Questions on Metals & ICP spectroscopy

1. Aluminium is produced by electrolysing a molten solution of alumina, Al₂O₃, in cryolite, Na₃AlF₆, using graphite electrodes.

(a) Explain why a molten solution of the alumina dissolved in cryolite is used rather than just using molten alumina.

(b) State the half-equations for the reactions occurring at (i) the cathode (negative electrode) and (ii) the anode (positive electrode) during the electrolysis.

(c) Explain why the anode has to be replaced periodically.

(d) State two ways in which the production of aluminium contributes to global warming.

2. A copper ornament was plated with silver. The copper ornament was made the cathode (negative electrode), the anode (positive electrode) was made of silver and the electrolyte was an aqueous solution of silver nitrate, AgNO₃(aq).

(a) State the half-equations for the reactions that took place at each electrode.

(b) Explain why a silver electrode was used as the anode, rather than an inert material, such as graphite.

(c) The copper ornament had a total surface area of 600 cm² and was uniformly covered by a layer of silver 3.00 x 10^-3 cm thick. Show that the mass of silver required to cover the copper ornament was 18.9 g. (Density of silver = 10.5 g cm^-3).

(d) Calculate the time it took to cover the copper ornament with the 3.00 x 10^-3 cm layer of silver if a steady current of 5.00 x 10^{− 1} A was used.

3. (a) Describe how you could determine practically whether a solid material consisting of a single pure substance is diamagnetic or paramagnetic.

(b) Identify whether the following are diamagnetic or paramagnetic.
(i) chlorine atoms
(ii) chloride ions
(iii) zinc metal
(iv) iron(III) ions

4. Trace amounts of metals in alloys can be determined using Inductively Coupled Plasma Spectroscopy together with either Optical Emission Spectroscopy or Mass Spectroscopy. These two processes are known as ICP-OES and ICP-MS.

(a) Plasma is often known as the fourth state of matter. Outline what plasma is.

(b) Explain how the radiation emitted by the plasma enables a trace metal to be identified both qualitatively and quantitatively by optical emission spectroscopy.

(c) A sample of brass (an alloy of copper and zinc) was analyzed by ICP-MS. The mass spectrum showed peaks with high intensity due to copper and zinc and peaks of low intensity with M/E values of 27, 55 and 75. Identify the impurities in the sample of brass.

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