Question

(a) What conditions must be met for a reduction potential to be a standard reduction potential? (b) What is the standard reduction potential of a standard hydrogen electrode? (c) Why is it impossible to measure the standard reduction potential of a single half reaction?

Short answer

(a) A reduction potential is considered standard when three conditions are met: (1) temperature is 25°C (298K), (2) participating solutes have a concentration of 1M, and (3) pressure of gaseous reactants or products is 1 atm. These conditions result in a standard reduction potential (E°). (b) The standard reduction potential of a standard hydrogen electrode (SHE) is 0 V, as it is used as a reference electrode. (c) Measuring the standard reduction potential of a single half-reaction is impossible because a complete electrochemical cell is needed for electron exchange in the redox reaction. Standard reduction potentials are calculated by comparing the half-reaction in question to a reference half-reaction (like the SHE).

Step-by-step solution

(a) Conditions for standard reduction potential

In order to classify a reduction potential as standard, there are three fundamental conditions that need to be met: 1. The temperature should be maintained at 25°C or 298K. 2. All participating solutes should have a concentration of 1 Molar (1M). 3. The pressure of any gaseous reactants or products should be fixed at 1 atmosphere (1 atm). When these conditions are satisfied, the reduction potential is expressed as the standard reduction potential (E°).

(b) Standard reduction potential of a standard hydrogen electrode

In electrochemistry, the standard hydrogen electrode (SHE) is used as a reference electrode, which is assigned a zero voltage, by definition. Therefore, the standard reduction potential of a standard hydrogen electrode is 0 V (volts).

(c) Impossibility of measuring the standard reduction potential of a single half-reaction

The reason why we cannot measure the standard reduction potential of a single half-reaction is that there needs to be a complete electrochemical cell to allow for an exchange of electrons which correspond to the redox reaction we are trying to quantify. In other words, an electron transfer between the half-reactions must occur, or else there will be no electrical potential generated. As a result, we need a complete redox reaction (which consists of both a reduction and an oxidation half-reaction) to establish an electrochemical cell. To measure a standard reduction potential, we generally compare the half-reaction in question to a reference half-reaction (like the standard hydrogen electrode). This way, we can calculate the standard reduction potential of a half-reaction relative to the reference electrode.

Key concepts

Electrochemistry

Electrochemistry is a branch of chemistry that deals with the relationship between electrical energy and chemical change. At the heart of this field is the study of redox reactions, where oxidation and reduction processes occur. These reactions involve the transfer of electrons between species, which can be harnessed to produce electrical energy.

In an electrochemical cell, such as a battery, two half-cells are connected by a salt bridge, allowing ions to flow and maintain charge balance as electrons are transferred from the oxidizing agent to the reducing agent. This flow of electrons constitutes an electric current that can be used to do work. To standardize comparisons between different electrochemical cells, certain conditions must be met, such as those described in your textbook exercise. By establishing these standard conditions, we can measure and tabulate the inherent tendencies of substances to gain or lose electrons, known as their standard reduction potentials.

Standard Hydrogen Electrode

The standard hydrogen electrode (SHE) serves as a universal reference for measuring the standard reduction potentials of other electrodes. It is essentially a platinum electrode coated with platinum black, immersed in a solution of 1 Molar acidic solution, and it is in contact with hydrogen gas at 1 atmosphere pressure.

The reason it holds such importance is that its standard reduction potential is defined to be zero volts under standard conditions, that is, at 25°C, 1 Molar concentration for solutions, and 1 atmosphere pressure for gases. This definition provides a baseline to which all other redox couples can be compared. Keep in mind that while SHE is a reference, it's still a half-cell and requires another half-cell to make a complete electrochemical cell. When SHE is paired with any other half-reaction, the potential of the complete cell is taken as the standard reduction potential of the non-standard electrode.

Redox Reaction

Redox reactions are chemical reactions involving the transfer of electrons from one substance to another. These are reactions where oxidation (loss of electrons) and reduction (gain of electrons) occur simultaneously. They play a crucial role in electrochemistry as they are fundamental to the operation of batteries and electrochemical cells.

To understand redox processes better, consider a simple redox reaction: when zinc metal is placed in a copper sulfate solution, zinc gets oxidized (loses electrons) and copper is reduced (gains electrons). In doing so, zinc gets dissolved, and copper gets deposited on the zinc surface. By using a voltmeter in a cell made up of zinc and copper half-reactions, we can measure their relative tendency to undergo reduction, which is known as reduction potential. Electrochemists list these values under standard conditions, leading to a table of standard reduction potentials used for predicting the course of chemical reactions and the spontaneity of redox reactions in electrochemical cells.