Electromotive force standard cell

The quantity q was obtained by measuring the electromotive force of cells containing standard solutions. The hydrogen ion concentration in these standard solutions was determined from conductance data, using the Arrhenius theory. Sorenson found the value... 

Define the following terms anode, cathode, cell voltage, electromotive force, standard reduction potential. 

A second source of standard free energies comes from the measurement of the electromotive force of a galvanic cell. Electrochemistry is the subject of other articles (A2.4 and B1.28). so only the basics of a reversible chemical cell will be presented here. For example, consider the cell conventionally written as... 

If electron flow between the electrodes is toward the sample half-cell, reduction occurs spontaneously in the sample half-cell, and the reduction potential is said to be positive. If electron flow between the electrodes is away from the sample half-cell and toward the reference cell, the reduction potential is said to be negative because electron loss (oxidation) is occurring in the sample halfcell. Strictly speaking, the standard reduction potential, is the electromotive force generated at 25°C and pH 7.0 by a sample half-cell (containing 1 M concentrations of the oxidized and reduced species) with respect to a reference half-cell. (Note that the reduction potential of the hydrogen half-cell is pH-dependent. The standard reduction potential, 0.0 V, assumes 1 MH. The hydrogen half-cell measured at pH 7.0 has an of —0.421 V.)... 

Table 6.11 lists, to the right of the arrows, reducing agents or disposition to electron loss or disposition to oxidation in order of increasing strength. Such a list is more popularly called the electromotive force, or emf, series. The maximum potential difference which can be measured for a given cell is called the electromotive force (abbreviated emf) and represented by the symbol Ecell. It may be recounted that the emf values reported in Table 6.11 are for those cells under specified standard conditions in which all the concentrations are 1 M and pressures are 1 atm. The emf of such a cell is said to be its standard electromotive force, and is given by the symbol E ell. 

While this potential cannot he determined for a single electrode, a potential can be derived if the potential of the other electrode in a cell is defined, i.e. the potential of the standard hydrogen electrode (SHE) is arbitrarily taken as 0.(XXX)V. In this way. a potential scale can then be devised for single electrode potentials - see Section 3.2. t The abbreviation emf , in upright script, is often used in other lextNmks as a direct , i.e. non-variable, acronym for the electromotive force. Note, however, that in this present text it is used to represent a variable (cell potential) and is therefore. shown in italic script. 

To recall the basic concepts of the thermodynamics of cell operation, such as the electrode potential E, the standard electrode potential and the electromotive force (emf). 

Nernst equation phys chem The relationship showing that the electromotive force developed by a dry cell is determined by the activities of the reacting species, the temperature of the reaction, and the standard free-energy change of the overall reaction. nernst i,kwa-zh3n )... 

Because, as we have already seen, the standard potential of hydrogen is zero, the electromotive force of the galvanic cell (eq. 8.161) directly gives the value of the standard potential for the Zn,Zn redox couple. Table 8.14 lists the standard potentials for various aqueous ions. The listed values are arranged in decreasing order and are consistent with the standard partial molal Gibbs free energies of table 8.13. 

Example 11.1. Given the following rather standard cell reaction and the electromotive force in the cell as... 

Over the last 20-30 years not too much effort has been made concerning the determination of standard potentials. It is mostly due to the funding policy all over the world, which directs the sources to new and fashionable research and practically neglects support for the quest for accurate fundamental data. A notable recent exception is the work described in Ref. 1, in which the standard potential of the cell Zn(Hg)jc (two phase) I ZnS O4 (aq) PbS O4 (s) Pb(Hg)jc (two phase) has been determined. Besides the measurements of electromotive force, determinations of the solubility, solubiKty products, osmotic coefficients, water activities, and mean activity coefficients have been carried out and compared with the previous data. The detailed analysis reveals that the uncertainties in some fundamental data such as the mean activity coefficient of ZnS04, the solubility product of Hg2S04, or even the dissociation constant of HS04 can cause uncertainties in the f " " values as high as 3-4 mV. The author recommends this comprehensive treatise to anybody who wants to go deeply into the correct determination of f " " values. 

Understanding voltaic cells, anodes, and cathodes Figuring standard reduction potentials and electromotive force Zapping current into electrolytic cells... 

In a similar though less diabolical manner, the electrons produced at the anode of a voltaic cell have a natural tendency to flow along the circuit to a location with lower potential the cathode. This potential difference between the two electrodes causes the electromotive force, or EMF, of the cell. EMF is also often referred to as the cell potential and is denoted fj.g,. The cell potential varies with temperature and concentration of products and reactants and is measured in volts (V). The standard cell potential, or E° gn, is the that occurs when concentrations of solutions ire all at 1 M and the cell is at standard temperature and pressure (STP). 

An equation relates standard cell potential to the electromotive force (EMF of the cell. This equation, called the Nernst equation, is expressed as follows ... 

The most simple practical application of CSZ is for the gauge of oxygen partial pressure, as mentioned in Sections 1.4.7 and 1.4.8. The oxygen partial pressure in the closed system as shown in Fig. 3.1 can be measured, taking the air as the standard oxygen pressure The electromotive force (EMF) of this concentration cell is expressed as... 

Cell potential, which is also called electromotive force (emf), is always positive. Compare with standard potential. 

The cell potential E (also called the cell voltage or electromotive force) is an electrical measure of the driving force of the cell reaction. Cell potentials depend on temperature, ion concentrations, and gas pressures. The standard cell potential E° is the cell potential when reactants and products are in their standard states. Cell potentials are related to free-energy changes by the equations AG = —nFE and AG° = —mFE°, where F = 96,500 C/mol e is the faraday, the charge on 1 mol of electrons. 

---