For each of the following balanced oxidation–reduction reactions, (i) identify the oxidation numbers for all the elements in the reactants and products and (ii) state the total number of electrons transferred in each reaction. (a) I₂O₅(s) + 5 CO(g) → I₂(s) + 5 CO₂(g) (b) 2 Hg²⁺(aq) + N₂H₄(aq) → 2 Hg(l) + N₂(g) + 4 H⁺(aq) (c) 3 H₂S(aq) + 2 H⁺(aq) + 2 NO₃^-(aq) → 3 S(s) + 2 NO(g) + 4 H₂O(l)

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Identify the oxidation numbers for each element in the reactants: Hg in Hg^{2+} is +2, N in N_2H_4 is -2, and H in N_2H_4 is +1.

Identify the oxidation numbers for each element in the products: Hg in Hg is 0, N in N_2 is 0, and H in H^+ is +1.

Determine which elements undergo a change in oxidation number: Hg changes from +2 to 0, and N changes from -2 to 0.

Calculate the change in oxidation number for each element: Hg changes by 2 per atom, and N changes by 2 per atom (for 2 N atoms, the total change is 4).

State the total number of electrons transferred: Since 2 Hg atoms are reduced and 2 N atoms are oxidized, the total number of electrons transferred is 4.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Oxidation Numbers

Oxidation numbers are assigned to atoms in a compound to indicate their degree of oxidation or reduction. They help in identifying which atoms are oxidized and which are reduced during a chemical reaction. The rules for assigning oxidation numbers include that the oxidation number of an element in its elemental form is zero, and for monoatomic ions, it equals the charge of the ion.

Oxidation and Reduction

Oxidation refers to the loss of electrons by a substance, resulting in an increase in its oxidation state, while reduction is the gain of electrons, leading to a decrease in oxidation state. In a redox reaction, one species is oxidized and another is reduced, and the total number of electrons lost in oxidation must equal the total number gained in reduction.

Balancing Redox Reactions

Balancing redox reactions involves ensuring that the number of atoms and the total charge are the same on both sides of the equation. This often requires identifying half-reactions for oxidation and reduction, balancing them separately, and then combining them while accounting for the electrons transferred. The total number of electrons transferred indicates the extent of the redox process.

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Textbook Question

Indicate whether each of the following statements is true or false: (c) If there are no changes in the oxidation state of the reactants or products of a particular reaction, that reaction is not a redox reaction.

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Textbook Question

Indicate whether each of the following statements is true or false: (a) If something is reduced, it is formally losing electrons. (b) A reducing agent gets oxidized as it reacts.

Textbook Question

Indicate whether each of the following statements is true or false: (c) An oxidizing agent is needed to convert CO into CO₂.

Textbook Question

For each of the following balanced oxidation–reduction reactions, (i) identify the oxidation numbers for all the elements in the reactants and products and (ii) state the total number of electrons transferred in each reaction. (a) 2 MnO₄^-(aq) + 3 S^2-(aq + 4 H₂O(l) → 3 S(s) + 2 MnO₂(s) + 8 OH^-(aq) (b) 4 H₂O₂(aq) + Cl₂O₇(g) + 2 OH^-(aq) → 2 ClO₂^-(aq) + 5 H₂O(l) + 4 O₂(g) (c) Ba²⁺(aq) + 2 OH^-(aq) + H₂O₂(aq) + 2 ClO₂(aq) → Ba(ClO₂)₂(s) + 2 H₂O(l) + O₂(g)

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Textbook Question

Indicate whether the following balanced equations involve oxidation–reduction. If they do, identify the elements that undergo changes in oxidation number. (a) PBr₃(l) + 3 H₂O(l) → H₃PO₃(aq) + 3 HBr(aq)

Textbook Question

Indicate whether the following balanced equations involve oxidation–reduction. If they do, identify the elements that undergo changes in oxidation number. (b) NaI(aq) + 3 HOCl(aq) → NaIO₃(aq) + 3 HCl(aq) (c) 3 SO(1g) + 2 HNO₃(aq) + 2 H₂O(l) → 3 H₂SO₄(aq) + 2 NO(g)