(c) Why is it impossible to measure the standard reduction potential of a single half-reaction?

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Standard reduction potentials are measured relative to a reference electrode, typically the standard hydrogen electrode (SHE).

A single half-reaction cannot occur independently; it must be paired with another half-reaction to form a complete redox reaction.

The standard reduction potential of a half-reaction is determined by measuring the voltage of a galvanic cell composed of the half-reaction and the SHE.

Without a complete redox reaction, there is no flow of electrons, and thus no potential difference to measure.

Therefore, it is impossible to measure the standard reduction potential of a single half-reaction in isolation.

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Key Concepts

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

Standard Reduction Potential

Standard reduction potential (E°) is a measure of the tendency of a chemical species to gain electrons and be reduced. It is defined under standard conditions (1 M concentration, 1 atm pressure, and 25°C) and is typically measured against a standard reference electrode, such as the standard hydrogen electrode. This potential is crucial for understanding redox reactions and predicting the direction of electron flow.

Half-Reaction

A half-reaction represents either the oxidation or reduction process in a redox reaction, showing the transfer of electrons. Each half-reaction can be written separately, but they cannot exist in isolation when measuring potentials. To determine the standard reduction potential, both half-reactions must be considered together, as they are interdependent in a complete redox process.

Reference Electrode

A reference electrode is a stable and well-defined electrode used as a comparison point for measuring the potential of other electrodes. The standard hydrogen electrode (SHE) is the most common reference, with a defined potential of 0 V. Since the potential of a half-reaction is relative, it is impossible to measure the potential of a single half-reaction without referencing it to another half-reaction, making it necessary to use a complete cell setup.

Related Practice

Textbook Question

Using standard reduction potentials (Appendix E), calculate the standard emf for each of the following reactions: (c) Fe1s2 + 2 Fe3+1aq2 ¡ 3 Fe2+1aq2

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Textbook Question

(b) Write the half-reaction that occurs at a hydrogen electrode in acidic aqueous solution when it serves as the anode of a voltaic cell.

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Textbook Question

A voltaic cell that uses the reaction PdCl₄^2-(aq) + Cd(s) → Pd(s) + 4 Cl^-(aq) + Cd²⁺(aq) has a measured standard cell potential of +1.03 V. (c) Sketch the voltaic cell, label the anode and cathode, and indicate the direction of electron flow