A mixture of ethanol, sodium dichromate, and water is allowed to drop gradually into a boiling solution containing one part concentrated H2SO
4 and the liberate gas is collected by cooling it with ice water to give a liquid. This liquid is?
ethanoic acid
acetaldehyde
ethyl acetate
methanol
ethylene
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The reaction described in the question is an oxidation of ethanol with sodium dichromate and sulfuric acid. The balanced chemical equation for this reaction is:
3 CH3CH2OH + Na2Cr2O7 + 8 H2SO4 → 3 CH3COOH + Cr2(SO4)3 + Na2SO4 + 11 H2O
In this reaction, ethanol (CH3CH2OH) is oxidized to acetic acid (CH3COOH), while the sodium dichromate (Na2Cr2O7) acts as an oxidizing agent, and the sulfuric acid (H2SO4) acts as a catalyst.
The liberated gas in this reaction is carbon dioxide (CO2), which is formed as a byproduct of the reaction.
When the gas is collected by cooling it with ice water, it condenses to form a liquid, which is a mixture of water, acetic acid, and sulfuric acid. The exact composition of the liquid will depend on the conditions of the reaction, including the concentrations of the reagents and the temperature and pressure at which the gas is collected
The reaction you’re referring to is the oxidation of ethanol using acidified sodium dichromate. In this reaction, ethanol is oxidized to form ethanal (acetaldehyde), and with further oxidation, ethanoic acid (acetic acid) is formed.
However, the product depends on the reaction conditions. If an excess amount of ethanol is used and the aldehyde is distilled off as soon as it forms, then the reaction stops at the aldehyde stage, producing ethanal1. If the reaction is allowed to continue, the ethanal can be further oxidized to ethanoic acid1.
Given that the gas is collected by cooling it with ice water to give a liquid, it suggests that the reaction was not allowed to proceed to the carboxylic acid stage. Therefore, the liquid is likely to be B. acetaldehyde (ethanal). However, without more specific information about the reaction conditions, it’s difficult to say definitively. Under different conditions, the product could potentially be ethanoic acid. But based on the given information, B. acetaldehyde is the most likely answer.
Understanding the Reaction
The question describes a classic organic chemistry reaction: the oxidation of ethanol using sodium dichromate in an acidic environment (sulfuric acid). Here's what's happening:
* Ethanol (CH3CH2OH): This is the starting material, a primary alcohol.
* Sodium Dichromate (Na2Cr2O7): This is a strong oxidizing agent. It provides the oxygen needed for the reaction.
* Sulfuric Acid (H2SO4): This creates an acidic environment, which is essential for the reaction to proceed. It also helps in the formation of chromic acid (H2CrO4), the active oxidizing agent.
* Heat: The boiling solution provides the necessary energy for the reaction to occur.
Oxidation of Alcohols
Primary alcohols, like ethanol, undergo oxidation in a stepwise manner:
* First Oxidation: Ethanol is oxidized to acetaldehyde (CH3CHO). This is an aldehyde.
* Second Oxidation: If a strong oxidizing agent is present (like in this case), the reaction doesn't stop at the aldehyde stage. Acetaldehyde is further oxidized to ethanoic acid (CH3COOH), also known as acetic acid. This is a carboxylic acid.
Why Ethanoic Acid is the Final Product
The key to understanding why the final product is ethanoic acid lies in the strength of the oxidizing agent and the reaction conditions:
* Strong Oxidizing Agent: Sodium dichromate in acidic solution is a powerful oxidizing agent. It can fully oxidize a primary alcohol to a carboxylic acid.
* Heat: The boiling temperature favors the complete oxidation to ethanoic acid.
* Excess Oxidizing Agent: The problem states "a mixture of ethanol, sodium dichromate, and water is allowed to drop gradually," implying that there's enough oxidizing agent to drive the reaction to completion.
Eliminating Other Options
* Acetaldehyde (B): While acetaldehyde is an intermediate product, it's not the final product under these conditions.
* Ethyl Acetate (C): Ethyl acetate is an ester, formed by the reaction of ethanol and ethanoic acid. This reaction would require different conditions and isn't favored in this scenario.
* Methanol (D): Methanol is a different alcohol (CH3OH). It's not involved in this reaction.
* Ethylene (E): Ethylene is an alkene (C2H4). It's formed by the dehydration of ethanol, which would require a different set of conditions (e.g., concentrated sulfuric acid at high temperature without an oxidizing agent).
Conclusion
Therefore, the liquid collected after cooling the liberated gas is ethanoic acid (A).
Alkanols are oxidized to alkanals (acetaldehyde) then further oxidation to alkanoic acids
The most likely answer is **B. acetaldehyde**.
Here's the reasoning behind this answer:
- Oxidation of Ethanol: Sodium dichromate in the presence of sulfuric acid is a powerful oxidizing agent. When ethanol is oxidized, the primary product is acetaldehyde (ethanal). Given the conditions described in the problem (dropping ethanol into a boiling solution with concentrated sulfuric acid and sodium dichromate), the expected oxidation product would be acetaldehyde.
- Distillation and Collection: When the mixture is added to a boiling solution, the resulting acetaldehyde (being a volatile compound with a lower boiling point) will be liberated as a gas. If this gas is collected and cooled, it will condense into liquid acetaldehyde.
Given these points, it's most likely that the liquid collected after cooling is acetaldehyde.
