Problem 33 - Chemistry The Central Science In Si Units

Question

(a) Write the half-reaction that occurs at an oxygen electrode in acidic aqueous solution when it serves as the cathode of a voltaic cell. (b) Write the half-reaction that occurs at an oxygen electrode in acidic aqueous solution when it serves as the anode of a voltaic cell. (c) What is standard about the standard oxygen electrode?

Step-by-Step Solution

Answer

(a) \(O_2 + 4H^+ + 4e^- \rightarrow 2H_2O\). (b) \(2H_2O \rightarrow O_2 + 4H^+ + 4e^-\). (c) "Standard" refers to measurement conditions: 1 atm, 25°C, 1 M concentration.

Step 1: Understanding the cathode reaction

At the cathode of a voltaic cell, reduction occurs. For an oxygen electrode in acidic solution, the relevant half-reaction involves the reduction of \({O_2}\). Oxygen will gain electrons to form water. The balanced half-reaction in acidic solution is: \(O_2(g) + 4H^+(aq) + 4e^- \rightarrow 2H_2O(l)\).

Step 2: Understanding the anode reaction

At the anode of a voltaic cell, oxidation occurs. For an oxygen electrode in acidic solution, the relevant half-reaction involves the oxidation of water to form oxygen gas. The balanced half-reaction in acidic solution is: \(2H_2O(l) \rightarrow O_2(g) + 4H^+(aq) + 4e^-\).

Step 3: Understanding the standard condition

A standard electrode, such as the standard oxygen electrode, is characterized by its conditions: 25°C (298 K), 1 atm pressure for gases, and 1 M concentration for solutions. These conditions ensure reproducibility of measurements. The standard hydrogen electrode (SHE) is also used as a reference point to measure standard electrode potentials.

Key Concepts

Understanding Half-Reactions

In a voltaic cell, chemical reactions are divided into two parts known as half-reactions. Each half-reaction takes place at different electrodes: cathode and anode. This helps in understanding the specific processes of oxidation and reduction.

For instance, during a reaction at the cathode, reduction of a substance occurs. In the context of an oxygen electrode in an acidic aqueous solution, the oxygen molecule undergoes reduction by gaining electrons and protons to form water. This specific half-reaction can be represented as follows:

• Oxygen gas, \(O_2(g)\), combines with hydrogen ions, \(4H^+(aq)\), and electrons, \(4e^-\), to produce water, \(2H_2O(l)\).

Conversely, at the anode, oxidation occurs. Here, water loses electrons and protons to produce oxygen gas. The half-reaction for oxidation in an acidic solution can be represented as:

• Water, \(2H_2O(l)\), is oxidized to form oxygen gas, \(O_2(g)\), hydrogen ions, \(4H^+(aq)\), and electrons, \(4e^-\).

These half-reactions are vital as they reveal how electrons are transferred during redox reactions in a voltaic cell.

The Significance of Standard Electrodes

A standard electrode provides a baseline to measure and compare the electrical potential of half-reactions under consistent conditions. These conditions usually include:

• Temperature of 25°C (or 298 K).
• Pressure of 1 atmosphere for any gases involved.
• Concentration of 1 Molarity for aqueous solutions.

Standard electrodes, such as the standard hydrogen electrode (SHE), serve as reference points providing a zero-electrode potential against which all other electrodes are measured. These standardized conditions ensure that measurements are reproducible and comparable across different experiments. When talking about the standard oxygen electrode, it is assumed that it is held under these conditions. The predictable and consistent nature of standard electrodes assists scientists in understanding the characteristics and potential of various chemical species when involved in redox reactions.

Exploring Redox Reactions in Voltaic Cells

Redox reactions are fundamental to the operation of voltaic cells. These included reactions are characterized by the transfer of electrons. The term 'redox' is a shorthand for reduction-oxidation reactions. Here’s what that means:

• Reduction refers to the gain of electrons by a chemical species.
• Oxidation refers to the loss of electrons by a chemical species.

In a voltaic cell, these two processes occur simultaneously at two different electrodes. The overall cell reaction is split into the two mentioned half-reactions: one for oxidation and one for reduction. At the molecular level, electrons get transferred from the substance getting oxidized to the substance undergoing reduction. For example, in an oxygen electrode within a voltaic cell:

• The oxygen at the cathode receives electrons—it's reduced, forming water.
• Water at the anode loses electrons—it’s oxidized to form oxygen gas.

Understanding these redox processes enables the analysis of how electrical energy is generated from chemical reactions, which is the very essence of how voltaic cells function.