Question

Write chemical equations for the following :
(I) Oxidation of ethanol by acidified K₂Cr₂O₇.
(II) Hydrogenation of ethene.

Hint:

(I) Ethanol oxidation by acidified K₂Cr₂O₇: Orange to green; ethanol \(\Rightarrow\) ethanal \(\Rightarrow\) ethanoic acid. (II) Ethene hydrogenation: CH₂=CH₂ + H₂ (Ni catalyst) \(\Rightarrow\) CH₃-CH₃ (ethane).

Answer 1

Part (I): Oxidation of ethanol by acidified K₂Cr₂O₇

Step 1: Nature of the oxidizing agent.
Acidified potassium dichromate (K₂Cr₂O₇ in presence of H₂SO₄) acts as a strong oxidizing agent.
During the reaction, dichromate ions (Cr₂O₇²⁻) are reduced to chromium(III) ions (Cr³⁺).

Step 2: Nature of ethanol.
Ethanol (CH₃CH₂OH) is a primary alcohol.
Primary alcohols are easily oxidized first to aldehydes and then to carboxylic acids under strong oxidizing conditions.

Step 3: Stepwise oxidation.
First, ethanol is oxidized to ethanal:
CH₃CH₂OH + [O] → CH₃CHO + H₂O
Then, ethanal is further oxidized to ethanoic acid:
CH₃CHO + [O] → CH₃COOH

Step 4: Observation during reaction.
The orange color of acidified dichromate changes to green.
This color change occurs due to the formation of Cr³⁺ ions.

Step 5: Overall balanced equation.
3CH₃CH₂OH + 2K₂Cr₂O₇ + 8H₂SO₄ → 3CH₃COOH + 2Cr₂(SO₄)₃ + 2K₂SO₄ + 11H₂O

Conclusion:
Ethanol is oxidized to ethanoic acid by acidified potassium dichromate, accompanied by a color change from orange to green.

Part (II): Hydrogenation of ethene

Step 1: Nature of ethene.
Ethene (CH₂=CH₂) is an unsaturated hydrocarbon containing a carbon–carbon double bond.
Because of the double bond, it undergoes addition reactions.

Step 2: Meaning of hydrogenation.
Hydrogenation is the addition of hydrogen (H₂) across a double or triple bond in the presence of a catalyst.

Step 3: Conditions required.
The reaction is carried out in the presence of a catalyst such as nickel (Ni), platinum (Pt), or palladium (Pd).
Moderate heating is required when nickel is used.

Step 4: Chemical equation.
CH₂=CH₂ + H₂ → CH₃–CH₃ (in presence of Ni catalyst)

Step 5: Explanation of reaction.
The double bond between carbon atoms breaks.
Each carbon atom forms a new bond with a hydrogen atom.
The product formed is ethane, which is a saturated hydrocarbon.

Step 6: Importance.
Hydrogenation is used in industries to convert vegetable oils into solid fats such as vanaspati ghee.