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Sodium sulfite and barium chloride are both white crystalline salts - Leaving Cert Chemistry - Question 3 - 2021
Question 3
Sodium sulfite and barium chloride are both white crystalline salts. Describe how a student could have carried out flame tests to distinguish between unlabelled sam... show full transcript
Worked Solution & Example Answer:Sodium sulfite and barium chloride are both white crystalline salts - Leaving Cert Chemistry - Question 3 - 2021
Step 1
Describe how a student could have carried out flame tests
Answer
To perform flame tests, the student should first ensure that they use a clean platinum (nichrome) wire. This can be done by dipping the wire in concentrated hydrochloric acid and placing it in a clean flame to remove any residual substances.
Next, the student should then take a small sample of sodium sulfite and place it at the end of the wire. The wire should then be held in the blue part of the Bunsen burner flame. A positive result for sodium sulfite would show a yellow flame.
For barium chloride, the same procedure should be followed. When the wire is dipped in the barium chloride sample and placed in the flame, a green color should be observed, indicating the presence of barium.
Step 2
Describe how the student could have first carried out a test on a sample of the sodium sulfite solution
Answer
The student should add dilute hydrochloric acid (HCl) to a small amount of the sodium sulfite solution. Observations should be made for any effervescence, indicating that sulfur dioxide (SO₂) gas is produced. This confirms that sulfite ions are present, as they will react with the acid to release SO₂.
Step 3
What was observed as bromine water was added dropwise to another sample of this sodium sulfite solution in a test-tube?
Answer
As bromine water is added dropwise to the sodium sulfite solution, a red to brown color may appear initially, indicating the presence of sulfate ions as the bromine oxidizes the sulfite ions to sulfate. The solution may also become colorless as the bromine is consumed in the reaction.
Step 4
How could the student have verified that the sulfite in this sample was oxidised to sulfate ion by the bromine water?
Answer
The student could verify the oxidation by performing a subsequent test. After adding bromine water, a small amount of barium chloride solution could be added to the sample. If a white precipitate of barium sulfate (BaSO₄) forms, it would confirm that sulfate ions were generated, indicating that sulfite ions were oxidized by bromine water.
Step 5
What information do these observations provide about the relative reducing abilities of magnesium metal and copper metal?
Answer
The observations suggest that magnesium is a stronger reducing agent than copper. This is evident as magnesium can reduce copper(II) sulfate, removing the blue color entirely, while copper cannot reduce magnesium ions due to its lower reactivity in comparison.
Step 6
What information do these observations provide about the relative reducing abilities of zinc metal and copper metal?
Answer
Similar to magnesium, zinc is also a stronger reducing agent compared to copper. When reacting with copper(II) sulfate, zinc will displace copper, proving its greater ability to reduce.
Step 7
Step 8
Suggest a reason why the blue colour completely disappeared in the case of adding magnesium to the copper(II) sulfate solution but only faded in the case of adding the zinc.
Answer
The blue color disappears completely with magnesium because it is a more reactive metal that fully reduces copper(II) ions to metallic copper, thus removing the copper from solution. In contrast, zinc only partially reduces copper(II) ions, leading to a fading of the color rather than complete disappearance, since it is less reactive compared to magnesium.