Question

a. What information is provided by the standard reduction potential of a given half-cell? b. What does the relative value of the reduction potential of a given half- reaction indicate about its oxidation-reduction tendency?

Short answer

Standard reduction potential shows a species' tendency to gain electrons. A more positive E° means a stronger reduction tendency; a more negative E° indicates a stronger oxidation tendency.

Step-by-step solution

Understand Standard Reduction Potential

The standard reduction potential, denoted as E°, is a measure of the tendency of a chemical species to gain electrons and be reduced. This potential is measured under standard conditions: 25°C, 1 M concentration for solutions, and 1 atm pressure for gases.

What E° Indicates

The value of E° is compared to the standard hydrogen electrode (SHE), which is assigned a potential of 0 V. If E° is positive, the half-cell has a greater tendency to gain electrons and be reduced compared to the SHE.

Relative Value of E° and Oxidation-Reduction Tendency

The relative value of the standard reduction potential indicates the reactivity of a substance in redox reactions. A more positive E° means a stronger tendency to gain electrons (be reduced), whereas a more negative E° indicates a stronger tendency to lose electrons (be oxidized).

Key concepts

oxidation-reduction reactions

Oxidation-reduction (redox) reactions are chemical processes in which electrons are transferred between substances. These reactions can involve the transfer of electrons from one element or compound to another, leading to changes in the oxidation states of the reacting species.

In a redox reaction, the substance that loses electrons becomes oxidized, while the substance that gains electrons becomes reduced. You can remember this with the mnemonic 'LEO the lion says GER', where LEO stands for 'Loss of Electrons is Oxidation' and GER stands for 'Gain of Electrons is Reduction'.

• The substance that is oxidized is called the reducing agent because it donates electrons.
• The substance that is reduced is called the oxidizing agent because it accepts electrons.

For example, in the reaction between hydrogen and oxygen to form water:
2H₂ + O₂ → 2H₂O
Here, hydrogen is oxidized (it loses electrons), and oxygen is reduced (it gains electrons). Understanding these principles helps make sense of how different substances interact in redox reactions.

half-cell reactions

A half-cell reaction is either the oxidation or reduction reaction component of a redox reaction. Each half-cell contains an electrode and an electrolyte solution. When connected, they form an electrochemical cell that can generate electrical energy.

In a standard half-cell, the electrodes are immersed into solutions with ion concentrations of 1 M. The potential difference between the electrode and its solution is measured. This potential, called the electrode potential, depends on the nature of the electrode material and the ions in the solution.

• Oxidation half-reaction: This is where the oxidation process occurs.
• Reduction half-reaction: This is where the reduction process occurs.

For example, consider a zinc-copper cell:

• At the zinc (anode): Zn → Zn²⁺ + 2e⁻ (oxidation)
• At the copper (cathode): Cu²⁺ + 2e⁻ → Cu (reduction)

By combining these half-cell reactions, we get the overall cell reaction:
Zn + Cu²⁺ → Zn²⁺ + Cu
Understanding half-cell reactions is crucial for analyzing and predicting the behavior of electrochemical cells.

standard hydrogen electrode

The standard hydrogen electrode (SHE) is a reference electrode used to measure the electrode potential of half-cells. It’s assigned a potential of 0 V under standard conditions: 25°C, 1 M H⁺ ion concentration, and 1 atm pressure for hydrogen gas.

The SHE consists of a platinum electrode in contact with 1 M H⁺ ions and bathed in hydrogen gas at 1 atm. This setup allows it to safely donate and accept electrons, making it a reliable reference for comparing other electrode potentials.

• In the SHE's reduction half-reaction: 2H⁺ + 2e⁻ → H₂
• In the SHE's oxidation half-reaction: H₂ → 2H⁺ + 2e⁻

When connected to another half-cell, the SHE’s potential is used to determine the standard reduction potential (E°) of the other cell.
If E° is positive, the tested half-cell has a greater tendency to gain electrons than the SHE. If E° is negative, it has a lower tendency. This concept helps us rank substances by their electron affinity, making the SHE a cornerstone in electrochemistry.