Electromotive force defined

The material changes in the cell are completely defined when we know the quantity of electricity passing through, for Faraday s law teaches us that for a quantity F there will always be a gram-equivalent of chemical change, independent of the electromotive force. 

The sign of the electrode potential is arbitrarily defined as follows. A kation electrode (e.g., Zn in ZnS04 aq.) is said to be positive when it is positive to a unimolar (f — 1) solution of its ions an anion electrode e.g., CI2 in KC1) is said to be positive when it is positive to a unimolar solution of its ions. If a cell is made up of electrodes reversible with respect to any kinds of ions, its electromotive force is the algebraic difference of its electrode potentials, provided the electromotive force at the contact of the two solutions, due to diffusion (cf. Jahn, Elcktro-chcmie) is neglected. 

While the voltage of the cell represents the potential difference between the two terminals of the battery, in reality it relates to the separation in energy between the two half-cells. We call this separation the emf where the initials derive from the archaic phrase electromotive force. An emf is defined as always being positive. 

All three of these terms have units of ohms as they are all measures of some form of resistance to electrical flow. The reactance of an inductor is high and comes specifically from the back electromotive force (EMF p. 46) that is generated within the coil. It is, therefore, difficult for AC to pass. The reactance of a capacitor is relatively low but its resistance can be high therefore, direct current (DC) does not pass easily. Reactance does not usually exist by itself as each component in a circuit will generate some resistance to electrical flow. The choice of terms to define total resistance in a circuit is, therefore, resistance or impedance. 

As shown in Fig. 6-3, it is also in the same TUPAC convention that a positive electric charge flows from the left hand electrode through the electrolyte to the right hand electrode, as the cell reaction proceeds in the direction as written in Eqn. 6-3. This defines the sign of the electromotive force of electrochemical cells. 

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. 

The lUPAC Commission for Analytical Nomenclature defines the calibration curve [138] as the dependence of the electromotive force of the given ISE -reference electrode cell on the logarithm of the activity or concentration of the given substance. It is recommended that the potential be plotted on the ordinate (the vertical axis) and the logarithmic function of the activity or concentration on the abscissa (the horizontal axis), with the concentration increasing from the left to the right. 

ACTIVITY COEFFICIENT. A fractional number which when multiplied by the molar concentration of a substance in solution yields the chemical activity. This term provides an approximation of how much interaction exists between molecules at higher concentrations. Activity coefficients and activities are most commonly obtained from measurements of vapor-pressure lowering, freezing-point depression, boiling-point elevation, solubility, and electromotive force. In certain cases, activity coefficients can be estimated theoretically. As commonly used, activity is a relative quantity having unit value in some chosen standard state. Thus, the standard state of unit activity for water, dty, in aqueous solutions of potassium chloride is pure liquid water at one atmosphere pressure and the given temperature. The standard slate for the activity of a solute like potassium chloride is often so defined as to make the ratio of the activity to the concentration of solute approach unity as Ihe concentration decreases to zero. 

The overall conclusion is that there is no objection in principle to extracting electromotive force from the tme vacuum, defined by the minimum value, a, which can be attained by A in the internal scalar space of the gauge theory, which is the theory underlying electromagnetic theory. 

The electromotive force (emf) of the cell is defined as the difference between the electrical potentials under open-circuit conditions (the requirement of reversibility is discussed in Section 12.6), so (" — ) equals 8 and... 

The relationship between the electromotive force and pH is defined as a straight line. To establish the slope and the intercept of a meter system at a given temperature, the meter must be calibrated with standard solutions prior to use. A typical field meter kit includes three calibration standards, which are buffer solutions with known pH values (usually pH 4.01, 7.00, and 10.01 at 25°C). By immersing the probe into the buffer solution with pH 7, we establish the intercept (also called offset or zero) of the probe. If the reading is different from 7.0 at this point, we must adjust it with a control knob labelled Offset or Zero. The buffer solutions with the pH values of 4.01 and 10.01 allow us to verify and adjust the slope (span) of the calibration line. 

Using Ohm s law the unit of voltage, called the volt (V), can be defined by the two precedent units as the potential difference required for a flow of current of one ampere through a conductor having a resistance of one ohm. In a closed electric circuit wo can write the equation E — IR, E meaning the electromotive force of the source of current connected to the circuit. Ohm s law can also be applied, however, to a part of an electric circuit then E = IR stands for the voltage or potential difference across the givon part of the conductor. 

A single electrode potential, if defined as the difference in electrostatic potential between the spaces just outside the metal and the solution, is definite, but it cannot be measured by merely connecting up the phases with wires, and adjusting a potentiometer, until no current flows for this connexion introduces more than one phase boundary. Practically all electrolytic cells consist of at least three phase boundaries and the terminals at which the electromotive force of the cell is measured are, finally, of the same metal. There may, of course, be any. greater number of phase boundaries. A simple type of cell consists of two metals, M and M, dipping into a solution 8 containing the ions of each metal. 

When we consider the difference in electric potential between two points instead of local electric forces, the quantity of electromotive force Atf/ is defined in a single direction by... 

Equation (11.28) resembles the theory of electrical circuits A max is the electromotive force, A is the voltage drop on a load, and XL is the voltage drop through internal resistance R,. External R and internal resistances Rt are defined by... 

One now defines the electromotive force (emf) fl by Ffl - 2(f2 — f ), where F is the Faradav (96,487 coulombs, the numerical value of the charge associated with one mole of electrons). More generally, the emf is related to the difference in the electrochemical potential for electrons that develops under open circuit conditions between the two electrodes of the cell under study. For the example considered here Ffl - (f2" - " ) + This may be revamped by noting that the... 

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. 

In general, the open-circuit potential measured between two reversible electrodes, which is also called electromotive force, /f 1, is defined by the Nernst equation. A simplified form of this equation for the electrochemical reaction (3) was given by Eq. (15). In general, the Nernst equation relates the activities (and/or fugacities) of the substances or species, a,-, in the cell s electrochemical reactions and the standard open-circuit potential, E°, of the cell as ... 

The electromotive force E or EMF) is defined as the maximum difference in potential between the two electrodes (right minus left) obtained when the cell current is zero. The ceE potential is measured usmg a potentiometer, of wliich the common pH meter is a special type. The direct-reading potentiometer is a voltmeter that measures the potential across the cell (between the two electrodes) however, to obtain an accurate potential measurement, it is necessary... 

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 ... 

The standard reduction potential Eq can be defined as the electromotive force (in volts) measured in a half-cell containing 1 M oxidant and 1 M reductant at 25 °C and pH 7.0, in equilibrium with a reference half-cell which can accept the electrons. 

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

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