Sergey Bylikin, Gary Horner and, Brian Murphy Solutions for Chapter: Redox Processes, Exercise 13: Questions

Which statement about the electrolysis of molten sodium chloride is correct?

What is the reducing agent in the reaction below?

$2{\mathrm{MnO}}_4^{-}\left(\mathrm{aq}\right)+{\mathrm{Br}}^{-}\left(\mathrm{aq}\right)+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)\to 2{\mathrm{MnO}}_2\left(\mathrm{s}\right)+{\mathrm{BrO}}_3^{-}\left(\mathrm{aq}\right)+2{\mathrm{OH}}^{-}\left(\mathrm{aq}\right)$.

Which changes could take place at the positive electrode (cathode) in a voltaic cell?

I. ${\mathrm{Zn}}^{2+}\left(\mathrm{aq}\right)$ to $\mathrm{Zn}\left(\mathrm{s}\right)$.

II. ${\mathrm{Cl}}_2\left(\mathrm{g}\right)$ to ${\mathrm{Cl}}^{-}\left(\mathrm{aq}\right)$.

III. $\mathrm{Mg}\left(\mathrm{s}\right)$ to ${\mathrm{Mg}}^{2+}\left(\mathrm{aq}\right)$.

Metal A is more reactive than metal B. A standard voltaic cell is made as shown figure.

Which statement is correct?

A $0.1337 \mathrm{g}$ sample of an alkali metal iodate, ${\mathrm{XIO}}_3$, was dissolved in water, acidified, and an excess of potassium iodide, $\mathrm{KI}$ added. The resulting iodine solution required $36.64 {\mathrm{cm}}^3$ of the sodium thiosulphate pentahydrate solution, ${\mathrm{Na}}_2{\mathrm{S}}_2{\mathrm{O}}_3.5{\mathrm{H}}_2\mathrm{O} \left(25.49 \mathrm{g} {\mathrm{dm}}^{-3}\right)$ for complete titration using starch solution as an indicator. Calculate the relative atomic mass of $\mathrm{X}$ and hence identify the metal. Deduce all relevant half-equations involved.

Describe how the dissolved oxygen concentration in a river would decrease if a car factory releases warm water into the river after using it for cooling. 

Describe how the dissolved oxygen concentration in a river would decrease if a farmer puts large quantities of a fertilizer on a field next to the river.

Describe how the addition of nitrates or phosphates to water can increase the BOD value of a water sample.