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

(a) Carbon shows catenation to the maximum extent because (1) the carbon atom is very small, so its nucleus holds the shared electron pairs strongly and the carbon–carbon bond is very strong and stable; and (2) carbon has a valency of four (tetravalency), so a carbon atom can bond with other carbon atoms and still have bonds left for more carbon atoms or other atoms, forming long chains, branched chains and rings. (b) (i) Ethane, ${\text{C}}_2{\text{H}}_6$: the two carbon atoms share one pair of electrons (a C–C single bond), and each carbon shares one pair with each of three hydrogen atoms, $7$ shared pairs in all. (ii) Ethene, ${\text{C}}_2{\text{H}}_4$: the two carbon atoms share two pairs of electrons (a $\text{C}=\text{C}$ double bond), and each carbon shares one pair with each of two hydrogen atoms, $6$ shared pairs in all. The electron dot structures are drawn in steps 3 and 4. (c) (i) ‘A’ is ethanoic acid (acetic acid), ${\text{CH}}_3\text{COOH}$: vinegar, a 5–8% solution of it in water, preserves pickles, and it reacts with sodium to give hydrogen gas, $2{\text{CH}}_3\text{COOH}+2\text{Na}\to 2{\text{CH}}_3\text{COONa}+{\text{H}}_2$. (ii) Its structural formula is $\text{H}-\underset{\underset{\text{H}}{|}}{\stackrel{\stackrel{\text{H}}{|}}{C}}-\stackrel{\stackrel{\text{O}}{\Vert }}{C}-\text{O}-\text{H}$.