Sciencemadness Discussion Board

manganese sulphate + sodium hypochlorite (dumb question)

learningChem - 23-8-2012 at 20:01

So, when sodium hypochlorite is added to a solution of manganese sulphate, manganese dioxide precipitates - chlorine is generated too. I can't figure out how the whole equation balances though. Could this be right?

MnSO4 + 2NaClO ---> MnO2 + Na2SO3 + Cl2

?

learningChem - 23-8-2012 at 20:03

oops - better

MnSO4 + 2NaClO ---> MnO2 + Na2SO4 + Cl2

woelen - 23-8-2012 at 22:28

Yes, that could be the right equation.

Do you really get a lot of Cl2? Does it bubble strongly and do you see the color of the gas?

learningChem - 24-8-2012 at 00:21

I did a very small test. Just a pinch of sulphate in ~2ml of water. I added a few drops of bleach (I don't know the concentration...) - I didn't see too much bubbling nor the color of the gas (but was foolish enough to sniff the test tube - ouch!)

I'm going to react 100 mmol of sulphate tomorrow and see how much oxide I get...

weiming1998 - 24-8-2012 at 00:56

Yes, that reaction works. It is actually an approximate two-step process:

First, the MnSO4 reacts with the NaClO to form Mn(ClO)2 and Na2SO4 as: MnSO4+2NaClO===>Mn(ClO)2+Na2SO4. This formula just makes it easier to visualize what is happening. Actually, in water (unless Mn(ClO)2 is insoluble), a solution of Mn2+ and ClO- ions will be created instead.

Then, the ClO- ion oxidizes the Mn2+ ion, forming MnO2 and Cl2 gas, which bubbles out.

First (assuming Mn(ClO)2 is soluble in water to some extent), reduction half-equation:

2ClO-+2H2O+2e- ---> Cl2+4OH-.

Oxidation half-equation : Mn(2+)+2OH-===>MnO2+2e-+2H+.

Overall: Mn(2+) +2ClO- ---->MnO2+Cl2.

This reaction actually occurs. I have tested it out and got a good amount of MnO2 from Mn acetate made by first dissolving old battery paste in NaHSO4, precipitation of MnCO3, then reaction of it with vinegar. This happens to solutions containing the Pb2+ ion and the ClO- ion as well, forming PbO2.

AJKOER - 25-8-2012 at 17:30

Weiming1998:

Note, under alkaline conditions, per Wikipedia (http://en.wikipedia.org/wiki/Hypochlorite ), to quote:

"Hypochlorite is the strongest oxidizing agent of the generalized chlorates[citation needed]. For example, it oxidizes Mn2+ to permanganate:

2 Mn (2+) + 5 ClO− + 6 OH− → 2 MnO (−4) + 3 H2O + 5 Cl− "
_________________________________________

[EDIT] For completion, here is the documented reaction of Mn (2+) with hypochlorite in the presence of a weak acid. First, the hypochlorite forms HOCl with the weak acid (examples include Acetic, Boric or even Carbonic acid). Next:

Mn (2+) + HOCl + H2O <----> MnO2 (s) + Cl(-) + 3 H(+)

Reference: Table 2.1 at http://scholar.lib.vt.edu/theses/available/etd-11012006-2153...

Recent work further indicates that Iron oxide is a catalyst for this reaction (See http://books.google.com/books/about/Mn_II_Oxidation_by_HOCl_... ). Note also that the reaction is reversible. Then with sufficient Hypochlorous acid, Chlorine can be formed as a consequence of:

HOCl + H(+) + Cl(-) <--> Cl2 + H2O

as was reported by Weiming1998. A possible net reaction is therefore:

Mn (2+) + 2 HOCl <----> MnO2 (s) + Cl2 (g) + 2 H(+)


[Edited on 26-8-2012 by AJKOER]

weiming1998 - 25-8-2012 at 20:08

Quote: Originally posted by AJKOER  
Weiming1998:

Note, under alkaline conditions, per Wikipedia (http://en.wikipedia.org/wiki/Hypochlorite ), to quote:

"Hypochlorite is the strongest oxidizing agent of the generalized chlorates[citation needed]. For example, it oxidizes Mn2+ to permanganate:

2 Mn (2+) + 5 ClO− + 6 OH− → 2 MnO (−4) + 3 H2O + 5 Cl− "


Yes, I have noticed that a solution of Mn2+ ions and bleach seems to go purple, indicating some formation of permanganates. This, however, can be reduced back to MnO2 by the addition of ethanol or sodium thiosulfate without reducing the MnO2 as well.