Question

Zinc oxide loses oxygen on heating according to the reaction.
$ZnO\stackrel{heat}{\to }Z{n}^{2+}+\frac{1}{2}{O}_2+2e^{-}$
It becomes yellow on heating because:

• A. $Z{n}^{2+}$ ions and electrons move to interstitial sites and F-centers are created
• B. oxygen and electrons move out of the crystal and ions become yellow
• C. $Z{n}^{2+}$ again combine with oxygen to give yellow oxide.
• D. $Z{n}^{2+}$ are replaced by oxygen.

Answer 1

The correct option is A $Z{n}^{2+}$ ions and electrons move to interstitial sites and F-centers are created

$ZnO$ shows non-stoichiometric defect under metal excess defect. When $ZnO$ is heated it loses oxygen as:

$ZnO\stackrel{△}{\to }Z{n}^{2+}+\frac{1}{2}{O}_2+2e^{-}$

The $Z{n}^{2+}$ and the two electrons formed move to interstitial sites of crystal. The electrons trapped in the interstitial sites are less tightly bound and gets promoted by absorption of photons in the violet region. So, we see the complementary color to violet which is yellow.

Hence, Option "A" is the correct answer.