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Difficulty: MediumDefinitions and Classical vs Modern Concepts of Redox
Consider the redox reaction represented by the following equation:
CuO(s)+H2(g)→Cu(s)+H2O(g)\text{CuO}_{(s)} + \text{H}_{2(g)} \rightarrow \text{Cu}_{(s)} + \text{H}_2\text{O}_{(g)}
Which of the following statements correctly describes hydrogen (H2\text{H}_2) from both classical and modern perspectives of redox?
  1. Classically, hydrogen is oxidized because it gains oxygen; modernly, it is oxidized because its oxidation state increases from 0 to +1.Answer
  2. B
    Classically, hydrogen is reduced because it gains oxygen; modernly, it is oxidized because it gains electrons.
  3. C
    Classically, hydrogen is oxidized because it accepts electrons; modernly, its oxidation state decreases from +1 to 0.
  4. D
    Classically, hydrogen is reduced because it acts as an oxygen donor; modernly, it is reduced because its oxidation state increases from 0 to +1.

Answer

Classically, hydrogen is oxidized because it gains oxygen; modernly, it is oxidized because its oxidation state increases from 0 to +1.
In the given reaction, hydrogen gas (H2\text{H}_2) combines with oxygen to form water. Classically, gaining oxygen is defined as oxidation. Modernly, the oxidation number of hydrogen increases from 0 (in H2\text{H}_2) to +1 (in H2O\text{H}_2\text{O}), which also represents oxidation (loss of electrons). Thus, both concepts agree that hydrogen undergoes oxidation.

Step-by-Step Solution

1
Analyze hydrogen under the classical concept of redox.
In CuO+H2→Cu+H2O\text{CuO} + \text{H}_2 \rightarrow \text{Cu} + \text{H}_2\text{O}, elemental hydrogen (H2\text{H}_2) gains oxygen to form water (H2O\text{H}_2\text{O}). Under the classical definition, oxidation is defined as the addition or gain of oxygen.
Classical redox concepts define oxidation as the addition of oxygen (or removal of hydrogen) to a substance.
2
Analyze hydrogen under the modern concept of redox.
The oxidation state of free elemental hydrogen (H2\text{H}_2) is 0. In water (H2O\text{H}_2\text{O}), hydrogen has an oxidation state of +1. The oxidation state increases from 0 to +1, which corresponds to a loss of electrons.
Modern redox concepts define oxidation as a loss of electrons or an increase in oxidation state.
3
Synthesize classical and modern findings to select the correct description.
Hydrogen is oxidized under both classical (gain of oxygen) and modern (increase in oxidation number from 0 to +1) definitions.
Both definitions consistently classify hydrogen as undergoing oxidation in this reaction.

Key Concept

Comparison of Classical (Oxygen/Hydrogen Transfer) and Modern (Electron Transfer/Oxidation Number) Concepts of Redox
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Topics

Definitions and Classical vs Modern Concepts of Redox

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