Three gas jars hold ethane, ethene and ethyne, but the labels have come off…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
Three gas jars hold ethane, ethene and ethyne, but the labels have come off. A student runs three tests on a sample from each jar.
Test 1: Bromine water. Jar X stays orange-brown. Jars Y and Z decolourise it.
Test 2: Ammoniacal silver nitrate. Jar Z gives a white precipitate. The others give none.
Test 3: Cold, dilute, alkaline permanganate. Jars Y and Z both turn it from purple to brown.
- Name the gas in each jar.
- How many moles of bromine does one mole of the gas of jar Z use up with excess bromine?
Need a reminder? The key ideaShow
Ozonolysis with zinc and water cuts C=C: gives methanal, an aldehyde and a ketone. Hot acidified permanganate gives carbon dioxide, an acid and a ketone. A terminal alkyne has an acidic hydrogen and gives precipitates with ammoniacal silver or copper(I). Hydration of an alkyne gives a ketone, or ethanal from ethyne.
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. X. It does not react with bromine or with permanganate: ethane.
Step 2. Z. The white precipitate with ammoniacal silver nitrate needs a hydrogen on a triple-bonded carbon: ethyne.
Step 3. Y. It is unsaturated but gives no precipitate: ethene.
Step 4. Bromine. Ethyne has two pi bonds. One mole adds two moles of bromine and gives 1,1,2,2-tetrabromoethane.
Answer: X is ethane, Y is ethene and Z is ethyne. One mole of Z uses up moles of bromine.
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