CBSE Class 10 Science 2020 · Paper 31/C/3 · Question 25(b)

(a) (i) ${\text{CH}}_4+{\text{Cl}}_2\stackrel{\text{Sunlight}}{\to }{\text{CH}}_3\text{Cl}+\text{HCl}$. Chloromethane and hydrogen chloride are formed: one hydrogen atom of methane is replaced by chlorine, a substitution reaction. (ii) $\text{R}-\underset{\underset{\text{R}}{|}}{C}=\underset{\underset{\text{R}}{|}}{C}-\text{R}+{\text{H}}_2\underset{\text{heat}}{\overset{\text{Ni}}{\to }}\text{R}-\underset{\underset{\text{R}}{|}}{\stackrel{\stackrel{\text{H}}{|}}{C}}-\underset{\underset{\text{R}}{|}}{\stackrel{\stackrel{\text{H}}{|}}{C}}-\text{R}$, that is, ${\text{R}}_2\text{C}={\text{CR}}_2+{\text{H}}_2\to {\text{R}}_2\text{CH}-{\text{CHR}}_2$. Hydrogen adds across the double bond, one hydrogen atom to each carbon, giving a saturated compound: an addition reaction (hydrogenation). (b) An oxidising agent is a substance that adds oxygen to another substance, that is, it brings about oxidation. When an oxidising agent such as alkaline potassium permanganate (or acidified potassium dichromate) is added to propanol and heated, propanol is oxidised to propanoic acid: ${\text{CH}}_3{\text{CH}}_2{\text{CH}}_2\text{OH}+2[\text{O}]\to {\text{CH}}_3{\text{CH}}_2\text{COOH}+{\text{H}}_2\text{O}$. The purple colour of the permanganate disappears as it is used up.