This set of Chemistry Multiple Choice Questions & Answers (MCQs) focuses on “Oxidation Reduction”.
1. Non-metals are extracted by oxidation.
Explanation:Usually, non-metals occur in the combined state in the reduced form. Therefore, they are generally extracted or isolated by the oxidation of their compounds. Isolation of chlorine is an example.
2. Which of the following is chlorine isolated from by oxidation?
a) The atmosphere
Explanation: A common example of extraction of non-metals based on oxidation is the isolation of chlorine from brine as chlorine is abundant in sea water as common salt. The oxidation of brine is carried out via electrolysis.
3. What is the voltage at which electrolysis of brine takes place?
a) < 2.2 V
b) > 2.2 V
c) 2.2 V
d) Brine cannot be electrolysed
Explanation: According to theoretical calculations, the minimum potential difference required for the electrolysis of brine is 2.2 V. But, due to overvoltage, actual electrolysis requires an emf greater than 2.2 V.
4. Leaching process is an oxidation reaction.
Explanation: Leaching process is an oxidation reaction usually involved in the extraction of gold and silver. In this process, silver (Ag) is oxidised to Ag+ and gold(Au) is oxidised to Au+ which then combine with CN– ions to form complexes.
5. Which of the following is a product formed during the electrolysis of brine?
Explanation: Electrolysis of brine occurs at a potential about 2.2 V. During the electrolysis process, Cl2 is liberated at the anode and H2 is liberated at the cathode. NaOH is obtained in the solution as one of the by-products of the process.
6. Which of the following is a complex formed during the extraction of silver via leaching?
Explanation: During the process of leaching, silver (Ag) is first oxidised to Ag+ and this Ag+ then combines with CN– ions to form a soluble complex. The complex formed by silver in this case is, [Ag(CN)2]–.
7. What is the process used to recover the metal from its soluble complex obtained after leaching?
d) It cannot be obtained
Explanation: Silver and gold form their respective complexes with CN– ions during leaching process. The metals are then obtained from these complexes by reduction or displacement method using a more electropositive metal.
8. Which of the following is used to reduce the metal complexes obtained during leaching process?
Explanation: To obtain the metals from their metal complexes, they are made to undergo a reduction process wherein the metal in the complex is displaced by a more electropositive metal. Zinc is used to reduce these complexes as it is a metal with higher electro positivity than gold and silver.
9. Which of the following processes involves the dissolution of the ore in a suitable reagent followed by precipitation of the metal by a more reactive metal?
a) Froth floatation
Explanation: The process of extraction of metals by dissolving the ore in a suitable reagent followed by precipitation or displacement of the metal by a more reactive or more electropositive metal is called hydrometallurgy.
10. Which of the following metals cannot be extracted via hydrometallurgy?
Explanation: For hydrometallurgy, the ore needs to be able to dissolve in a reagent and the metal should form a soluble complex. Copper, gold and silver ores are more suitable to be obtained via hydrometallurgy and iron is usually obtained via the blast furnace.
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