Sodium Chloride

Sodium chloride (NaCl) is the standard example of an ionic compound. It appears in bonding questions, in the electrolysis of brine, and as the salt made when sodium hydroxide neutralises hydrochloric acid.

Key facts

Property	Value
Formula	NaCl
Mr	23 + 35.5 = 58.5
Bonding	Ionic — giant lattice of Na+ and Cl- (each ion surrounded by 6 of the other)
Melting point	801 C — strong electrostatic forces throughout the lattice
Conductivity	Conducts when molten or in solution, not as a solid
Main uses	Brine feedstock (Cl2, H2, NaOH); seasoning; de-icing roads

Where sodium chloride appears in 0620

• Ions and ionic bonds: dot-and-cross diagrams and the properties of ionic compounds.
• Electrolysis of aqueous solutions: concentrated NaCl (brine) is the key example.
• Preparation of salts: NaCl from the titration of NaOH with HCl.
• Tests for ions: the chloride test and the sodium flame test.

Properties explained by ionic bonding

Property	Explanation
High melting point	Strong electrostatic attraction between oppositely charged ions needs a lot of energy to overcome
Does not conduct as a solid	Ions are held in fixed positions and cannot move
Conducts when molten/dissolved	Ions become free to move and carry charge
Brittle	A knock lines up like charges, which repel and split the crystal

Identifying sodium chloride

• Chloride ion: add dilute nitric acid, then aqueous silver nitrate — a white precipitate confirms chloride: Ag+(aq) + Cl-(aq) -> AgCl(s)
• Sodium ion: flame test gives a yellow flame.
• Formation: NaOH(aq) + HCl(aq) -> NaCl(aq) + H2O(l)

Electrolysis of brine (concentrated NaCl solution)

An industrially important process giving three products:

Electrode	Product	Half-equation	Use
Anode (+)	Chlorine	2Cl-(aq) -> Cl2(g) + 2e-	Water treatment, PVC, bleach
Cathode (-)	Hydrogen	2H+(aq) + 2e- -> H2(g)	Hydrogenation, fuel
Solution	Sodium hydroxide	Na+ and OH- remain	Soap, paper, bleach

Chlorine forms because the solution is concentrated; hydrogen forms because sodium is more reactive than hydrogen, so H+ is discharged in preference. See electrolysis of aqueous solutions.

Molten sodium chloride and concentrated aqueous sodium chloride are each electrolysed with inert electrodes. Name the product formed at the cathode in each case and explain why they differ. (4 marks)

Mark scheme

• Molten sodium chloride: sodium (metal) is produced at the cathode [1]; because sodium ions are the only positive ions present (there is no water) [1]
• Concentrated aqueous sodium chloride: hydrogen is produced at the cathode [1]; because water supplies H+ ions and sodium is more reactive than hydrogen, so H+ is discharged in preference to Na+ [1]

Examiner note: at the cathode of an aqueous solution the less reactive ion is discharged, so a reactive metal like sodium stays in solution and hydrogen comes off instead. Molten salts contain no water, so the metal itself is always deposited.

Common exam mistakes

• Writing that solid sodium chloride conducts electricity. It conducts only when molten or dissolved, once the ions are free to move.
• Mixing up concentrated and dilute brine: concentrated NaCl gives chlorine at the anode, whereas dilute NaCl gives oxygen.
• Writing a formula such as Na2Cl or referring to “molecules” of NaCl. It is a giant lattice in a fixed 1:1 ratio, with no molecules.
• Drawing shared electrons in the dot-and-cross diagram. Ionic bonding is electron transfer — Na loses one electron to Cl — and both ions are drawn in brackets with their charges.