O-LevelChemistryElectrolysis5070/22 · May/June 2016 · Q710 Marks

The formula of lead(II) nitrate is Pb(NO3)2.

(a)Describe how a pure sample of lead(II) nitrate crystals can be prepared from lead(II) oxide, which is insoluble in water.[4]
(b)Aqueous potassium iodide is added to a sample of aqueous lead(II) nitrate. A precipitate of lead(II) iodide is formed. Construct the ionic equation, with state symbols, for this reaction.[2]
(c)(i)Aqueous lead(II) nitrate is electrolysed using graphite electrodes. Bubbles of colourless gas are formed at both electrodes. Identify the gas formed at each electrode. negative electrode (cathode) .................................................................................... positive electrode (anode) .......................................................................................[2]
(c)(ii)Construct the equation for the reaction at the cathode.[1]
(d)On heating, lead(II) nitrate decomposes to form PbO, NO2 and O2. Construct the equation for this reaction.[1]

Mark scheme

Marking points from the official mark scheme (May/June 2016). The mark or mark type is shown in brackets.

[1] Use of excess lead(II) oxide
[1] Use of nitric acid
[1] Warm the solution/use warm/hot nitric acid
[1] Filter mixture to get the solution
[1] Evaporate some of the solution and leave/leave to crystallise/warm to crystallisation point/leave on window sill (to crystallise)/evaporate solution then cool
[1] Pb2+(aq) + 2I⁻(aq) → PbI2(s) (Correct formulae and balance)
[1] State symbols – dependent on correct formulae
[1] Cathode - hydrogen/H2
[1] Anode - oxygen/O2
[1] 2H⁺ + 2e⁻ → H2
[1] 2Pb(NO3)2 → 2PbO + 4NO2 + O2

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