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: Identify the oxidizing agent.
Acidified potassium dichromate (K₂Cr₂O₇ / H⁺) is a strong oxidizing agent. It contains dichromate ions (Cr₂O₇²⁻) which get reduced to chromium(III) ions (Cr³⁺) during the reaction.
Step 2: Oxidation product of ethanol.
Ethanol (C₂H₅OH) is a primary alcohol. On oxidation, it first forms acetaldehyde (ethanal, CH₃CHO) and then further oxidized to acetic acid (ethanoic acid, CH₃COOH) under controlled conditions.
Step 3: Observation during reaction.

• The orange color of dichromate solution turns green due to formation of Cr³⁺ ions.
• The pungent smell of acetaldehyde is observed initially, followed by the vinegar-like smell of acetic acid.

Step 4: Balanced chemical equation.
The overall reaction for complete oxidation of ethanol to acetic acid is: \[ \boxed{3\text{CH}_3\text{CH}_2\text{OH} + 2\text{K}_2\text{Cr}_2\text{O}_7 + 8\text{H}_2\text{SO}_4 \rightarrow 3\text{CH}_3\text{COOH} + 2\text{Cr}_2(\text{SO}_4)_3 + 2\text{K}_2\text{SO}_4 + 11\text{H}_2\text{O}} \]
Step 5: Simpler ionic equation.
The ionic equation can be written as: \[ 3\text{CH}_3\text{CH}_2\text{OH} + 2\text{Cr}_2\text{O}_7^{2-} + 16\text{H}^+ \rightarrow 3\text{CH}_3\text{COOH} + 4\text{Cr}^{3+} + 11\text{H}_2\text{O} \]
Step 6: Stepwise oxidation (for reference).
Step 1: Ethanol \(\Rightarrow\) Ethanal \[ \text{CH}_3\text{CH}_2\text{OH} + [O] \rightarrow \text{CH}_3\text{CHO} + \text{H}_2\text{O} \]
Step 2: Ethanal \(\Rightarrow\) Ethanoic acid \[ \text{CH}_3\text{CHO} + [O] \rightarrow \text{CH}_3\text{COOH} \] Part (II): Hydrogenation of ethene
Step 1: Identify the reactants.
Ethene (C₂H₄) is an unsaturated hydrocarbon containing a carbon-carbon double bond. Hydrogenation means addition of hydrogen (H₂) across the double bond.
Step 2: Conditions for hydrogenation.
The reaction requires:

• A catalyst (usually nickel, platinum, or palladium)
• Moderate temperature (about 200-300°C with Ni catalyst)

Step 3: Product formed.
Hydrogenation of ethene produces ethane (C₂H₆), which is a saturated hydrocarbon.
Step 4: Balanced chemical equation.
\[ \boxed{\text{CH}_2=\text{CH}_2 + \text{H}_2 \xrightarrow{\text{Ni catalyst}} \text{CH}_3-\text{CH}_3} \]
Step 5: Explanation.
The double bond breaks, and one hydrogen atom adds to each carbon atom, converting the double bond into a single bond. This reaction is an example of addition reaction.
Step 6: Importance.
Hydrogenation is used industrially to convert unsaturated oils into saturated fats (e.g., vegetable oils into vanaspati ghee).