Which of the following compounds has difficulty in breaking of $$\mathrm{C}-\mathrm{X}$$ bond during nucleophilic substitution reaction?

What is the change in oxidation number of $$\mathrm{Cr}$$ in the following redox reaction?

$$3 \mathrm{H}_2 \mathrm{O}_{2(\mathrm{aq})}+\mathrm{Cr}_2 \mathrm{O}_{7 \text { (aq) }}^{2-}+8 \mathrm{H}_{(\mathrm{aq})}^{+} \longrightarrow 3 \mathrm{O}_{2(\mathrm{~g})}+2 \mathrm{Cr}^3+7 \mathrm{H}_2 \mathrm{O}$$

Equal masses of $$\mathrm{H}_{2(\mathrm{~g})}$$ and $$\mathrm{He}_{(\mathrm{g})}$$ are enclosed in a container at constant temperature. The ratio of partial pressure of $$\mathrm{H}_2$$ to $$\mathrm{He}$$ is

Identify the reaction intermediate of following reaction.