Question

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

Short answer

(a) When the hydrogen electrode serves as the cathode in an acidic aqueous solution, the half-reaction is: \[2H^+ (aq) + 2e^- ⟶ H_2(g)\] (b) The standard hydrogen electrode is considered standard because it operates under certain constant conditions: 25°C temperature, 1 atm hydrogen gas pressure, and 1 M hydrogen ion concentration. Its electrode potential is defined as 0 V and serves as a reference electrode. (c) The platinum foil in a standard hydrogen electrode acts as a catalyst for redox reactions, is inert to the chemicals involved, has high electrical conductivity, and provides a large surface area for the reactions to occur efficiently.

Step-by-step solution

a) Half-reaction at the hydrogen electrode in an acidic aqueous solution

When the hydrogen electrode serves as a cathode in an acidic aqueous solution, it means that it is a site of reduction, and hydrogen ions (H⁺) from the solution will gain electrons to form hydrogen gas (H₂). Therefore, the half-reaction occurring at the hydrogen electrode is as follows: \[2H^+ (aq) + 2e^- ⟶ H_2(g)\]

b) What is standard about the standard hydrogen electrode?

The term "standard" in the standard hydrogen electrode refers to certain standard conditions that the electrode operates under. These conditions are held constant and include the following: 1. The temperature is set at 25°C (298 K). 2. The pressure of hydrogen gas (H₂) is maintained at 1 atm. 3. The concentration of hydrogen ions (H⁺) in the acidic aqueous solution is set at 1 M. When these conditions are met, the electrode is said to be under standard conditions, and the electrode potential of the standard hydrogen electrode is defined as 0 V. It serves as the reference electrode, which other electrode potentials are measured against.

c) The role of the platinum foil in a standard hydrogen electrode

The platinum foil plays a significant role in the standard hydrogen electrode. Its main functions are: 1. Catalyst: Platinum is an excellent catalyst for the redox reactions occurring at the electrode surface. It speeds up the transfer of electrons between the hydrogen ions from the solution and the hydrogen gas, thus facilitating the redox reaction. 2. Inertness: Platinum is an inert metal, meaning it doesn't react with the chemicals involved in the redox reaction. This ensures that only the desired chemical species participate in the redox reaction and the electrode remains unchanged. 3. Conductivity: Platinum has high electrical conductivity, which allows it to effectively transfer electrons between the electrode and the external circuit. This efficient electron transfer is crucial for the proper functioning of the electrode. 4. Surface area: The platinum foil provides a large surface area for the redox reactions to take place, allowing for efficient electron transfer and faster reaction rates.