Dehydration of Ethanol to Ethene | Class 10 Science

Quick answer: When ethanol is heated at 443 K with excess concentrated sulphuric acid, it loses a molecule of water and forms ethene: $\mathrm{CH_3CH_2OH \xrightarrow{\text{hot conc. } H_2SO_4,\ 443\ K} CH_2{=}CH_2 + H_2O}$. This is called dehydration. Concentrated sulphuric acid acts as a dehydrating agent: it removes water from the ethanol.

Ethanol is a saturated compound: all its carbon–carbon bonds are single bonds. When it is heated with concentrated sulphuric acid, a molecule of water is removed from each ethanol molecule, and an unsaturated compound, ethene, is formed. Ethene has a carbon–carbon double bond.

In CBSE Class 10 Science, Chapter 4 (Carbon and its Compounds), the dehydration of ethanol and the role of concentrated sulphuric acid are frequently asked board exam questions.

What You Will Learn

• What a dehydration reaction is
• The equation for converting ethanol to ethene
• The role of concentrated sulphuric acid
• Why the reaction conditions (443 K, excess acid) matter
• How to show that the gas formed is unsaturated
• How dehydration differs from the oxidation of ethanol

1. What Is Dehydration?

Definition: A dehydration reaction is one in which a molecule of water is removed from a compound.

In the dehydration of ethanol, an $\mathrm{-OH}$ group is removed from one carbon atom and an $\mathrm{H}$ atom from the neighbouring carbon atom. Together they form $\mathrm{H_2O}$, and a double bond forms between the two carbon atoms.

2. Ethanol to Ethene

Equation

$\mathrm{CH_3{-}CH_2{-}OH \xrightarrow{\text{hot conc. } H_2SO_4,\ 443\ K} CH_2{=}CH_2 + H_2O}$

Conditions

• Temperature: $443\text{ K}$ ($170^\circ\text{C}$)
• Reagent: excess concentrated sulphuric acid

3. Role of Concentrated Sulphuric Acid

Common board question: What is the role of concentrated sulphuric acid when ethanol is heated with it at 443 K?

Answer: Concentrated sulphuric acid acts as a dehydrating agent. It removes a molecule of water from ethanol, converting it into ethene.

4. Why the Conditions Matter (Extra)

Extra fact (beyond the Class 10 textbook): The product depends on the conditions. At about $443\text{ K}$ with excess acid, ethene is the main product. At a lower temperature, about $413\text{ K}$, with excess ethanol, a different compound called diethyl ether ($\mathrm{C_2H_5{-}O{-}C_2H_5}$) is formed instead.

So in your answer, always write 443 K and concentrated $\mathrm{H_2SO_4}$ with the reaction arrow.

5. Showing That Ethene Is Unsaturated (Extra)

Ethene has a carbon–carbon double bond, so it takes part in addition reactions. A common laboratory test is to bubble the gas through bromine water, which is reddish-brown. Ethene adds bromine across its double bond, and the colour disappears:

$\mathrm{CH_2{=}CH_2 + Br_2 \longrightarrow CH_2Br{-}CH_2Br}$

Saturated compounds such as ethanol do not decolourise bromine water in this way.

6. Dehydration vs Oxidation of Ethanol

7. Summary Table

8. Exam Tips and Common Mistakes

Exam Tip: Always write the conditions with the arrow: hot conc. $\mathrm{H_2SO_4}$, 443 K.

Exam Tip: Use the term dehydrating agent when describing the role of concentrated sulphuric acid.

Common Mistake: Confusing dehydration with oxidation. Dehydration gives ethene; oxidation gives ethanoic acid.

Common Mistake: Writing the product as ethane ($\mathrm{C_2H_6}$). The product is ethene ($\mathrm{C_2H_4}$), which has a double bond.

Frequently Asked Questions

What happens when ethanol is heated with concentrated sulphuric acid at 443 K?

Ethanol loses a molecule of water and forms ethene: CH₃CH₂OH → CH₂=CH₂ + H₂O.

What is the role of concentrated sulphuric acid in this reaction?

It acts as a dehydrating agent, removing water from ethanol.

What is a dehydration reaction?

A reaction in which a molecule of water is removed from a compound.

What is the product of oxidation of ethanol?

Ethanoic acid, using alkaline potassium permanganate or acidified potassium dichromate.

Is ethene saturated or unsaturated?

Ethene is unsaturated, because it has a carbon–carbon double bond.