Why is the oxidation of #Mn^(2+)# in ACIDIC media represented differently to the oxidation of #Mn^(2+)# in an ALKALINE medium? What do we mean by #H^+# or #H_3O^+#?

1 Answer
anor277
Mar 24, 2017

Because chemistry follows experiment...........

Explanation:

#H^+# is very much a conceptual particle. When we write #H^+# in a reaction we mean the hydronium ion, #H_3O^+#, which AGAIN is a conceptual species. The actual species is (probably) a cluster of 3-4 water molecules, with an extra proton to give #H_7O_3^+#, or #H_9O_4^+#. The clusters exchange the proton between themselves. We write #H^+# or #H_3O^+# for convenience.

When we write a redox equation, of course we balance mass and charge, and the use of #H^+# or #H_3O^+# is a means to this end. It happens that in ACIDIC medium, #Mn^(2+)#, may be oxidized to #MnO_4^(-)#:

#Mn^(2+) +4H_2O rarr MnO_4^(-) + 8H^(+) + 5e^-#

In alkaline media, #Mn^(2+)# may be oxidized to #"manganate ion"#, #MnO_4^(2-)#, which is #Mn(VI+)#:

#Mn^(2+) +8HO^(-) rarr MnO_4^(2-) + 4H_2O + 4e^-#

#"Manganates"# give green salts.

Mass and charge are balanced (I think) as required. Note that this representation follows the actual experiment, not vice versa.

If there is a further issue or query, fire away.

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