More than three million carbon compounds have been discovered in the field of chemistry. The diversity of these compounds is due to the capacity of carbon atoms for bonding with one another as well as with other atoms. Most of the carbon compounds are poor conductors of electricity and have low melting and boiling points. (a) Write the molecular formula of first two members of homologous series having functional group –Br. (b) Given below are the formulae of some functional groups : (i) -C(=O)H (aldehyde group drawn as –C(=O)H) and (ii) -C(=O)- (ketone group drawn as –C(=O)–). Write the name of these functional groups. (c) What would be observed on adding a 5% alkaline potassium permanganate drop by drop to some warm ethanol taken in a test tube ? State the role of KMnO_4 in the reaction and write the chemical equation for the reaction involved. (c) Write the name of the compound formed when ethanol is heated at 443 K temperature with excess of conc. H_2SO_4. What is the role of conc. H_2SO_4 in the reaction ? Write the chemical equation for the reaction involved.

(a) CH_3Br, C_2H_5Br; (b) (i) aldehyde, (ii) ketone; (c)(a) purple colour of KMnO_4 disappears, KMnO_4 is an oxidising agent, CH_3CH_2OH → CH_3COOH; (c)(b) ethene, conc. H_2SO_4 is a dehydrating agent, CH_3CH_2OH → CH_2=CH_2 + H_2O

ScienceCBSE Class 10 · Board exam 2024, set 31/3/1 · 4 marks · Case-based

CBSE Class 10 Science 2024: question 37

Set in the CBSE Class 10 board exam of 2024, paper 31/3/1, section E, question 37, for 4 marks. Try it first; the hints and our step-by-step solution are below.

Q37 · 4 marks · Case-basedCompetency-basedCarbon and its Compounds

More than three million carbon compounds have been discovered in the field of chemistry. The diversity of these compounds is due to the capacity of carbon atoms for bonding with one another as well as with other atoms. Most of the carbon compounds are poor conductors of electricity and have low melting and boiling points.

(a)
Write the molecular formula of first two members of homologous series having functional group –Br.
1 mark
(b)
Given below are the formulae of some functional groups : (i) $- C (= O) H$ (aldehyde group drawn as –C(=O)H) and (ii) $- C (= O) -$ (ketone group drawn as –C(=O)–). Write the name of these functional groups.
1 mark
(c)
What would be observed on adding a 5% alkaline potassium permanganate drop by drop to some warm ethanol taken in a test tube ? State the role of $KMn O_{4}$ in the reaction and write the chemical equation for the reaction involved.
2 marks
OR
(c)
Write the name of the compound formed when ethanol is heated at 443 K temperature with excess of conc. $H_{2} S O_{4}$ . What is the role of conc. $H_{2} S O_{4}$ in the reaction ? Write the chemical equation for the reaction involved.
2 marks

Figure in the paper: Two functional group structures: (i) a carbon atom double-bonded to O, with a single bond to H and a single bond to the rest of the molecule (aldehyde group, –CHO). (ii) a carbon atom with a double bond to O and two single bonds to the rest of the molecule (ketone group, –CO–). See it in the official paper.

Source: official question paper from cbse.gov.in (PDF). See it in the full 2024 board paper, set 31/3/1.

Hints

Open one hint at a time and try again before the next.

Hint 1

Successive members of a homologous series differ by $C H_{2}$ . Start from methane and replace one H with Br.

Hint 2

Check whether the carbonyl carbon is attached to a hydrogen or to two carbon atoms.

Hint 3

Alkaline $K M n O_{4}$ adds oxygen, while hot conc. $H_{2} S O_{4}$ removes water. Think about what each does to ethanol.

Step-by-step solution

Each step shows the marks it earns, as an examiner would award them. Checked by an independent re-solve from the question paper.

(a) First member: In a homologous series, each member differs from the next by a $- C H_{2} -$ unit. The first member with the –Br group is bromomethane, $C H_{3} Br$ .
½ mark
(a) Second member: The second member is bromoethane, $C_{2} H_{5} Br$ ( $C H_{3} C H_{2} Br$ ).
½ mark
(b)(i) $- CHO$ (carbonyl carbon bonded to H): aldehyde group.
½ mark
(b)(ii) $- CO -$ (carbonyl carbon bonded to two carbons): ketone group.
½ mark
(c)(a) Observation: The purple (pink) colour of alkaline $KMn O_{4}$ disappears as it is added to warm ethanol. Once excess is added, the colour no longer goes.
½ mark
(c)(a) **Role of $KMn O_{4}$ : It acts as an oxidising agent**. It supplies oxygen to convert ethanol into ethanoic acid, and is used up itself in the process, so its colour is lost.
½ mark
(c)(a) Equation:
$C H_{3} C H_{2} OH alkaline KMn O_{4} + heat C H_{3} COOH$
1 mark
(c)(b) OR alternative (marked instead of (c)(a) if chosen) – Product: Ethene ( $C H_{2} = C H_{2}$ ) is formed. [½ mark]
(c)(b) **Role of conc. $H_{2} S O_{4}$ : It is a dehydrating agent**; it removes a water molecule from ethanol. [½ mark]
(c)(b) Equation: [1 mark]
$C H_{3} C H_{2} OH hot conc. H_{2} S O_{4} 443 K C H_{2} = C H_{2} + H_{2} O$

Answer: (a) $C H_{3} B r$ , $C_{2} H_{5} B r$ ; (b) (i) aldehyde, (ii) ketone; (c)(a) purple colour of $K M n O_{4}$ disappears, $K M n O_{4}$ is an oxidising agent, $C H_{3} C H_{2} O H \to C H_{3} C O O H$ ; (c)(b) ethene, conc. $H_{2} S O_{4}$ is a dehydrating agent, $C H_{3} C H_{2} O H \to C H_{2} = C H_{2} + H_{2} O$

Where the marks are

$C H_{3} B r$ and $C_{2} H_{5} B r$ (½ + ½ marks)
Aldehyde and ketone (½ + ½ marks)
(c) Colour of $K M n O_{4}$ disappears (½ mark); acts as oxidising agent (½ mark); correct equation (1 mark)
OR (c) Ethene (½ mark); dehydrating agent (½ mark); correct equation (1 mark)

Revise the chapter

Chapter: Carbon and its Compounds (notes, formulas, where marks are lost and every past question)

Practise this chapterBoard-style questions from Carbon and its Compounds, one at a time, marked against the answer key and marking points.Back to the full paper2024 board paper, set 31/3/1: every question, by section.

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CBSE Class 10 Science 2024: question 37

Hints

Step-by-step solution

Revise the chapter

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