Consider the following statements regarding oxidation numbers : I. The oxidation number of oxygen in peroxides and superoxides is assigned as -1 and -1/2 respectively. II. In all its compounds, fluorine has an oxidation number of -1. III. In a reduction half-reaction, the oxidation number of the reductant decreases. Which of the statements given above are correct?
- A.I and II only
- B.II and III only
- C.I and III only
- D.I, II and III
Correct answer
Explanation
The correct answer is A, I and II only. Statements I and II are standard rules for oxidation numbers, but Statement III reverses what happens to a reducing agent.
Statement I is correct: in peroxides such as H2O2 and Na2O2 the O–O link gives each oxygen −1, and in superoxides such as KO2 each oxygen carries −1/2, against the usual −2. Statement II is correct: fluorine is the most electronegative element, so it always takes −1 in its compounds, even in OF2, where oxygen becomes +2. Statement III is wrong: the reductant is the substance that loses electrons and is itself oxidised, so its oxidation number increases. It is the oxidant whose oxidation number decreases.
B is wrong because it includes the incorrect Statement III and leaves out the correct Statement I.
C is wrong because it includes Statement III and leaves out Statement II.
D is wrong because Statement III is not correct.
Exam tip: reductant loses electrons and its oxidation number rises; oxidant gains electrons and its number falls.
Practice Questions
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