Experimental techniques and chemical analysis - Identification of ions and gases (3)
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1.
Question 2
Describe tests to identify the following gases:
- (d) Hydrogen, H2, using a lighted splint.
- (e) Oxygen, O2, using a glowing splint.
(d) Hydrogen (H2):
- A small amount of hydrogen gas is collected in a test tube.
- A lighted splint is brought to the mouth of the test tube.
- If the hydrogen gas ignites with a 'pop' sound, it indicates the presence of hydrogen.
Explanation: Hydrogen is flammable and reacts with oxygen to produce water. The 'pop' sound is the ignition of hydrogen with oxygen.
(e) Oxygen (O2):
- A glowing splint is held in a test tube containing oxygen gas.
- If the glowing splint re-ignites, it indicates the presence of oxygen.
Explanation: Oxygen supports combustion. A glowing splint will re-ignite when exposed to oxygen.
2.
(c) Describe the tests to identify the nitrate, NO3-, ion by reduction with aluminium foil and aqueous sodium hydroxide, then testing for ammonia gas.
Test 1: Reduction with Aluminium and Sodium Hydroxide
- A small amount of the unknown solution is mixed with a piece of aluminium foil in dilute aqueous sodium hydroxide (NaOH).
- The mixture is allowed to react.
- The aluminium reduces the nitrate ions (NO3-) to ammonia (NH3).
- The reaction is as follows: 4NO3-(aq) + 3Al(s) + 6H2O(l) → 3NH3(g) + 4Al(OH)3(aq)
Test 2: Testing for Ammonia Gas
- The gas evolved in the first test is passed into damp litmus paper.
- If ammonia gas is present, the damp litmus paper will turn blue.
- Alternatively, the gas can be passed into lime water (calcium hydroxide solution, Ca(OH)2). If ammonia is present, the lime water will turn milky due to the formation of calcium ammonium hydroxide.
- The balanced chemical equation for the reaction with lime water is: NH3(g) + Ca(OH)2(aq) → Ca(OH)2(aq) + NH3(aq)
Conclusion: If the aluminium foil reacts vigorously with sodium hydroxide and ammonia gas is produced, it confirms the presence of the nitrate ion.
3.
(e) Describe the tests to identify the sulfite, SO32-, ion by reaction with acidified aqueous potassium permanganate(VII).
Test: Reaction with Acidified Potassium Permanganate(VII)
- To the unknown solution, add a few drops of dilute nitric acid (HNO3) to acidify the solution.
- Then, add a few drops of acidified potassium permanganate (KMnO4) solution.
- Observe the solution for a colour change.
Observation and Conclusion:
- If the purple colour of the acidified potassium permanganate disappears, it confirms the presence of the sulfite ion. Reaction: 2SO32-(aq) + 2KMnO4(aq) + 2H+(aq) → 2Mn2+(aq) + 2K+(aq) + 2SO42-(aq) + 2H2O(l)
- If the purple colour remains, the sulfite ion is not present.