NEET 2020 Chemistry Question 126 Solution

In the yellow chromate ion ($$\text{CrO}_4^{2-}$$), oxygen has an oxidation state of $$-2$$: $$x + 4(-2) = -2$$ $$x - 8 = -2 \implies x = +6$$ Thus, chromium has an oxidation state of $$+6$$.

In the orange dichromate ion ($$\text{Cr}_2\text{O}_7^{2-}$$), oxygen has an oxidation state of $$-2$$: $$2x + 7(-2) = -2$$ $$2x - 14 = -2 \implies 2x = +12 \implies x = +6$$ Thus, chromium also has an oxidation state of $$+6$$.

Chromate and dichromate ions interconvert depending upon pH without any change in oxidation state: $$2\text{CrO}_4^{2-} \text{(yellow)} + 2\text{H}^+ \rightleftharpoons \text{Cr}_2\text{O}_7^{2-} \text{(orange)} + \text{H}_2\text{O}$$ Because chromium remains in the $$+6$$ oxidation state in both species, Statement (4) is undeniably false.

- Statement (1): $$Cr^{2+}$$ ($$d^4$$) is reducing because $$d^4 \to d^3$$ ($$t_{2g}^3$$, half-filled stable configuration in aqueous medium), whereas $$Fe^{2+}$$ ($$d^6 \to d^5$$) has a positive standard potential ($$E^\circ_{\text{Cr}^{3+}/\text{Cr}^{2+}} = -0.41\text{ V}$$, $$E^\circ_{\text{Fe}^{3+}/\text{Fe}^{2+}} = +0.77\text{ V}$$). True. - Statement (2): Transition metals are catalysts due to multiple oxidation states and complex formation. True. - Statement (3): Interstitial compounds form when small non-metal atoms (H, C, N) occupy interstitial voids. True.