I'm currently struggling with understanding why electrochemical reactions take place the way they do.

For example, in an alkaline-manganese battery manganese-dioxide is getting reduced by reacting with water whereby the electron is actually absorbed by the hydroxide molecule (?):

$$\ce{MnO2 + H2O + e- -> MnO(OH) + OH-}$$

Essentially, my question is why does manganese give up the bond with the two oxygens and instead bonds with one oxygen which itself is bonded with an hydroxide and why does the other hydroxide absorb the electron instead of the other molecules.

Would manganese-dioxide and water also react without the transferred electron?


Chemistry can be difficult because it has lots of pieces of different importance, some of which are not depicted in equations. In this case, the important pieces are the ones that undergo the greatest change. The hydrogens don't change much at all - they are always bonded to oxygen. The oxygens have their outer shells filled, and they sit in a crystal lattice (MnO2) or a liquid (H2O); there's a change, but it's moderate (in this case, anyway).

But the manganese changes from +4 to +3! Big change! It took a lot of energy to rip 4 electrons off the Mn, so there is a driving force to get at least one back. The MnO2 would become MnO2-; this is a nice place for a proton. The proton comes from a water molecule, leaving OH-.

MnO2 + H2O don't react together. MnO2 is stable unless it can oxidize something and water is quite inert.


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