Oxidisers and reducers part C answer questions please?

Question

Part C: Potassium permanganate

potassium permanganate, KMnO4, is used as a disinfectant on cuts and abrasions. The crystals dissolve to produce a bright purple solution. In acidic solutions, the MnO4- ion acts as an oxidiser and is reduced to the colourless Mn2+ ion.

Aim: to observe the oxidation of Fe2+ to Fe3+ by permanganate ions in acidified aqueous solution.

apparatus: two tesat tubes, test tube rack, access to fresh solutions of iron(II) nitrate and acidified potassium permanganate.

method: to about 5ml of iron(II) nitrate solution add 1ml of potassium permanganate solution. Note and record the colour changes.

Questions:

1. The orange colour formed is due to the Fe3_(aq) ion. If the reduction half equation is:

MnO4-(aq) + 8H+(aq) + 5e- ---> Mn2+ (aq) + 4H2O(l)

write a blanced overall equation for the reaction.

2. why did the permanganate solution have to be acidified?

Answer 1

1.  Start by writing the oxidation half reaction:

Fe2+ ---> Fe3+ + e-

The electrons used in the reduction half reaction must come from the oxidation reaction, so the number of electrons involved in each half reaction in the balanced equation must be the same.  To make this true, we multiply the half reactions by coefficients as needed.  In this case, we multiply the oxidation half reaction by 5:

5Fe2+ ---> 5Fe3+ + 5e-

Next, add the multiplied equations:

MnO4- + 8H+ + 5e- + 5Fe2+ ---> Mn2+ + 4H2O + 5Fe3+ + 5e-

Finally, cancel anything that occurs on both the reactant and product side.  In this case it's only the electrons (if you have done the above steps correctly, the electrons will always cancel).

Final, balanced equation:

MnO4- + 5Fe2+ + 8H+ ---> Mn2+ + 5Fe3+ + 4H2O

2.  As we can see in the equation, we had to "get rid" of the oxygens on the MnO4-.  In acidic solution, excess oxide ions are dealt with by reacting them with H+ to make water.

If we wanted to get out aqueous Mn2+ and Fe3+ from the reaction, then the acidification is necessary.  However, the reaction will also proceed in basic solution, forming MnO2 in place of Mn2+, and possibly forming hydroxides of Fe3+ rather than Fe3+ ions.