8.4.2. Measuring Standard Electrode Potentials

It is impossible to measure the absolute potential of a single electrode. Therefore, electrode potentials are measured relative to a standard reference electrode: the Standard Hydrogen Electrode (SHE).

The SHE consists of a platinum electrode immersed in a 1.0 mol dm⁻³ solution of H⁺ ions, with hydrogen gas at 100 kPa bubbling over the electrode, all at 298 K. The standard electrode potential of the SHE is arbitrarily defined as exactly 0.00 V.

• Standard Hydrogen Electrode (SHE) half-reaction: 2H⁺(aq, 1M) + 2e⁻ ⇌ H₂(g, 100 kPa) E° = 0.00 V

The standard electrode potential (E°) serves as a reference point for measuring the potential of other electrodes. The Standard Hydrogen Electrode (SHE) is used for this purpose and is defined under standard conditions: a 1.0 M solution of hydrogen ions (H⁺) at a pressure of 100 kPa and a temperature of 298 K. Its potential is set to 0.00 V.

When we refer to an electrode's potential, we compare it to the SHE. For example, if another half-cell is connected to the SHE, we can measure how much voltage (potential) exists between the two. This comparison is essential for understanding how conducive a material is to gaining or losing electrons in electrochemical reactions.

Think of the Standard Hydrogen Electrode as a neutral benchmark, similar to sea level in geography. Just as we measure the height of mountains and valleys relative to sea level, we measure the electrode potentials relative to the SHE. If a half-cell 'rises above' the SHE, its potential is positive, akin to a mountain peak, while if it 'falls below' it, the potential is negative, like a valley.

To measure the standard electrode potential of a half-cell, it is connected to a SHE. The voltage measured by the voltmeter is the standard electrode potential (E°) of that specific half-cell.

• If the reaction with SHE causes the SHE to be oxidized (i.e., the other half-cell is reduced), the measured E° is positive.
• If the reaction with SHE causes the SHE to be reduced (i.e., the other half-cell is oxidized), the measured E° is negative.

To find the standard electrode potential of any half-cell, we utilize a voltmeter connected to the SHE. Depending on the direction of electron flow, we can determine whether the half-cell is a better oxidizer or reducer than the SHE. If the half-cell reduces the ions in the SHE, it shows a positive potential, indicating a strong oxidizing effect. Conversely, if it oxidizes the SHE, it indicates negative potential, thus representing a stronger reducing effect.

Imagine the SHE is a well-known competitive swimmer, and other swimmers are trying to either outrun or get outrun by this champion. If another swimmer is fast enough to beat the SHE, they represent a positive electrode potential, meaning they can reduce (gain electrons). If another swimmer can't beat the SHE, they are slower, representing a negative electrode potential instead.

A table of standard electrode potentials (reduction potentials) lists various half-reactions in order of their tendency to be reduced.

• More positive E° value: Indicates a stronger tendency for the substance to be reduced (i.e., it is a stronger oxidizing agent).
• More negative E° value: Indicates a stronger tendency for the substance to be oxidized (i.e., it is a stronger reducing agent).

The standard electrode potential values are compiled in a table that helps chemists assess the likelihood of various reactions. A more positive E° value suggests that the half-reaction is more favorable towards reduction, indicating it's a strong oxidizing agent. Similarly, a more negative E° value suggests a stronger tendency toward oxidation, marking the half-reaction as a strong reducing agent, which is useful for predicting reaction outcomes and designing electrochemical cells.

Think of the E° table as a leaderboard in a competition. The athletes at the top (those with more positive values) are very effective at their sports (reduction), while those at the bottom (more negative values) are not as proficient but still play a crucial role (oxidation). Knowing who ranks where helps coaches (chemists) strategize their teams (reactions) effectively.

The standard cell potential of a galvanic cell is the potential difference between the two half-cells when all components are in their standard states. It can be calculated from the standard electrode potentials of the two half-cells:

E°_cell = E°_reduction (cathode) - E°_reduction (anode)

Alternatively, one can reverse the sign of the anode's reduction potential to make it an oxidation potential and then add the two potentials:

E°_cell = E°_reduction (cathode) + E°_oxidation (anode)

• Spontaneity: A positive E°_cell indicates a spontaneous reaction (voltaic cell). A negative E°_cell indicates a non-spontaneous reaction (electrolytic cell requires energy input).

The standard cell potential (E°_cell) represents the electrical potential difference between the cathode and anode in a galvanic cell under standard conditions. This value can help predict the spontaneity of a reaction. If the E°_cell is positive, it indicates that the reaction can occur spontaneously, generating electrical energy as a voltaic cell. A negative value means that external energy is required to drive the reaction, indicative of electrolytic cells. Calculating E°_cell involves using the reduction potentials of both half-cells, which can be rearranged for clarity.

Consider E°_cell like the elevation difference between two locations, representing energy gradients. If the energy (cell potential) difference is positive, it’s like flowing downhill, indicating a spontaneous reaction. A negative difference indicates moving uphill, needing an external push (energy) to achieve the reaction.