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Difficulty: HardDefinitions and Classical vs Modern Concepts of Redox
Consider the following chemical reaction:
H2S(g)+Cl2(g)→2HCl(g)+S(s)\text{H}_2\text{S}_{(g)} + \text{Cl}_{2(g)} \rightarrow 2\text{HCl}_{(g)} + \text{S}_{(s)}
Which of the following statements correctly interprets the chemical behavior of hydrogen sulfide (H2S\text{H}_2\text{S}) using both classical and modern concepts of oxidation and reduction?
  1. It is oxidized according to the classical definition because it loses hydrogen, and acts as a reducing agent according to the modern definition because sulfur increases its oxidation state from −2-2 to 00.Answer
  2. B
    It is reduced according to the classical definition because it loses hydrogen, and acts as an oxidizing agent according to the modern definition because sulfur gains electrons.
  3. C
    It acts as an oxidizing agent according to the classical definition because it donates hydrogen to chlorine, and is reduced according to the modern definition because sulfur loses electrons.
  4. D
    It is oxidized according to the modern definition because the oxidation number of hydrogen increases, and acts as an oxidizing agent according to the classical definition by gaining chlorine.

Answer

Hydrogen sulfide (H2S\text{H}_2\text{S}) is oxidized according to the classical definition because it loses hydrogen, and acts as a reducing agent according to the modern definition because sulfur increases its oxidation state from −2-2 to 00.
The correct option correctly applies both definitions: under the classical concept, loss of hydrogen from hydrogen sulfide (converting it to elemental sulfur) is oxidation. Under the modern electronic concept, the sulfur atom in hydrogen sulfide changes its oxidation number from -2 to 0, which represents a loss of electrons (oxidation), thereby acting as a reducing agent.

Step-by-Step Solution

1
Analyze H2S\text{H}_2\text{S} using the classical concept of redox (hydrogen transfer).
In the reaction, H2S\text{H}_2\text{S} is converted into elemental sulfur (S\text{S}). Since H2S\text{H}_2\text{S} loses hydrogen atoms, it undergoes oxidation under the classical definition.
Classically, oxidation is defined as the addition of oxygen or the removal of hydrogen.
2
Determine the oxidation state of sulfur in reactants and products.
In H2S\text{H}_2\text{S}, hydrogen has an oxidation number of +1+1, so sulfur has an oxidation number of −2-2. In elemental sulfur (S\text{S}), the oxidation number is 00.
Uncombined elements have an oxidation number of zero.
3
Analyze H2S\text{H}_2\text{S} using the modern concept of redox (electron transfer and oxidation number change).
Sulfur goes from −2-2 to 00, which is an increase in oxidation state (loss of 2 electrons per sulfur atom).
An increase in oxidation state (loss of electrons) is oxidation. The substance that undergoes oxidation causes reduction in the other reactant, making H2S\text{H}_2\text{S} the reducing agent.

Key Concept

Classical vs. Modern Redox Concepts
Estimated Time:2m 0s
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Topics

Definitions and Classical vs Modern Concepts of Redox

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