Inside a coastal recycling depot, technicians recover lead from crushed boat-battery parts. A sample of lead(II) bromide compound is placed in a heat-resist...
Question 1 Report
Inside a coastal recycling depot, technicians recover lead from crushed boat-battery parts. A sample of lead(II) bromide compound is placed in a heat-resistant crucible and warmed to 400 degrees C. At this temperature it is molten. The crucible is connected to a d.c. power supply using two carbon electrodes. Fig. 1 shows the arrangement. The workers do not add water, acid or sodium chloride solution because the aim is to separate the ions in the molten compound only.
The electrodes are graphite, so they conduct but do not take part in the reaction. Lead ions move towards one electrode and bromide ions move towards the other.
(a) Name the electrodes labelled X and Y. [2]
(b) Write the two half-equations for the reactions at the electrodes. [2]
(c) Describe two observations a worker should make while the current is passing through the molten compound. [2]
(d) Explain why the lead bromide must be molten, rather than solid, and explain one safety measure needed for this process. [4]
Answer Details
(a) X is the anode, the positive electrode. Y is the cathode, the negative electrode. [2]
(b) At the cathode, lead ions gain electrons:
\[\text{Pb}^{2+}+2e^-\rightarrow\text{Pb}\]
At the anode, bromide ions lose electrons:
\[2\text{Br}^-\rightarrow\text{Br}_2+2e^-\]
[2]
(c) Grey or silvery lead forms at the cathode. Orange-brown bromine gas is produced at the anode, so bubbles are also seen there. Any two observations are acceptable. [2]
(d) In solid lead bromide, ions are fixed in a lattice and cannot move to carry charge. When molten, lead and bromide ions are free to move to the electrodes. This allows lead ions to be reduced at the cathode and bromide ions to be oxidised at the anode. Use a fume cupboard because bromine is toxic; alternatively wear eye protection because molten material can splash, or keep water away because it can turn to steam and spit molten material. [4]
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