Question Video: Identifying Which Species Are Oxidized and Reduced by Determining the Changes in Oxidation Numbers Chemistry • Second Year of Secondary School

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Consider the redox reaction 2Cu₂O (s) + Cu₂S (s) → 6Cu (s) + SO₂ (g), Which of the following statements is correct? [A] Copper is reduced from +1 to 0, and sulfur is oxidized from −2 to +4. [B] Copper is oxidized from +1 to 0, and sulfur is reduced from −2 to +4. [C] Copper is reduced from 0 to +1, and sulfur is oxidized from +4 to −2. [D] Copper is oxidized from 0 to +1, and sulfur is reduced from +4 to −2.

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Video Transcript

Consider the redox reaction two Cu2O solid plus Cu2S solid react to form six Cu solid plus SO2 gas. Which of the following statements is correct? (A) Copper is reduced from positive one to zero, and sulfur is oxidized from negative two to positive four. (B) Copper is oxidized from positive one to zero, and sulfur is reduced from negative two to positive four. (C) Copper is reduced from zero to positive one, and sulfur is oxidized from positive four to negative two. (D) Copper is oxidized from zero to positive one, and sulfur is reduced from positive four to negative two.

A redox reaction involves the transfer of electrons. In a redox reaction, both oxidation and reduction occur. Species that lose electrons are undergoing oxidation. Species that gain electrons during the reaction are undergoing reduction. To help keep track of how electrons move during a redox reaction, chemists use oxidation numbers. Oxidation leads to an increase in the oxidation number. On the other hand, reduction leads to a decrease in the oxidation number. In the reactant copper(I) oxide, copper has an oxidation number of positive one and oxygen has an oxidation number of negative two. Similarly, in the reactant copper(I) sulfide, copper has an oxidation number of positive one and sulfur has an oxidation number of negative two.

Moving to the product side of the chemical equation, we see that the first product is elemental copper. Atoms in their elemental form have an oxidation number of zero. The oxidation number of oxygen in most compounds is negative two. In sulfur dioxide, there is one sulfur atom and two oxygen atoms. And the oxidation numbers of these atoms must add up to zero. Therefore, the oxidation number of sulfur must be positive four.

Now, we can see that the oxidation number of copper decreased from positive one to zero. Therefore, copper was reduced. We can also see that the oxidation number of sulfur increased from negative two to positive four. This means sulfur was oxidized. We probably also noticed that the oxidation number of oxygen did not change in this reaction. In conclusion, the statement that is correct is “copper is reduced from positive one to zero, and sulfur is oxidized from negative two to positive four,” or answer choice (A).