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If water, $\ce{H2O}$ is a covalent compound formed by sharing of electrons, why is it said (in case of fuel cells) that formation of water from hydrogen and oxygen is a redox reaction (transfer of electrons, not sharing). This would make it an ionic compound.

Any insights? Note that, the concept of water being formed as a result of redox reaction is highlighted in fuel cells (which is the basic principle of fuel cells - generating electricity by exploiting this transfer of electrons).

I know how fuel cells work, and it is indeed a redox reaction; but my doubt is - can water be formed by both covalent/ ionic means? (In textbooks, or in exams when asked to identify bonding present in water); when both covalent and ionic type can be achieved in its formation? Is both "covalent" and "ionic" correct while describing the formation of $\ce{H2O}$ molecule?

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    $\begingroup$ Why can't you have covalent compound formed via redox reaction ? $\endgroup$ – Babounet Feb 7 '15 at 16:29
  • $\begingroup$ Perhapse it is referring to the polar nature? (vs organic solvents which are nonpolar) $\endgroup$ – Dale Nov 11 '15 at 18:25
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Water is a covalent compound. But as we know no compound is 100% ionic or 100% covalent. Since the electronegative difference (END) between hydrogen and oxygen is very high it is a strongly polarised covalent compound therefore the electron cloud is shifted towards the oxygen atom and therefore water has a very high % ionic character.

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Water is certainly a covalent compound. Being produced from electron transfer doesn't mean that it is ionic. In a fuel cell, hydrogen is first ionized: $\ce{H2 <=> 2 H+ + 2 e- }$
Next, the electron moves to the other electrode, reducing oxygen while doing electric work: $\ce{O2 + 4e- -> 2O^2^-}$
this step is the step that generates electricity, because the oxygen has a higher electrode potential than hydrogen.
Finally, hydrogen ions and oxide ions unite to form water: $\ce{2H+ + O^2^- -> H2O}$
in this step, it is essentially similar to the reaction of an ionic oxide with an acid. Although the electron is initially transferred to the oxygen atom, as the ions of the opposite charge merge, the hydrogen ions gain their electrons back and share them with oxygen atoms, thus forming covalent bonds.

Note, however, that water can self-ionize and become ionic, although these ions have only a low concentration of $10^{-7} M$ in pure water under room conditions. The presence of solutes and temperature of water affects the amount of water ionized at any given moment.

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