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 a standard reduction potential when: 1) the temperature is 298 K (25°C), 2) the pressure is 1 atm (101.325 kPa), and 3) the concentration of each ion in the reaction is 1 M (1 mol L^-1). (b) The standard reduction potential of a standard hydrogen electrode (SHE) is defined as 0 Volts (V) under standard conditions (298 K, 1 atm, and 1 M concentration). (c) It is impossible to measure the standard reduction potential of a single half-reaction because an electrochemical cell requires two half-reactions (connected in a complete redox reaction) for a potential difference to exist and be measured. Comparing the measured potential difference between the unknown half-reaction and the SHE directly gives the standard reduction potential of the unknown half-reaction.

Step-by-step solution

a) Conditions for standard reduction potential

To be considered a standard reduction potential, a reduction potential must meet the following conditions: 1. Temperature: The measurement must be taken at a temperature of 298 K (25 degrees Celsius). 2. Pressure: The pressure must be set at 1 atmosphere (atm) or 101.325 kPa. 3. Concentration: The concentration of each ion in the reaction must be 1 M (1 mol L^-1). By keeping these conditions constant, it is possible to compare the reduction potentials of various half-reactions.

b) Standard reduction potential of a standard hydrogen electrode

The standard hydrogen electrode (SHE) is a common reference electrode used in electrochemistry. It consists of a platinum wire coated with a thin layer of platinum black, and it is immersed in an acidic solution (pH = 0) containing hydrogen ions (H+) with a concentration of 1 M. Hydrogen gas (H2) is continuously bubbled through the solution at a pressure of 1 atm. By definition, the SHE is assigned a standard reduction potential of 0 Volts (V) under standard conditions (298 K, 1 atm, and 1 M concentration). This value serves as a reference point for measuring and comparing the standard reduction potentials of other electrodes.

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

In electrochemistry, a half-reaction represents either the reduction or oxidation process in an overall redox reaction. The standard reduction potential of a half-reaction is the potential difference between the half-reaction and the SHE under standard conditions. However, it is impossible to measure the standard reduction potential of a single half-reaction because there is no flow of electrons between the half-reaction and SHE when they are isolated from each other. The flow of electrons occurs only when both half-reactions are connected to form a complete redox reaction within an electrochemical cell. To measure the standard reduction potential of a half-reaction, the half-reaction must be paired with another half-reaction to form a complete redox reaction. By connecting the two half-reactions to form an electrochemical cell, a potential difference can be measured between them. This potential difference represents the sum of the standard reduction potentials of the two half-reactions. Since the standard reduction potential of the SHE is defined as 0 V, comparing the measured potential difference between the unknown half-reaction and the SHE directly gives the standard reduction potential of the unknown half-reaction. Thus, it is impossible to measure the standard reduction potential of a single half-reaction because an electrochemical cell requires two half-reactions (connected in a complete redox reaction) for a potential difference to exist and be measured.

Key concepts

Electrochemistry

Electrochemistry is a branch of chemistry that studies the movement of electrons, using it to understand and harness chemical reactions that produce or consume electric current. It's the science behind batteries, electrolysis, and corrosion protection. By exploring how electrons are transferred in chemical processes, scientists can create devices that store and release energy. This field blends chemistry and physics, and it plays a crucial role in developing technologies for energy generation, storage, and conversion.

Some key concepts include:

• Redox Reactions: These involve the transfer of electrons between substances, with one substance being reduced and another being oxidized.
• Electrochemical Cells: Devices that convert chemical energy into electrical energy or vice versa.

Understanding electrochemistry helps explain how batteries work and how we can develop better, more efficient energy systems.

Standard Hydrogen Electrode

The Standard Hydrogen Electrode (SHE) serves as a reference point in electrochemistry. It is used as the baseline to measure the standard reduction potentials of other electrodes. The SHE is carefully constructed to maintain standard conditions: it uses a platinum electrode immersed in a 1 M acid solution with hydrogen ions while hydrogen gas is bubbled through it at 1 atm pressure and a temperature of 298 K.

The SHE is essential because:

• Its standard reduction potential is defined as 0 Volts, providing a universal standard for comparisons.
• It helps standardize measurements across different laboratories and experiments, ensuring consistent data.

This electrode is fundamental in establishing the relative tendencies of various agents to gain or lose electrons.

Redox Reactions

Redox reactions are chemical reactions in which reduction and oxidation processes occur simultaneously. One substance donates electrons (is oxidized) while another accepts electrons (is reduced). The electron transfer process is what drives these chemical changes.

There are two main aspects to redox reactions:

• Oxidation: This involves the loss of electrons, resulting in an increase in oxidation state.
• Reduction: This involves the gain of electrons, decreasing the oxidation state of the substance.

Understanding these reactions is crucial for working with electrochemical cells and for processes such as energy production, metal extraction, and even biological systems.

Half-Reaction

A half-reaction represents either the oxidation or reduction component of a full redox reaction. In electrochemistry, when studying these reactions, understanding half-reactions helps to comprehend how electrons are shared or transferred.

Key points about half-reactions:

• They show the specific changes in oxidation states and electron transfer for a given substance.
• Each half-reaction must be balanced in terms of the number of electrons for the overall reaction to occur effectively.

Usually, two half-reactions combine in an electrochemical cell to produce a potential difference and allow electron flow.

Electrochemical Cell

An electrochemical cell is a device that facilitates the conversion between chemical energy and electrical energy through redox reactions. There are two main types of electrochemical cells: galvanic cells (which generate electric current from spontaneous chemical reactions) and electrolytic cells (which drive non-spontaneous reactions using external electrical energy).

Important components of electrochemical cells:

• Anode: The electrode where oxidation occurs, releasing electrons into the circuit.
• Cathode: The electrode where reduction occurs, accepting electrons from the circuit.
• Salt bridge: Connects the two halves of the cell and helps maintain electrical neutrality.

Electrochemical cells are the fundamental units of batteries, powering countless devices by exploiting redox reactions.