Problem 29 - Chemistry: The Central Science

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?

Step-by-Step Solution

Answer

(a) The half-reaction occurring at a hydrogen electrode in acidic aqueous solution when it serves as the cathode of a voltaic cell is: \[2H^{+}(aq) + 2e^{-} \rightarrow H_{2}(g)\] (b) The standard hydrogen electrode (SHE) is a reference electrode with the following characteristics: 1. Hydrogen ion concentration of \(1M\). 2. Partial pressure of hydrogen gas at \(1 atm\). 3. Temperature at \(298 K\) (or \(25^{\circ}C\)). Its standard electrode potential is defined as exactly zero volts (E° = 0.000 V). (c) The platinum foil in a standard hydrogen electrode serves as an inert conductor that allows the flow of electrons in and out of the electrode. It also provides a surface for hydrogen gas to adsorb onto, helping maintain the required equilibrium between dissolved hydrogen ions and gaseous hydrogen molecules.

Step 1: (a) Half-reaction of a hydrogen electrode in acidic aqueous solution as cathode

To find the half-reaction occurring at the hydrogen electrode, we must remember that as the cathode, it will be the site of reduction. The reduction half-reaction for hydrogen in acidic aqueous solution can be written as: \[2H^{+}(aq) + 2e^{-} \rightarrow H_{2}(g)\]

Step 2: (b) Standard hydrogen electrode characteristics

The standard hydrogen electrode (SHE) is a reference electrode used in electrochemical experiments, such as measuring the standard electrode potentials of other electrodes. Its characteristics are: 1. In a standard hydrogen electrode, the concentration of hydrogen ions (protons) in the solution is \(1M\). 2. The partial pressure of hydrogen gas is \(1 atm\). 3. The temperature at which the electrode operates is \(298 K\), or \(25^{\circ}C\). This electrode is used as a reference because its standard electrode potential is defined as exactly zero volts (E° = 0.000 V). In other words, all other electrode potentials are compared to the potential of the standard hydrogen electrode.

Step 3: (c) Role of platinum foil in a standard hydrogen electrode

The platinum foil in a standard hydrogen electrode serves as an inert conductor that allows the flow of electrons in and out of the electrode. Platinum foil is chosen for the electrode because it is unreactive, has high conductivity, and does not oxidize. This way, platinum can facilitate the electrochemical reaction without getting consumed or altering the reaction. Additionally, it provides a surface for hydrogen gas to adsorb onto, helping maintain the required equilibrium between dissolved hydrogen ions and gaseous hydrogen molecules.