Science — CBSE Class 10 board question

(a) A homologous series is a series of carbon compounds having the same functional group and similar chemical properties, where successive members differ by a –CH₂ unit (14 mass units).

Two consecutive members of the homologous series of ketones:

• Propanone: CH₃COCH₃
• Butanone: CH₃COC₂H₅

(b)

(i) Ethanoic acid + Sodium hydroxide:

$$\text{CH}_3\text{COOH} + \text{NaOH} \rightarrow \text{CH}_3\text{COONa} + \text{H}_2\text{O}$$

Products: Sodium ethanoate and water

(ii) Ethanoic acid + Ethanol (in presence of acid catalyst):

$$\text{CH}_3\text{COOH} + \text{C}_2\text{H}_5\text{OH} \xrightarrow{\text{H}^+} \text{CH}_3\text{COOC}_2\text{H}_5 + \text{H}_2\text{O}$$

Products: Ethyl ethanoate (ester) and water

(c) Structure of benzene (C₆H₆):

```
H H
\ /
H—C C—H
‖ ‖
H—C C—H
\ /
C=C
```

Benzene is a cyclic compound with alternating single and double bonds between six carbon atoms, each bonded to one hydrogen atom.

Source: Chapter 4 — Carbon and its Compounds

---

• (a) Examiners want the definition to include: same functional group, similar properties, differ by –CH₂/14u. For ketones, the functional group is C=O between two carbon atoms; propanone (3C) and butanone (4C) are the simplest consecutive pair.
• (b) Both equations must be balanced with correct formulae. Always name the products — easy marks. The esterification reaction requires mentioning the acid catalyst (H⁺).
• (c) A clean hexagonal ring with alternating double bonds is expected. Label all H atoms for full marks. Benzene's structure is a standard diagram to memorise.