Electromotive force of a cell

The electromotive force of a cell with solutions of given concentrations may be calculated by subtracting the electrode potentials so obtained. 

The electromotive force of a cell can be related to the Gibbs free energy change for the cell reaction by combining equations (9.5), (9.90), and (3.96). We recall that... 

As has already been mentioned, the EMF the electromotive force) of a cell is given by the potential difference between leads of identical metallic material. In view of this, a galvanic cell is represented schematically as having identical metallic phases at either end. 

In this cell, the following independent phases must be considered platinum, silver, gaseous hydrogen, solid silver chloride electrolyte, and an aqueous solution of hydrogen chloride. In order to be able to determine the EMF of the cell, the leads must be made of the same material and thus, to simplify matters, a platinum lead must be connected to the silver electrode. It will be seen in the conclusion to this section that the electromotive force of a cell does not depend on the material from which the leads are made, so that the whole derivation could be carried out with different, e.g. copper, leads. In addition to Cl- and H30+ ions (further written as H+), the solution also contains Ag+ ions in a small concentration corresponding to a saturated solution of silver chloride in hydrochloric acid. Thus, the following scheme of the phases can be written (the parentheses enclose the species present in the given phase) ... 

In potentiometry with ion-selective electrodes (ISE), as a rule, it is the electromotive force of a cell such as the following that is measured. 

In general the electromotive force of a cell operating under reversible conditions is referred to as emf ( ), while that observed when conditions are irreversible is termed a voltage. In other words, emf, 8, is the maximum possible voltage that a galvanic cell can produce. 

As seen in the above, the electromotive force of a cell is an intensive property. Like temperature and pressure it is independent of the size of the system. 

E Voltage (in volts), mostly electromotive force of a cell. 

The maximum of energy obtainable during the galvanic cell operation is expressed by the product of the quantity of electricity passed through the cell and the voltage across the electrodes, called the electromotive force of a cell (EMF). 

The valne (E°) for the standard electromotive force of a cell in which hydrogen nnder standard conditions is oxidized to hydroninm ions (solvated protons) at the left-hand electrode. This value is used as a standard to measure electrode potentials. 

ELECTROMOTIVE FORCE OF A CELL AND POTENTIAL OF A HALF-CELL... 

Since the electromotive force of a cell depends on the activity of the ions of the electrolyte it is possible to determine activities potentiometrically, in particular that of the hydrogen ion which is a measure of acid strength. Strictly, however, the activity of a single ionic species is not measurable, for positive ions always exist in the presence of negative ions. Nevertheless, the mean activity of HCl which is measurable in a cell does conveniently indicate the mean activity of the ions which carry the current, and —logjQ of this activity is the pH of the solution. The basic principle of potentiometric pH determination may be seen by considering a cell divided into two halves by a silver wall coated on both sides with silver chloride ... 

According to its definition, the standard (reduction) potential of the A/A couple is the standard electromotive force of a cell in which an A/A electrode (where the activities of A and A are made unity) is opposed to an NHE (normal hydrogen electrode) whose potential is assigned to zero by convention. 

The addition of organic solvents to water should modify acid-base phenomena, but assessment of such effects poses many problems, as only the measured pH of aqueous solutions can be interpreted in terms of hydrogen ion concentrations. The quantitative comparison of the acidities of partially aqueous solutions is therefore a problem of far greater complexity than the measurements of pH values in aqueous media. As mentioned earlier, a proton activity (paH) is defined in such a way that — log paH is equal to pH when the medium is water, and its value can be measured both by the electromotive force of a cell with liquid junction and by the spectrophotometry of colored indicators. 

The electrode potential (reduction potential) for a redox couple is defined as the couple s potential measured with respect to the standard hydrogen electrode, which is set equal to zero (see hydrogen electrode later). This potential, by convention, is the electromotive force of a cell, where the standard hydrogen electrode is the reference electrode (left electrode) and the given half-cell is the indicator electrode (right electrode). The reduction potential for a given redox couple is given by the Nernst equation ... 

