equation for SO₃ reacting with water and give the pH of the solution. (2)

Answer 6

• SO₃ + H₂O → H₂SO₄
• pH ≈ 0–1 (strong acid)

Question 7

Why does Al₂O₃ show amphoteric behaviour? (2)

Answer 7

• Al₂O₃ reacts with both acids and bases
• It can donate and accept protons (acts as acid or base)

Question 8

Write the equation for Al₂O₃ reacting with HCl.

Answer 8

• Al₂O₃ + 6HCl → 2AlCl₃ + 3H₂O

Question 9

Write the equation for Al₂O₃ reacting with hot sodium hydroxide.

Answer 9

• Al₂O₃ + 2NaOH + 3H₂O → 2Na[Al(OH)₄]

Question 10

Explain why Al₂O₃ has both ionic and covalent character. (3)

Answer 10

• Al³⁺ has a high charge density
• It polarises O²⁻ ions
• This distorts the electron cloud → partial covalent character

Question 11

Explain why SiO₂ does not react with water but is still acidic. (3)

Answer 11

• SiO₂ is a giant covalent structure – insoluble in water
• It reacts with bases like NaOH → forms silicate salts
• Acts as an acid in those reactions