1 markVery Short AnswerSection A

Write the structural formula of ethanol. What happens when it is heated with excess of conc. $H_2SO_4$ at 443 K? Write the chemical equation for the reaction stating the role of conc. $H_2SO_4$ in this reaction.

Q: Write the structural formula of ethanol. What happens when it is heated with excess of conc. $H_2SO_4$ at 443 K? Write the chemical equation for the reaction stating the role of conc. $H_2SO_4$ in this reaction.

The structural formula of ethanol is: $$\text{H}-\overset{\underset{\lvert}{\text{H}}}{\underset{\lvert}{\text{C}}}-\overset{\underset{\lvert}{\text{H}}}{\underset{\lvert}{\text{C}}}-\text{O}-\text{H}$$ When ethanol is heated with excess concentrated $H_2SO_4$ at $443\text{ K}$, it undergoes dehydration to form ethene (an unsaturated hydrocarbon) and water. **Chemical Equation:** $$CH_3CH_2OH \xrightarrow{\text{Conc. } H_2SO_4, 443\text{ K}} CH_2=CH_2 + H_2O$$ **Role of concentrated $H_2SO_4$:** Concentrated sulphuric acid acts as a **dehydrating agent** because it removes a molecule of water from ethanol.

Chemical Reactions and Equations

Dehydration of Ethanol

Official Answer

The structural formula of ethanol is:

$\text{H}-\overset{\underset{\lvert}{\text{H}}}{\underset{\lvert}{\text{C}}}-\overset{\underset{\lvert}{\text{H}}}{\underset{\lvert}{\text{C}}}-\text{O}-\text{H}$

When ethanol is heated with excess concentrated $H_2SO_4$ at $443\text{ K}$ , it undergoes dehydration to form ethene (an unsaturated hydrocarbon) and water.

Chemical Equation:

$CH_3CH_2OH \xrightarrow{\text{Conc. } H_2SO_4, 443\text{ K}} CH_2=CH_2 + H_2O$

Role of concentrated $H_2SO_4$ :

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

structural formulaethenedehydrating agentCH2=CH2443 Kconc. H2SO4

Marking Scheme

10.5 marks for drawing the correct structural formula of ethanol.
20.5 marks for stating that ethene is formed.
31.0 mark for writing the complete, balanced chemical equation with correct reaction conditions ( $443\text{ K}$ , conc. $H_2SO_4$ ).
41.0 mark for correctly identifying the role of conc. $H_2SO_4$ as a dehydrating agent.

Hint

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

Quick Oral Answer

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

Analysis & Explanation

Ethanol ( $CH_3CH_2OH$ ) is a saturated alcohol. When treated with a strong acid like concentrated $H_2SO_4$ at a high temperature of $443\text{ K}$ ( $170^\circ\text{C}$ ), an intramolecular elimination reaction occurs. A hydrogen atom from the $\beta$ -carbon and the hydroxyl group ( $-\text{OH}$ ) from the $\alpha$ -carbon are eliminated as a water molecule ( $H_2O$ ), creating a double bond between the two carbon atoms to form ethene ( $CH_2=CH_2$ ). Concentrated sulphuric acid facilitates this removal, acting as a powerful dehydrating agent.

Common Mistakes

1Writing the molecular formula ( $C_2H_5OH$ ) instead of the structural formula.
2Writing the product as ethane ( $CH_3-CH_3$ ) instead of ethene ( $CH_2=CH_2$ ).
3Omitting the reaction conditions (concentrated $H_2SO_4$ and $443\text{ K}$ ) on the arrow.
4Confusing the role of the acid as an oxidizing agent instead of a 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!

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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.