Understanding the Concept: Identifying the Strongest Oxidizing Agent

An oxidizing agent is a substance that gains electrons and is reduced in a chemical reaction. The strength of an oxidizing agent is determined by its standard reduction potential (E⁰). A higher (more positive) E⁰ value indicates a stronger oxidizing agent because it has a greater tendency to gain electrons and be reduced.

Step 1: List the Given Half-Reactions and Their E⁰ Values

We are given the standard reduction potentials for four half-reactions:

1. Reduction of Cr³⁺ to Cr: ${\mathrm{Cr}}^{3+}+3e^{-}\to \mathrm{Cr}$ ; $E_{{\mathrm{Cr}}^{3+}/\mathrm{Cr}}^0=-0.74V$
2. Reduction of MnO₄⁻ to Mn²⁺: ${\mathrm{MnO}}_4^{-}+8H^{+}+5e^{-}\to {\mathrm{Mn}}^{2+}+4H_{2}O$ ; $E_{{\mathrm{MnO}}_4^{-}/{\mathrm{Mn}}^{2+}}^0=1.51V$
3. Reduction of Cr₂O₇²⁻ to Cr³⁺: ${\mathrm{Cr}}_2{O}_7^{2-}+14H^{+}+6e^{-}\to 2{\mathrm{Cr}}^{3+}+7H_{2}O$ ; $E_{{\mathrm{Cr}}_2{O}_7^{2-}/{\mathrm{Cr}}^{3+}}^0=1.33V$
4. Reduction of Cl₂ to Cl⁻: ${\mathrm{Cl}}_2+2e^{-}\to 2{\mathrm{Cl}}^{-}$ ; $E_{\mathrm{Cl}/{\mathrm{Cl}}^{-}}^0=1.36V$

Step 2: Compare the E⁰ Values

Let's list the E⁰ values in descending order to find the most positive one:

• MnO₄⁻ / Mn²⁺: 1.51 V
• Cl / Cl⁻: 1.36 V
• Cr₂O₇²⁻ / Cr³⁺: 1.33 V
• Cr³⁺ / Cr: -0.74 V

Step 3: Identify the Strongest Oxidizing Agent

The species with the highest (most positive) standard reduction potential is the strongest oxidizing agent. From the list above, ${\mathrm{MnO}}_4^{-}$ (Permanganate ion) has the highest E⁰ value of 1.51 V.

Final Answer

The strongest oxidizing agent is MnO₄⁻.

Related Topics and Formulae

Key Theory

• Electrochemical Series: A list of elements and ions arranged in order of their standard electrode potentials. Oxidizing agents (substances that are reduced) are on the left side of the half-reactions.
• Standard Reduction Potential (E⁰): The tendency of a chemical species to gain electrons and thereby be reduced. It is measured under standard conditions (1 M concentration, 1 atm pressure, 25°C).
• Spontaneity of Reaction: A redox reaction is spontaneous if the E⁰_cell is positive. E⁰_cell = E⁰_cathode - E⁰_anode, where the cathode is the half-cell with the higher reduction potential.

Important Formulae

• Standard Cell Potential: $E_{cell}^0=E_{red}^0-E_{oxd}^0$
• Relationship to Gibbs Free Energy: $\mathrm{\Delta }{G}^0=-nF{E}_{cell}^0$ (where n is the number of electrons and F is the Faraday constant). A negative ΔG⁰ indicates a spontaneous reaction.