O-LevelChemistryElectrolysis5070/22 · May/June 2017 · Q46 Marks

Sodium oxide, Na₂O, is an ionic compound.

(a)State the electronic configuration for each of the ions in sodium oxide. sodium ion oxide ion[2]
(b)When molten sodium oxide is electrolysed, sodium and oxygen are formed. Construct equations for the two electrode reactions. reaction at the negative electrode reaction at the positive electrode[2]
(c)Explain how molten sodium oxide conducts electricity.[1]
(d)Sodium oxide reacts with water to give sodium hydroxide. Construct the equation for this reaction.[1]

Mark scheme

Marking points from the official mark scheme (May/June 2017). The mark or mark type is shown in brackets.

A4(a) Sodium ion: 2.8 (1) Oxide ion: 2.8 (1)
A(b) Negative electrode: Na⁺ + e⁻ → Na (1) Positive electrode: 2O²⁻ → O₂ + 4e⁻ (1)
A(c) Ions move / mobile ions / ions free to move
A(d) Na₂O + H₂O → 2NaOH

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Topic: Electrolysis, O-Level Chemistry (syllabus 5070). Structured question, 6 marks.
Source: O-Level Chemistry, May/June 2017 Paper 2 Variant 2, question 4.

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