Question

Which of the following will not act as oxidizing agents?

• A. $W{O}_3$
• B. $Cr{O}_3$
• C. $Mo{O}_3$
• D. $Cr{O}_4^{2-}$

Answer 1

Answer: (A), (C)

$Mo{O}_3$

Analysing the given Options:

Option a:

In $Cr{O}_3$, $Cr$ is present in $+6$ oxidation state (highest oxidation state for $Cr$). Since chromium is a lighter element, its $+6$ oxidation state is not as stable as that of heavier members of the same group.

Thus, it undergoes self-reduction by gaining electrons to achieve $+3$ oxidation state (a stable state) and hence acts as an oxidizing agent.

Option b:

The oxidation state of $Mo$ in $Mo{O}_3$ is $+6$.

$Mo$ is comparatively a heavier element, so its higher oxidation states, like $+6$, are quite stable and it is reluctant to undergo self-reduction to achieve lower oxidation states like $Cr{O}_3$.

That’s why $Mo{O}_3$ is quite stable and does not act as an oxidizing agent like $Cr{O}_3$.

Option c:

The oxidation state of $W$ in $W{O}_3$ is $+6$.

$W$ is comparatively a heavier element, so its higher oxidation states, like $+6$, are quite stable and therefore it does not undergo self-reduction to achieve lower oxidation states like $Cr{O}_3$.

That’s why $W{O}_3$ is quite stable and does not act as an oxidizing agent like $Cr{O}_3$.

Option d:

In the chromate ion $Cr{O}_4^{2-}$, $Cr$ is in $+6$ oxidation state.

The $+6$ oxidation state of $Cr$ is not as stable as that of heavier members of the same group.

So $Cr{O}_4^{2-}$ acts as an oxidizing agent and gets reduced (by gaining electrons), to $+3$ oxidation state.

Hence, the correct options are (b) and (c).