Galvanic cells work done

Knowledge of the Volta potential of a metal/solution interface is relevant to the interpretation of the absolute electrode potential. According to the modem view, the relative electrode potential (i.e., the emf of a galvanic cell) measures the value of the energy of the electrons at the Fermi level of the given metal electrode relative to the metal of the reference electrode. On the other hand, considered separately, the absolute value of the electrode potential measures the work done in transferring an electron from a metal surrounded by a macroscopic layer of solution to a point in a vacuum outside the solotion. ... 

Both of these processes are carried out in electrochemical cells which are forced to operate in the reverse , or non-spontaneous direction, as indicated by the negative for the above cell reaction. The free energy is supplied in the form of electrical work done on the system by the outside world (the surroundings). This is the only fundamental difference between an electrolytic cell and the galvanic cell in which the free energy supplied by the cell reaction is extracted as work done on the surroundings. 

Although we can never actually realize the maximum work from a cell reaction, its value is still useful for evaluating the efficiency of a real process based on the cell reaction. For example, suppose a certain galvanic cell has a maximum potential of 2.50 V. In a particular experiment 1.33 moles of electrons passes through this cell at an average actual potential of 2.10 V. The actual work done is... 

The electromotive force of a galvanic cell is a measure of the electrical work which can be obtained from the reaction in the cell. The total or maximum work which can be obtained from the cell reaction includes also the work which is done against the external forces owing to the changes in volume (formation of gas, etc.) of the reacting substances. From the definition of affinity (p. 318) it follows, therefore, that the electromotive... 

The potential difference A% is positive for a galvanic cell, so meiec is negative in this case and net electrical work is performed by the galvanic cell. In an electrolytic cell, in contrast, A% is negative and rngiec is positive, corresponding to net electrical work done on the system by an external source such as an electric generator. 

In the same cell operated irreversibly (i.e., with a large current permitted to flow), less electrical work is accomplished. The maximum electrical work is done by the galvanic cell when it is operated reversibly. 

The electrical work done by a galvanic cell on the surroundings is tt eiec = -QA%. 

The decrease of Gibbs free energy is therefore equal to the reversible non-mechanical work done by the system at constant temperature and pressure. In this book the only non-mechanical work considered is electrical energy. For a galvanic cell this has already been shown, page 99, to be equal to EnF, so that... 

The Daniell cell is an example of a galvanic cell, in this type of electrochemical cell, electrical work is done by the system. The potential difference, between the two half-cells can be measured (in volts, V) on a voltmeter in the circuit (Figure 7.1) and the value of is related to the change in Gibbs energy for the cell reaction. Equation 7.9 gives this relationship under standard conditions, where is°ceu is the standard cell potential. 

Recall that free energy is related to the maximum possible amount of work that can he done hy the system. In the case of a galvanic cell, the work done is electrical work, so we can state the relationship between fi-ee energy and the cell potential as... 

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. 

The enthalpy change of a closed system is thus equal to the heat absorbed under ttuo restrictive conditions, namely, that there is constancy of pressure and that the only form of work is due to the volume change of the system. In a galvanic cell, where electrical work is done, the heat absorbed is not equal to the change in enthalpy. 

It may be noted from (2 21) that if Fg < F the work ti is numerically larger than w (and the decrease in O is larger than the decrease in A). For example, if a reaction in a galvanic cell takes place with decrease of volume, the work done on the cell by the atmosphere contributes (very slightly) to the amount of electrical energy which is obtainable from the reaction. 

Think About It If you ever calculate a negative voltage for a galvanic cell potential, you have done something wrong—check your work. Under standard-state conditions, the overall cell reaction will proceed in the direction that gives a positive El. ... 

Emf and work. The emf of a galvanic cell is 0.80 V. (a) How much work, in joules, is done by this cell when 5.0 C of charge pass fmm one electrode to the other (b) When a current of 0.10 A flows, how much power (in watts) is the cell producing (1 watt= 1 joule/second) (c) How much work, in joules and in calories, does the cell do when 1 faraday passes through the circuit Assume that the emf of the cell is unaffected by the rate at which current is drawn. 

The heat produced on merely burning 1 mole H2, with no work obtained directly, is — AH = 68.32 kcal. This is the <7h in the expression for efficiency the heat that is put into the working substance in a heat engine when the fuel bums. If the reaction can be made to take place in a galvanic cell, the maximum electrical work available (when the cell operates reversibly) per mole Hj is —AG = 56.69 kcal. This is the w in the expression for efficiency the work done by the device. The maximum efficiency of the cell then is... 

A galvanic cell does electrical work as the reaction drives electrons through an external circuit. The work done by a given transfer of electrons depends on the potential difference between the two electrodes. When the potential difference is large (for instance, 2 V), a given number of electrons travehng between the electrodes can do a lot of electrical work. When the potential difference is small (such as 2 mV), the same number of electrons can do only a httle work. An electrochemical cell in which the reaction is at equilibrium can do no work and the potential difference between its electrodes is zero. 

Negative values of E° mean that the reactions do not take place by themselves. An external source of energy needs to do electrical work to make the reaction take place. In that case, we say the reaction is taking place in an electrolytic cell instead of a galvanic cell. The difference between a cell that does electrical work and a cell that needs electrical work to be done on it is a crucial distinction. 

If a reversible galvanic cell (q.v.), of e.mi. E, drives a perfectly reversible motor which delivers all the electrical energy it receives as work, then for n Faradays of electricity passing through the cell and motor, the work done is nFE joules. This is the maximum net work (excluding work due to change in volume) obtainable from the reaction which takes place in the cell hence,... 

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