(a) Describe the phenomenon of dynamic equilibrium as it applies to oxidation and reduction electrochemical reactions. (b) What is the exchange current density?

Dynamic equilibrium occurs when the rate of the forward reaction equals the rate of the reverse reaction in a chemical reaction. In the context of oxidation and reduction electrochemical reactions, dynamic equilibrium refers to the simultaneous movement of electrons between the redox species which occurs at electrodes. At an electrode, there can be a movement of electrons between the oxidized and reduced species without a net change in the concentration of these species. The electrode potential will remain constant. The dynamic equilibrium can be represented by the following equation: Oxidized species + M electrons <-> Reduced species As the forward reaction takes place, the oxidized species lose M number of electrons, and the reduced species are formed at the same rate as that of the reverse reaction where reduced species gain M electrons resulting in the oxidized species. This phenomenon is an essential aspect of electrochemical cells, as it ensures that there is no accumulation of a charged species in the electrode or the electrolyte, allowing the cell to work in a steady mode, achieving the maximum efficiency.

Exchange current density is an essential parameter in electrochemistry that represents the rate of an electrode reaction under dynamic equilibrium conditions, when there's no net transfer of charge, and the cell's voltage is zero (it's unbiased). It is the measure of the kinetics of the redox reaction under equilibrium conditions and is denoted by i0 (i_zero). It depends on factors such as, the concentration of the reactants, temperature, and surface properties of the electrode. In general, exchange current density can vary significantly for different redox couples and electrodes depending on the easiness at which the redox reactions are taking place. A high exchange current density implies faster electron transfer rates and a less polarized system, meaning that the potential difference needed to provoke the reaction will be smaller. Therefore, smaller overpotential would be required for the reactions to take place. The exchange current density can be calculated using Faraday's law. i0 = nFAk[Oxidized species] [Reduced species] Here, i0 represents the exchange current density, n is the number of electrons transferred per molecule during the reaction, F is Faraday's constant, A is the electrode surface area, k is the rate constant, and [Oxidized species] and [Reduced species] represent the concentrations of the respective species involved in the redox reaction.