MediumChemistryHigh School (11–12)Organic Chemistry: Some Basic Principles and Techniques › Purification and Analysis of Organic CompoundsConceptualA-LevelIBNEET
A student adds silver nitrate straight to the sodium fusion extract of a compound that has both…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
A student adds silver nitrate straight to the sodium fusion extract of a compound that has both nitrogen and sulphur, and sees a black precipitate. The student writes that the compound holds a halogen. Is that right? Explain, and say what should have been done first.
Need a reminder? The key ideaShow
Carbon is shown by carbon dioxide turning lime water milky, and hydrogen by water turning white copper sulphate blue. Fusion with sodium turns N, S and the halogens into CN, S and X ions. Nitrogen: Prussian blue. Sulphur: black PbS or violet with nitroprusside. Nitrogen and sulphur together: blood red . Halogens: boil with nitric acid first, then silver nitrate. Phosphorus: a yellow colour with ammonium molybdate.
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. The black solid. The extract holds cyanide and sulphide ions. Sulphide gives black silver sulphide, , and cyanide gives a white precipitate of silver cyanide.
Step 2. The conclusion. A black precipitate is a sign of sulphur. It says nothing about a halogen.
Step 3. The fix. Boil the extract with dilute nitric acid first. It drives off hydrogen cyanide and hydrogen sulphide. Only a halide is then left to react with silver ions.
Answer: no, the claim is wrong. The extract should have been boiled with dilute nitric acid first.
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