O-LevelChemistryBonding & Structure5070/22 · Oct/Nov 2014 · Q610 Marks

The structures of sodium chloride and chlorine are shown below. [Figure showing a sodium chloride lattice structure (Cl⁻ Na⁺ ions) and two chlorine molecules (Cl-Cl)]

(a)The melting point of sodium chloride is 801 °C. The melting point of chlorine is -101 °C. Explain, in terms of structure and bonding, the difference between the melting points of these two substances.[4]
(b)Explain why molten sodium chloride conducts electricity but solid sodium chloride does not.[1]
(c)Draw a 'dot-and-cross' diagram for sodium chloride, showing all the electron shells.[2]
(d)The electrode reactions occurring when molten sodium chloride is electrolysed are shown below. negative electrode Na+ + e- → Na positive electrode 2Cl- → Cl2 + 2e- Refer to these equations to explain why this electrolysis involves both oxidation and reduction.[2]
(e)Chlorine reacts with excess ammonia, NH3, to form hydrogen chloride and nitrogen. Construct an equation for this reaction.[1]

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Mark scheme

Marking points from the official mark scheme (Oct/Nov 2014). The mark or mark type is shown in brackets.

[1] B6 (a) sodium chloride is giant ionic structure/has a continuous structure of ions/ions in lattice (1)
[1] B6 (a) strong (attractive) forces between the ions/lot of energy needed to break ionic bond (1)
[1] B6 (a) chlorine is a (simple) molecule/chlorine has simple covalent structure (1)
[1] B6 (a) chlorine has weak forces between the molecules/small amount of energy required to separate molecules/not much energy needed to break intermolecular forces/chlorine has weak van der Waals' forces (1)
[1] B6 (b) in molten sodium chloride ions can move but ions can't move in solid/ions can only move in molten sodium chloride (1)
[1] B6 (c) sodium ion 2, 8 and + charge (1)
[1] B6 (c) chloride ion 2, 8, 8 and – charge (1)
[1] B6 (d) at the negative electrode/cathode reduction takes place which is gain of electrons (by sodium) (1)
[1] B6 (d) at the positive electrode/anode oxidation takes place which is loss of electrons (by chloride) (1) OR sodium ions are reduced because they gain electrons (1) chloride ions are is oxidised because they lose electrons (1) OR sodium is reduced because oxidation number of sodium decreases (1) chloride/chlorine is oxidised because the oxidation number of chlorine increases (1)
[1] B6 (e) 2NH3 + 3Cl2 → N2 + 6HCl (1)

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