Standard electrochemical conditions

Standard electrochemical conditions 1 M concentration for solution species, 1 atm partial pressure for gases, and pure sohds and liquids. 

In answering Exercises 54-73, justify each answer by appropriate calculations. Assume that each reaction occurs at standard electrochemical conditions. 

HOCl(aq) + H2S(aq) + Zn(s) Consult a table of standard reduction potentials, and determine which of the foUowing reactions are spontaneous under standard electrochemical conditions. 

Electrochemical cells can be constructed using an almost limitless combination of electrodes and solutions, and each combination generates a specific potential. Keeping track of the electrical potentials of all cells under all possible situations would be extremely tedious without a set of standard reference conditions. By definition, the standard electrical potential is the potential developed by a cell In which all chemical species are present under standard thermodynamic conditions. Recall that standard conditions for thermodynamic properties include concentrations of 1 M for solutes in solution and pressures of 1 bar for gases. Chemists use the same standard conditions for electrochemical properties. As in thermodynamics, standard conditions are designated with a superscript °. A standard electrical potential is designated E °. 

To define an electrochemical "sea level," chemists have chosen a reference halfcell called the standard hydrogen electrode (S.H.E., shown in Figure 18.4). It consists of a platinum electrode in contact with H2 gas and aqueous H + ions at standard-state conditions [1 atm H2(g), 1 M H+ aq), 25°C]. The corresponding half-reaction, written in either direction, is assigned an arbitrary potential of exactly 0 V ... 

Standard cell — An electrochemical cell composed of two - half-cells containing electrodes built according to standard (normal) conditions. Frequently the term is also used for electrochemical cells showing a well-defined, reproducible, and stable cell voltage suitable for calibration purposes (- Weston cell, - Clark cell). 

Under standard reaction conditions, the mechanism of the Heck reaction is more complicated than the textbook pathway shown in Scheme 5. The active catalyst responsible for oxidative addition of the ArX substrate is an anionic Pd species, either I PdCl or L2Pd(OAc), depending on the starting palladium compound, where L is triphenylphosphine. Reduction of the starting palladium salt to a Pd species is carried out by the phosphine. A mechanism that is consistent with spectroscopic and electrochemical data is given in Scheme 13. 

Potentiometric transducers measure the potential under conditions of constant current. This device can be used to determine the analytical quantity of interest, generally the concentration of a certain analyte. The potential that develops in the electrochemical cell is the result of the free-energy change that would occur if the chemical phenomena were to proceed until the equilibrium condition is satisfied. For electrochemical cells containing an anode and a cathode, the potential difference between the cathode electrode potential and the anode electrode potential is the potential of the electrochemical cell. If the reaction is conducted under standard-state conditions, then this equation allows the calculation of the standard cell potential. When the reaction conditions are not standard state, however, one must use the Nernst equation to determine the cell potential. Physical phenomena that do not involve explicit redox reactions, but whose initial conditions have a non-zero free energy, also will generate a potential. An example of this would be ion-concentration gradients across a semi-permeable membrane this can also be a potentiometric phenomenon and is the basis of measurements that use ion-selective electrodes (ISEs). 

Under electrochemical conditions and T, P = constant, adsorption isotherms can be derived using standard statistical considerations to calculate the Gibbs energy of the adsorbate in the interphase and the equilibrium condition for the electrochemical potentials of the adsorbed species i in the electrolyte and in the adsorbed state (eq. (8.15) in Section 8.2). A model for the statistical considerations consists of a 2D lattice of arbitrary geometry with Ns adsorption sites per unit area. In the case of a 1/1 adsorption, each adsorbed particle can occupy only one adsorption site so that the maximal number of adsorbed particles per unit area in the compact monolayer is determined by A ax = Ng. Then, this model corresponds to the simple Ising model. The number of adsorbed particles, A ads< and the number of unoccupied adsorption sites, No, per unit area are given by... 

Under standard-state conditions, any species on the left of a given half-cell reaction will react spontaneously with a species that appears on the right of any halfcell reaction located above it in Table 19.1. This principle is sometimes called the diagonal rule. In the case of the Cu/Zn electrochemical cell... 

As the following examples show. Table 19.1 enables us to predict the outcome of redox reactions under standard-state conditions, whether they take place in an electrochemical cell, where the reducing agent and oxidizing agent are physically separated from each other, or in a beaker, where the reactants are all mixed together. 

We omit the subscript cell because this reaction is not carried out in an electrochemical cell.) Since E° is negative, we conclude that mercury is not oxidized by hydrochloric acid under standard-state conditions. 

We have seen how to use standard reduction potentials to calculate for cells. Real cells are usually not constructed at standard state conditions. In fact, it is almost impossible to make measurements at standard conditions because it is not reasonable to adjust concentrations and ionic strengths to give unit activity for solutes. We need to relate standard potentials to those measured for real cells. It has been found experimentally that certain variables affect the measured cell potential. These variables include the temperature, concentrations of the species in solution, and the number of electrons transferred. The relationship between these variables and the measured cell emf can be derived from simple thermodynamics (see any introductory general chemistry text). The relationship between the potential of an electrochemical cell and the concentration of reactants and products in a general redox reaction... 

In the case of other alloys, such as Al-base precipitation age-hardened materials, a range of environments have also been proposed (see Table 2) and test methods standardized [52, 53]. The NaCl-H202 environment probably induces widespread pitting. In this case, the local chemistry and electrochemical conditions that develop in the pit environment then induce IGC of exposed grain boundaries within the pit. 

The reversible potentials can be used to predict the corrosion tendency of the metal when the metal and the electrolyte are under standard thermodynamic conditions described in Chapter 2, Section 2.12.2. Table 6.1 is written as reduction reactions following the guidelines suggested by the International Union of Pure and Applied Chemistry (lUPAC) during the Stockholm Convention in 1953. The procedure for estimating half-cell potential is presented in Chapter 2. In an electrochemical ceU, the electrode with a smaller standard potential in Table 6.1 undergoes oxidation and transfer electrons to the electrode with a larger standard potential, which is reduced at the interface. In redox sys-... 

Extraction of quantitative chemical information from SECM requires a mathematical model of the interaction of the tip and substrate. Such modeling typically involves numerical solution of a reaction-diffusion equation with the boundary conditions appropriate to the interfacial kinetics. Simulation of SECM experiments is computationally much more demanding than for standard electrochemical experiments (discussed in Chapter 1.3). This is because diffusion in at least two dimensions must be considered and the discontinuity in the boundary condition between the tip metal and insulating sheath necessitates a fine mesh. 

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