O-LevelChemistryElectrolysis5070/22 · Oct/Nov 2020 · Q212 Marks

Part of the structures of chlorine and sodium chloride are shown. [Figure showing representations of chlorine molecules and a sodium chloride lattice structure]

(a)Explain in terms of structure and bonding why chlorine has a low boiling point and sodium chloride has a high boiling point.[3]
(b)Write the electronic configuration of a chlorine atom.[1]
(c)(i)The electrolysis of molten sodium chloride is carried out using graphite electrodes. State the meaning of the term electrolysis.[1]
(c)(ii)State the direction of movement of both the positive and negative ions when molten sodium chloride is electrolysed. positive ions negative ions[1]
(c)(iii)State one observation that can be made at the positive electrode when molten sodium chloride is electrolysed.[1]
(c)(iv)Give the formulae of the two negative ions present in aqueous sodium chloride.[1]
(d)When aqueous sodium chloride is electrolysed, hydrogen is produced at the negative electrode. Explain, in terms of transfer of electrons, why hydrogen and not sodium is produced at the negative electrode.[1]
(e)(i)A 36.3 g sample of a compound contains 14.4 g carbon, 0.600g hydrogen and 21.3g chlorine. Calculate the empirical formula of this compound.[2]
(e)(ii)The relative molecular mass of this compound is 181.5. Deduce the molecular formula of this compound.[1]

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Mark scheme

Marking points from the official mark scheme (Oct/Nov 2020). The mark or mark type is shown in brackets.

[1] 2(a) chlorine is a (simple) molecule and sodium chloride is ionic (1)
[1] 2(a) for chlorine: weak (attractive) forces between molecules / weak intermolecular forces(1)
[1] 2(a) for sodium chloride: strong (attractive) forces between ions (1)
[1] 2(b) 2,8,7
[1] 2(c)(i) decomposition of the electrolyte by an electric current
[1] 2(c)(ii) (positive ions move to the) negative electrode AND (negative ions move to the) positive electrode
[1] 2(c)(iii) green gas formed / green fumes formed / solution goes green (near electrode) / bubbles formed / fizzes / effervescence
[1] 2(c)(iv) Cl- AND OH-
[1] 2(d) hydrogen ions (from water) gain electrons more easily than sodium ions / sodium ions gain electrons less easily than hydrogen ions
[1] 2(e)(i) mol C = 14.4 / 12 mol Cl = 21.3 / 35.5 mol H= 0.600 / 1 (1) OR mol C = 1.20 mol Cl = 0.60 mol H = 0.60 (1)
[1] 2(e)(i) empirical formula is C2HCl (1)
[1] 2(e)(ii) C6H3Cl3

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