Amphoteric Compounds

\(\require{mhchem}\)

Amphoteric is a fancy-sounding word, but it has a simple meaning. The word comes from the Greek ἀμφότερος (amphóteros, “each of two”) which gives a clue to the behaviour. An amphoteric compound is capable of acting as an acid or an alkali.

We don't see this very often but it crops up most in the chemistry of aluminium, zinc, lead, tin, and beryllium. Let's have an example: aluminium, the metal, will react with an acid

$$\ce{aluminium + hydrochloric~acid -> aluminium~chloride + hydrogen}$$ This is our old friend: $$\ce{acid + metal -> salt + hydrogen}$$ $$\begin{equation} \ce{2Al(s) + 6HCl(aq) -> 2AlCl3(aq) + 3H2(g)} \end{equation}$$

It also reacts with alkalis: $$\ce{aluminium + sodium~hydroxide -> sodium~aluminate + hydrogen}$$ $$\begin{equation} \ce{Al(s) + 2NaOH(aq) + 2H2O(l) → 2NaAlO2(aq) + 3H2(g)} \end{equation}$$ Zinc oxide is another amphoteric compound. It reacts with acid: $$\ce{zinc~oxide + sulfuric~acid -> zinc~sulfate + water}$$ $$\begin{equation} \ce{ZnO(s) + H2SO4(aq) -> ZnSO4(aq) + H2O(l)} \end{equation}$$ and with alkali: $$\ce{zinc~oxide + sodium~hydroxide -> hydrated~sodium~zincate}$$ $$\begin{equation} \ce{ZnO(s) + 2NaOH(aq) + H2O(l) -> Na2[Zn(OH)4](aq)} \end{equation}$$

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