Question
Electrolysis and Winkler titrations are both applications of redox reactions.
(a) An electrolytic cell was set up using inert electrodes and molten magnesium chloride, \(MgCl_2 (l)\).
(i) Identify the product formed at the cathode.
(ii) Annotate the diagram to show the movement of electrons.
(iii) Graphite rods are sometimes used as inert electrodes. Describe the structure of graphite and explain why graphite conducts electricity.
(b) Winkler titrations can be used to determine the biochemical oxygen demand, BOD, of a water sample. One set of equations for the reactions occurring is:
150\(cm^3\) of a water sample was tested using a Winkler titration. 36.0\(cm^3\)
of 0.00500mol \(dm^{−3}\) sodium thiosulfate solution, \(Na_2S_2O_3\) (aq), was required to reach the end point.
(i) Determine the concentration, in mol \(dm^{−3}\) , of oxygen dissolved in the water sample.
(ii) Outline how the BOD of the water sample could be determined.
(iii) Suggest what a low BOD value indicates about a water sample.
Answer/Explanation
Answer:
(a) (i) magnesium/Mg «metal»
(ii) 
electron flow from anode to battery OR from battery to cathode
(iii) layers «of carbon atoms in a giant structure»
delocalized electrons «flow along layers»
(b) (i) 
(ii) titrate/measure dissolved oxygen in «another» water sample «stored under
controlled conditions five days» later
difference between two values «is BOD»
(iii) low levels of «organic/oxygen consuming» water pollution
Question
Redox reactions can be used to produce electricity.
(a) State the oxidation state of sulfur in copper(II) sulfate.
(b) A voltaic cell was constructed using a copper(II) sulfate/copper half-cell and a zinc sulfate/zinc half-cell.
(i) Outline why electrons flow from zinc to copper when these half cells are connected with a wire. Use section 25 of the data booklet.
(ii) Formulate equations for the reactions taking place at each electrode.
Anode (negative electrode): . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Cathode (positive electrode): . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Answer/Explanation
Answer:
(a) +6/VI
(b) (i) Zinc more reactive/ <<better>> reducing agent/ <<more>> easily oxidized/loses electrons <<more>> easily.
(ii) Anode (negative electrode):
\(Zn_{(s)} \rightarrow Zn^{2+}_{(aq)} + 2e^-\)
Cathode (positive electrode):
\(Cu^{2+}_{(aq)} + 2e^- \rightarrow Cu_{(s)}\)