Consider the following pairs of reactions and properties : Reaction — Property I. Fe(s) + 2HCl(aq) → FeCl₂(aq) + H₂(g) — Fe is reductant II. Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s) — Cu is being reduced III. Br₂(l) + 2I⁻(aq) → 2Br⁻(aq) + I₂(s) — Br₂ is reductant IV. CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l) — C atom in CH₄ is oxidized Which of the above pairs are correct?
- A.I and II only
- B.II, III and IV
- C.I, II and IV
- D.I, III and IV
Correct answer
Explanation
The correct answer is C, I, II and IV. Pairs I, II and IV describe the electron transfer correctly; pair III calls bromine a reductant when it is the oxidant.
A reductant loses electrons and is oxidised; an oxidant gains electrons and is reduced. In I, iron goes from 0 to +2, so it is the reductant. In II, copper goes from +2 in copper sulphate to 0 in copper metal, so copper is reduced. In III, bromine goes from 0 to −1: it gains electrons and oxidises iodide to iodine, so it is the oxidising agent. In IV, carbon goes from −4 in methane to +4 in carbon dioxide, so it is oxidised.
A is wrong because it leaves out pair IV, which is correct.
B is wrong because it includes pair III and leaves out pair I.
D is wrong because it includes pair III and leaves out pair II.
Exam tip: OIL RIG: oxidation is loss, reduction is gain of electrons; the substance reduced is the oxidant.
Practice Questions
View allThe potential difference between the terminals of an electrical heating device is 40 V, when it draws a current of 2 A from the source. Which one of the following is the correct value of current drawn by the heating device if the potential is increased to 140 V?
- A.7 A
- B.5 A
- C.4 A
- D.2 A
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By Ohm's law the heater's resistance is 40 V divided by 2 A, which is 20 ohms, so at 140 V it draws 140 divided by 20, or 7 A. Ohm's law states that, at constant temperature, the current through a conductor is directly proportional to the potential difference across…
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- A.1 only
- B.2 only
- C.Both 1 and 2
- D.Neither 1 nor 2
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- B.Short-sight defect can be corrected by a converging spectacle lens.
- C.For a person suffering from long-sight defect, the image of a near object is focused behind the retina.
- D.Long-sight defect can be corrected by a diverging spectacle lens.
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- A.Reflection
- B.Diffraction
- C.Refraction
- D.Dispersion
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Read the full explanation →Which one of the following statements is correct?
- A.The number of electrons in alpha particle is 2.
- B.The coordination number of EDTA is 6.
- C.The charge of beta (β) particle is +2.
- D.Both carbon and silicon form ionic compounds.
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