Question
Lead can be removed from waste water by precipitation, for example as lead(II) sulfide, PbS, or lead(II) hydroxide, \(Pb(OH)_2\).
(a) The unit cell of lead(II) sulfide is shown:
(i) State the coordination number of the lead(II) ion.
(ii) There are 14 sulfide ions shown in the diagram. Explain why the unit cell is only considered to contain 4 sulfide ions.
(iii) Identify one technique that could be used to determine the structure of lead(II) sulfide.
(b) Lead hydroxide is best precipitated at pH 9. Determine the concentration of lead(II) ions present in an aqueous solution of a lead(II) salt at pH 9. Use section 32 of the data booklet.
(c) An alternative method of removing lead(II) ions from aqueous solutions is chelation.
A possible ligand for this is the anion of iminodiacetic acid.
Draw circles around the atoms on the following ligand, that would bond to the lead(II) ion when it forms a complex ion.
Answer/Explanation
Answer:
(a) (i) 6
(ii) 6 on faces «shared between two unit cells» so 6×1⁄2 = 3
8 on corners «shared between eight unit cells» so 8×1⁄8 = 1
(iii) X-ray crystallography/diffraction
(b) 
(c) 
Question
Most metals have to be extracted from an ore. The way in which this is carried out depends on the reactivity of the metal.
(a) Identify a metal produced by reacting its oxide with carbon or carbon monoxide.
Use section 25 of the data booklet.
(b) Aluminium is produced by electrolytic reduction of a solution of aluminium oxide, \(Al_2O_3\), in molten cryolite, \(Na_3AlF_6\).
(i) Write the half equation for the reaction at the electrode where aluminium is formed.
(ii) Calculate the atom economy for the production of aluminium from its oxide, assuming the products do not react with the electrodes. Use section 1 of the data booklet.
(iii) Suggest one factor, other than atom economy, that indicates the production of aluminium from its ore has a significant environmental impact.
(iv) Deduce why pure molten aluminium oxide is a poor conductor of electricity.
Use sections 8 and 29 of the data booklet.
(c) Inductively coupled plasma (ICP) techniques can be used to estimate the concentration of other metals in the aluminium produced.
(i) Describe the plasma state.
(ii) Explain how different metals are identified, and their concentrations determined, if ICP is coupled with Optical Emission Spectroscopy (OES).
Identification:
Concerntration:
(d) An aluminium matrix can be reinforced with carbon nanotubes. Outline why carbon nanotubes are so strong and rigid.
Answer/Explanation
Answer:
(a) Any one of:
Zn, Cr, Fe, Cd, Co, Ni, Sn, Pb, Sb, As, Bi, Cu, Ag, Pd, Hg, Pt
(b) (i) \(Al^{3+} + 3 e^-\) → Al(l)
(ii) 〈〈\(\frac{2 \times 26.98}{2 \times 26.98 + 3 \times 16.00} \times 100\) = 〉〉 52.92%
(iii) high energy consumption «that has environmental implications»
OR
large amounts of waste «produced by mining and purification of the ore»
OR
mining has negative impact on landscape
OR
greenhouse gas/pollution from transport/machinery
(iv) average electronegativity 2.5 AND
electronegativity difference 1.8
border between ionic and «polar» covalent
(c) (i) electrons AND «positive» ions «in gaseous state»
(ii) Identification:
«emit» light/photons of characteristic frequencies
Concentration:
intensity/brightness of light «proportional to concentration»
(d) «held together by strong» covalent bonds «and defect free/regular 2D/3D»