After breathing in a test tube that contains acidified K 2 Cr 2 O 7 , a man noticed a change in the colour of…

Analysis

The question describes a classic chemical test used to identify specific functional groups in organic chemistry. We are observing a color change in acidified potassium dichromate(VI) ($K_2Cr_2O_7$) from orange to green.

Concept Explanation

• The Reagent: Acidified potassium dichromate ($K_2Cr_2O_7$) is a strong oxidizing agent.
• The Reaction: When it reacts with certain organic compounds, the dichromate ion ($Cr_2O_7^{2-}$, which is orange) is reduced to the chromium(III) ion ($Cr^{3+}$, which is green).
• The Functional Group: This specific color change occurs when the reagent is reduced by a substance that can be oxidized. Alkanols (primary and secondary alcohols) are easily oxidized by acidified $K_2Cr_2O_7$. • Primary alcohols are oxidized to alkanals (aldehydes) and then to alkanoic acids. • Secondary alcohols are oxidized to alkanones (ketones). • Alkanals can also be oxidized, but in the context of breathalyzer tests (which this question describes), the test is specifically designed to detect the presence of ethanol (an alkanol) in the breath.

Step-by-Step Solution

1. Identify the reactant: The man is breathing into the test tube. If he has consumed alcohol, his breath will contain ethanol vapor ($C_2H_5OH$).
2. Analyze the chemical change: Ethanol is a primary alkanol. When it comes into contact with the acidified $K_2Cr_2O_7$, the ethanol is oxidized to ethanal and then to ethanoic acid.
3. Observe the color change: During this redox reaction, the chromium in the dichromate is reduced from an oxidation state of +6 (orange) to +3 (green).
4. Evaluate the options: • Alkanoic acids and alkanones are generally resistant to further oxidation by this reagent under these conditions. • Alkanols are the primary targets for this oxidation test.

Conclusion

The change from orange to green confirms the presence of an alkanol (specifically ethanol in the context of a breath test).

Exam Tip

Remember the mnemonic "Alcohol is Green" for this test. If you see "acidified potassium dichromate" and "orange to green," immediately look for an alcohol (alkanol) in your options. This is a very common question in JAMB Chemistry!