1. A Student has run as electrolysis reaction of an aqueous solution of MgBr2 (a

Question

1. A Student has run as electrolysis reaction of an aqueous solution of MgBr2 (aq) in a petri dish using inert electrodes and a 9v battery. Observations are made and the following data is recorded:

* At the electrode attatched to the (-) terminal of the battery, bubbles are observed. A white precipitate forms around the electrode.

* At the electrode attatched to the (+) terminal of the battery, a small amount of bubbles are also observed. The solution near the electrode turned yellowish. When a few drops of the yellowish liquid are mixed with cyclohexane, an orange layer forms at the top of the mixture,

* When a few drops of phenopthalein are added to the petri dish, a magenta pink polor is observed near the electrode attached to the (-) terminal of the battery.

Account for all of the observations above. write the redox reactions which occured and account for the colors, precipitate and bubbles.

2. Suppose that molten MgBr2 (l) is electrolized. What reactions woul take place, and what is the minimum EMF required for this reaction?

Explanation / Answer

In case of MgBr2(aq) the redox reactions and the electrode potentials which occur are as follows

Mg(s) ==> Mg2+(aq) + 2e At anode Eooxidation = 2.356 V

2H2O(l) + 2e- ===> H2(g) + 2 OH-(aq) At cathode Eoreduction = - 0.41 V

Overall redox reaction is as follows

Mg(s) + 2H2O(l) ==> Mg2+(aq) + H2(g) + 2 OH-(aq)

2. Molten MgBr2 (l) is electrolized. Reactions that would take place, and the minimum EMF required for this reaction are as follows :

MgBr2 -----> Mg2+ + 2Br- electrolysis

Mg2+ + 2e- -----> 2Mg At cathode Eocell = –2.37 V
2Br- -----> Br2 + 2e- At anode Eocell = - 1.07 V

Eocell = Eocathode - Eoanode

= -2.37 -(-1.07)

Minimum Eocell = -1.3 V

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