This served as the guiding principle of extended researches by Berthelot (1827-1907) and by Thomsen (1826-1909), to whom the greater part of our data on thermochemistry is due. However, it can be seen from equation (38) that the available work (not including mechanical work which is usually very small) can only be equal to the heat of reaction (a) at absolute zero or (b) if the Gibbs free energy does not change with the temperature. This latter statement also means, of course, that the electromotive force of a cell in which the reaction takes place does not change with the temperature, i.e.,... 

By means of the Gibbs-Helmholtz equation (38) it is obviously possible to compute the heats of the reaction, — AH, from the electromotive force of a cell in which the reaction takes place, and the temperature coefficient of the electromotive force of the cell. If the temperature range is not too great equation (38) can be replaced by... 

Electrode potential - The electromotive force of a cell in which the electrode on the left is the standard hydrogen electrode and that on the right is the electrode in question. [2]... 

No valid method exists for determining the absolute potential of an electrode. All potential measurements require a second electrode, and are therefore relative potentials. It is then necessary to choose one particular electrode to be arbitrarily assigned the zero position on the potential scale. By convention, the standard hydrogen electrode (SHE) is defined to have a potential of exactly 0 V, and is the reference point from which the potentials of all other electrodes are stated (Sec. 2.3). The electromotive force of a cell is the potential difference between two electrodes and is independent of the particular reference-electrode scale used. 

E = the electromotive force of a cell containing the unknown solution = the electromotive force of a cell containing a standard reference buffer solution of known or defined pH, that is, pH,... 

Before we discuss standard electrode potential, we will talk about electromotive force (emf). The electromotive force of a cell is the potential difference between the two electrodes. This can be measured using a voltmeter. The maximum voltage of a cell can be calculated using experimentally determined values called standard electrode potentials. By convention, the standard electrode potentials are usually represented in terms of reduction half-reactions for 1 molar solute concentration. The standard electrode potential values are set under ideal and standard-state conditions (latm pressure and 25°C temperature). From the MCAT point of view, you can assume that the conditions are standard, unless stated otherwise. Table 12-1 shows a list of standard electrode potentials (in aqueous solution) at 25°C. 

The electrode potential of any single redox couple is defined as the electromotive force of a cell consisting of the standard hydrogen electrode and the electrode in question, written in the following manner ... 

Although Braun made some use of the second law of thermodynamics, his applications of it were neither correct nor fruitful, and the first quantitative theory of the electromotive force of a cell was given independently, in different forms, by Gibbs and Helmholtz. Apart from external work Gibbs s equation is ... 

The decrease in free energy of the system in a spontaneous redox reaction is equal to the electrical work done by the system on the surroundings, or AG = nFE. The equilibrium constant for a redox reaction can be found from the standard electromotive force of a cell. 10. The Nernst equation gives the relationship between the cell emf and the concentrations of the reactants and products under non-standard-state conditions. Batteries, which consist of one or more galvanic cells, are used widely as self-contained power sources. Some of the better-known batteries are the dry cell, such as the Leclanche cell, the mercury battery, and the lead storage battery used in automobiles. Fuel cells produce electrical energy from a continuous supply of reactants. 

Electromotive force n. This force is defined as that which causes a flow of current. The electromotive force of a cell is measured by the maximum difference of potential between its plates. The electromagnetic unit of potential difference is that against which 1 erg of work is done in the transfer of electromagnetic unit quantity. The volt is that potential difference against which 1 J of work is done in the transfer of 1 C. One volt is equivalent to 10 electromagnetic units of potential. The international volt is the electrical potential which when steadily applied to a conductor whose resistance is one international ohm will cause a current of one international ampere to flow. The international volt = 1.00033 absolute volts. The electromotive force of a Weston standard cell is 1.0183 int. volts at 20°C. Dimensions T ],... 

The measurement of the absolute activity of an ion of a strong electrolyte (salts, strong acids and bases) is based on the complete dissociation of that electrol into ions, and uses the measurement of the electromotive force of a cell involving that ion. 

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