Question 31 markVery Short AnswerSection A

Write the structural formula of ethanol. What happens when it is heated with excess of conc. H2SO4H_2SO_4 at 443 K? Write the chemical equation for the reaction stating the role of conc. H2SO4H_2SO_4 in this reaction.

Chemical Reactions and Equations
Dehydration of Ethanol

Official answer

The structural formula of ethanol is:

H−C∣H∣−C∣H∣−O−H\text{H}-\overset{\underset{\lvert}{\text{H}}}{\underset{\lvert}{\text{C}}}-\overset{\underset{\lvert}{\text{H}}}{\underset{\lvert}{\text{C}}}-\text{O}-\text{H}


When ethanol is heated with excess concentrated H2SO4H_2SO_4 at 443 K443\text{ K}, it undergoes dehydration to form ethene (an unsaturated hydrocarbon) and water.


Chemical Equation:

CH3CH2OH→Conc. H2SO4,443 KCH2=CH2+H2OCH_3CH_2OH \xrightarrow{\text{Conc. } H_2SO_4, 443\text{ K}} CH_2=CH_2 + H_2O


Role of concentrated H2SO4H_2SO_4:

Concentrated sulphuric acid acts as a dehydrating agent because it removes a molecule of water from ethanol.

Answer keywords:structural formulaethenedehydrating agentCH2=CH2443 Kconc. H2SO4

Marking scheme

  1. 1.0.5 marks for drawing the correct structural formula of ethanol.
  2. 2.0.5 marks for stating that ethene is formed.
  3. 3.1.0 mark for writing the complete, balanced chemical equation with correct reaction conditions (443 K443\text{ K}, conc. H2SO4H_2SO_4).
  4. 4.1.0 mark for correctly identifying the role of conc. H2SO4H_2SO_4 as a dehydrating agent.

Hint

Ethanol has two carbon atoms. Heating it with concentrated acid at 443 K443\text{ K} removes water to form a double-bonded hydrocarbon (alkene).

Quick oral answer

When ethanol is heated with excess concentrated sulphuric acid at 443 K443\text{ K}, it dehydrates to form ethene gas. In this reaction, concentrated sulphuric acid acts as a dehydrating agent.

Common mistakes

  1. 1.Writing the molecular formula (C2H5OHC_2H_5OH) instead of the structural formula.
  2. 2.Writing the product as ethane (CH3−CH3CH_3-CH_3) instead of ethene (CH2=CH2CH_2=CH_2).
  3. 3.Omitting the reaction conditions (concentrated H2SO4H_2SO_4 and 443 K443\text{ K}) on the arrow.
  4. 4.Confusing the role of the acid as an oxidizing agent instead of a dehydrating agent.

Analysis and explanation

Ethanol (CH3CH2OHCH_3CH_2OH) is a saturated alcohol. When treated with a strong acid like concentrated H2SO4H_2SO_4 at a high temperature of 443 K443\text{ K} (170∘C170^\circ\text{C}), an intramolecular elimination reaction occurs. A hydrogen atom from the β\beta-carbon and the hydroxyl group (−OH-\text{OH}) from the α\alpha-carbon are eliminated as a water molecule (H2OH_2O), creating a double bond between the two carbon atoms to form ethene (CH2=CH2CH_2=CH_2). Concentrated sulphuric acid facilitates this removal, acting as a powerful dehydrating agent.

Interesting facts

This reaction is an industrial method for producing ethene from bio-ethanol, which serves as a renewable pathway to make plastics like polyethylene without relying on fossil fuels!

Question details

Q3 of 28 questions

Board
CBSE
Class
Class 10
Subject
Science
Year
2017
Section
Section A
Marks
1
Type
Very Short Answer
Chapter
Chemical Reactions and Equations
Topic
Dehydration of Ethanol
Bloom's level
Understanding
Syllabus topic
Properties of ethanol and its dehydration

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Frequently asked questions

How many marks does this question carry in CBSE Class 10 Science 2017?

This question carries 1 mark in the CBSE Class 10 Science 2017 examination.

Which chapter does this question come from in Science?

This question is from the chapter "Chemical Reactions and Equations" in the CBSE Class 10 Science syllabus.

What topic does this question cover in Science?

This question covers the topic "Dehydration of Ethanol" from CBSE Class 10 Science.

What type of question is this in the CBSE Class 10 Science 2017 paper?

This is a Very Short Answer question from Section A in the CBSE Class 10 Science 2017 paper.