1. In school textbook

$$\ce{Cd +2Ni(OH)3->CdO + 2Ni(OH)2 + H2O}$$

  1. Wikipedia

$$\ce{2NiO(OH) + Cd + 2H2O->2Ni(OH)2 + Cd(OH)2}$$

  1. Chemwiki

$$\ce{Cd +NiO2 + 2H2O->Cd(OH)2 + 2Ni(OH)2}$$

Which reaction should I follow?

The ChemWiki page got updated on 22nd Jan 2017, an year after this question was posted, and the reaction in question has since been updated. The reaction on the ChemWiki page is now identical with that on the Wikipedia page. This question now has no significance whatsoever but for the sake of the time and effort of the answerer, I am keeping it undeleted

  • $\begingroup$ If you're studying a course, follow the textbook. There will be multiple possible cell configurations for a "Ni-Cd" cell, depending on their configuration they may have different properties such as how the voltage varies during discharge. Your "chemwiki" equation doesn't balance, check this and this $\endgroup$ Jan 23, 2016 at 14:49

1 Answer 1


Note that the three equations are different only in whether the nickel and cadmium oxides are hydrated (and thus where water appears in the equation). Also, the Chemwiki equation is not balanced because nickel oxide is $\ce{Ni2O3}$ and not $\ce{NiO2}$.

The correct equation for the ChemWiki version is:

$$\ce{Ni2O3 + Cd + 3H2O -> 2Ni(OH)2 + Cd(OH)2}$$

Many metal oxides react with water to form hydroxides. This kind of behavior is probably most frequently taught with the alkali metals and alkaline earth metals:

$$\begin{align} \ce{2NaO + H2O} &\ce{-> 2NaOH}\\ \ce{CaO + H2O} &\ce{-> Ca(OH)2}\end{align} $$

However, other metals undergo these kinds of reactions. For example, the chemistry of rust involves the equilibrium between iron oxides and iron oxide hydroxides: $$\begin{align} \ce{FeO + H2O} &\ce{<=> Fe(OH)2}\\ \ce{Fe2O3 + 3H2O} &\ce{<=> 2Fe(OH)3}\end{align} $$


The three ionic nickel species in the reactants are different only on the degree of hydration:

$$\begin{align} \ce{Ni2O3 + H2O} &\ce{<=> 2NiO(OH)}\\ \ce{NiO(OH) + H2O} &\ce{<=> Ni(OH)3}\end{align} $$


The two ionic cadmium species in the products differ only by degree of hydration:

$$ \ce{CdO2 + H2O<=> Cd(OH)2} $$


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