Question 62 marksShort AnswerSection A

Write the structural formula of ethanol. What happens when it is heated with excess of conc. H₂SO₄ at 443 K? Write the chemical equation for the reaction stating the role of conc. H₂SO₄ in this reaction.

Carbon and its Compounds
Chemical Properties of Carbon Compounds

Official answer

1. Structural Formula of Ethanol

HH∣∣H−C−C−O−H∣∣HH\begin{array}{ccccc} \text{H} & & \text{H} & & \\ | & & | & & \\ \text{H}-\text{C} & - & \text{C} & - & \text{O}-\text{H} \\ | & & | & & \\ \text{H} & & \text{H} & & \end{array}

(or represented as CH3−CH2−OHCH_3-CH_2-OH)


2. Reaction with Excess Conc. H2SO4H_2SO_4 at 443 K443\text{ K}

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


3. Chemical Equation

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


4. Role of Conc. H2SO4H_2SO_4

Concentrated sulphuric acid acts as a dehydrating agent in this reaction, as it removes a water molecule from ethanol.

Answer keywords:ethanol structural formulaethenedehydrationdehydrating agentCH2=CH2443 K

Marking scheme

  1. 1.Correct structural formula of ethanol: 0.5 mark
  2. 2.Statement of what happens (formation of ethene): 0.5 mark
  3. 3.Balanced chemical equation with conditions: 0.5 mark
  4. 4.Stating the role of conc. H2SO4H_2SO_4 as a dehydrating agent: 0.5 mark

Hint

Heating ethanol with concentrated acid at 443 K443\text{ K} removes a water molecule to form an alkene.

Quick oral answer

When ethanol is heated with excess concentrated sulphuric acid at 443 K443\text{ K}, it gets dehydrated to form ethene. Here, concentrated sulphuric acid acts as a dehydrating agent.

Common mistakes

  1. 1.Writing the temperature incorrectly (e.g., 413 K413\text{ K} instead of 443 K443\text{ K}, which yields ether instead of ethene).
  2. 2.Forgetting to specify that the sulphuric acid must be concentrated and hot.
  3. 3.Drawing the structure of ethane instead of ethene as the product.

Analysis and explanation

Ethanol (CH3CH2OHCH_3CH_2OH) contains an −OH-OH group. When treated with a strong dehydrating agent 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 methyl carbon and the hydroxyl group from the adjacent 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).

Interesting facts

Concentrated sulphuric acid has a very strong affinity for water, so much so that it can tear apart hydrogen and oxygen atoms from organic molecules to form water, leaving behind carbon or unsaturated compounds.

Question details

Q6 of 33 questions

Board
CBSE
Class
Class 10
Subject
Science
Year
2017
Section
Section A
Marks
2
Type
Short Answer
Chapter
Carbon and its Compounds
Topic
Chemical Properties of Carbon Compounds
Bloom's level
Applying
Syllabus topic
Properties of ethanol and ethanoic acid (only properties and uses)

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Frequently asked questions

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

This question carries 2 marks in the CBSE Class 10 Science 2017 examination.

Which chapter does this question come from in Science?

This question is from the chapter "Carbon and its Compounds" in the CBSE Class 10 Science syllabus.

What topic does this question cover in Science?

This question covers the topic "Chemical Properties of Carbon Compounds" from CBSE Class 10 Science.

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

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