Questions and Answers
What volume of filtrate is used in the nitrogen detection test?
3 ml
How many drops of ferrous sulfate solution are added during the nitrogen detection test?
4 drops
To make the solution basic, what percentage of NaOH solution is used?
10%
For how long should the solution be heated to boil during the nitrogen detection test?
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What indicates a positive test for nitrogen?
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What is the role of acetic acid in the detection of sulfur?
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Which compound forms a black precipitate indicating the presence of sulfur?
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If lead acetate is omitted from the experiment, what would be the observed result?
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Which product is NOT formed in the reaction involving sodium sulfide and lead acetate?
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What is the chemical formula for lead sulfide?
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In the preparation of sodium ferrocyanide, what role does NaOH play in the reaction?
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Which compound is formed as an intermediate before sodium ferrocyanide during the preparation steps?
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What is the overall charge of the ferric ferrocyanide complex Fe4[Fe(CN)6]3?
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Which of the following statements is incorrect regarding the structure of sodium ferrocyanide?
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During the preparation of ferric ferrocyanide, what is the by-product formed along with ferric ferrocyanide?
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What is the primary purpose of adding alcohol to the test tube containing excess unreacted sodium before breaking it with heating?
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Why should the test tube be cautiously dropped into a beaker containing 20ml of D.W.?
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What is the expected appearance of the filtrate after the solution is heated to boiling and filtered?
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Which chemical reaction correctly describes the formation of sodium alkoxide in the experiment?
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What should be done if the test tube does not break down after being dropped into the solution?
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Study Notes
Steps for the Experiment
- Cautiously drop the hot test tube into a beaker containing 20ml of distilled water to break it down.
- Use a glass rod if the tube doesn't break down.
- Heat the resulting solution to boiling and filter it.
- The filtrate should be colorless and is used for specific tests.
Removing Excess Sodium
- Add a small quantity of alcohol to the test tube before breaking it with heating to react with excess sodium, producing sodium alkoxide.
- The chemical equation for this reaction is 2Na + 2CH₃CH₂OH → 2NaOCH₂CH₃ + H₂.
Detection of Nitrogen
- Add 4 drops (0.2 gm) of ferrous sulfate solution to 3 ml of the filtrate.
- Make the solution basic by adding enough NaOH solution (10%).
- Heat the solution for 30 seconds and then make it acidic by adding drops of dilute H₂SO₄.
- A Prussian blue precipitate indicates a positive test for nitrogen.
Preparation of Ferrocyanides
- Sodium cyanide (NaCN) is prepared by fusion of sodium, carbon, and nitrogen from an organic compound.
- NaCN reacts with ferrous sulfate to form ferricyanide and sodium sulfate in the presence of basic NaOH.
- Ferricyanide reacts with sodium cyanide to form sodium ferrocyanide.
- Sodium ferrocyanide reacts with ferric sulfate to form ferric ferrocyanide, which has a blue color and is formed during boiling.
Chemical Structure
- The structure of ferric ferrocyanide is a square planar complex with a central iron atom bonded to four cyanide ions.
- Each cyanide ion is bonded to the nitrogen atom of a neighboring cyanide ion.
- The overall charge of the complex is -4, which is balanced by four sodium ions.
Chemical Formula
- The chemical formula for ferric ferrocyanide is [Fe(CN)₆]³⁻.
- The chemical formula for sodium ferrocyanide is [Fe(CN)₆]⁴⁻.
Detection of Sulfur
- Acidify 2 ml of the filtrate with dilute acetic acid.
- Add 5 drops of lead acetate solution to the acidified filtrate.
- A black precipitate of lead sulfide indicates the presence of sulfur.
- The chemical equation for this reaction is Na₂S + (CH₃COO)₂Pb → PbS + 2CH₃COONa.
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