Bromide + chlorate

if i add sodium chlorate(I) to a solution of bromide ions the reaction proceeds thus:

Br2 + Cl- + 2OH- --> ClO- + H2O + 2Br-

The solutions were colourless to begin with and after reacting became slightly yellow. My teacher said Br2 was produced (meaning the reaction went the other way) but according to the electrode potentials it should proceed as written above. The yellow colour was not dark enough to be Br2(aq) so what caused it?

ps If I add HCl to aqueous manganate, is it Cl2 thats given off?

Reply 1

charco

Study Forum Helper

If you are adding chlorate (I) to bromide ions then the equation has no choice but to go in reverse (you only have the species on the right hand side!)

ClO-(aq) + H2O(l) + 2e- --> Cl-(aq) + 2OH-(aq) Eº = 0.9
Br2(l) + 2e- --> 2Br-(aq) Eº = 1.7

If you assume the forward reaction then the Bromine would be reduced and the chloride oxidised
E = E(red state) - E(oxidised state) = 1.7 - 0.9 = + 0.8 suggesting a feasible reaction (as you say)

This prediction however is only valid under standard conditions. If the concentrations are not molar or if the temperature is not 25º then it is possible that the value of E will fall to equilibrium levels and that some bromine will be seen (pale yellow in dilute solution)

If you add HCl to potassium permanganate chlorine is indeed given off.

A lab tech at my school didn't have any sulphuric acid to hand when making up a solution of KMnO4 for some redox titration practicals (about 5 litres) and thought that no-one would notice if she used HCl instead. As KMnO4 is notoriously slow to dissolve she mixed them together in a large beaker on a hotplate with automatic stirrer and went off to lunch....

Came back to find the school evacuated and the fire brigade in gas masks dealing with it!

Salutary tale - and 100% true!