On operation principles of zinc-manganese alkaline batteries

  • #1
The Wikipedia article on alkaline batteries says that they operate at the expense of the oxidation of zinc metal to zinc oxide and the reduction of manganese dioxide to Mn2O3. Anode half reaction is written as

Zn + 2 OH → ZnO + H2O + 2 e.

However we know that metals reacting with water solutions produce hydroxides rather than oxides. So I would write the above reaction as

Zn + 2 OH → Zn(OH)2 + 2 e.

The other Wikipedia article on zinc hydroxide says that it is a rather stable compound that decomposes at 125°C. My first question is where zinc oxide comes from in alkaline batteries?

Gas production is an unwanted process in batteries. It is known that zinc react with sodium hydroxide and water producing hydrogen gas (see chemiday.com):

Zn + 2 NaOH + 2 H2O → Na2[Zn(OH)4] + H2.

My second question is why we have not a similar reaction with KOH in alkaline batteries?
 

Answers and Replies

  • #2
Borek
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What we really get is a mixture of Zn2+ complexes with OH-, depending on the concentration and amount of water some of them can decompose into ZnO, but it will also dissolve back when there is a lot of free OH-. The only thing that really matters is that the Zn gets oxidized to Zn2+, counterions are selected to make the reaction as practical as possible.
 

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