Spontaneous processes work available from

If the process is irreversible, q m spontaneous change, weieo will be less than the best available, which is AG. We will see later that the maximum electrical work available from an electrical cell will be obtained under reversible conditions, where the cell e.m.f. is opposed by an infinitesimally smaller potential. The electrons are made to work their passage around the external circuit to the maximum of their ability. Under these conditions, the electrical work depends on the equilibrium voltage, E, and on the number of electrons made to go through the circuit, corresponding to nF coulombs F is a unit of charge, the Faraday=96 485 coulombs/mol of electrons and / =numbcr of moles of electrons or equivalents . This is expressed as ... 

Achieving the maximum work available from a spontaneous process can occur only via a hypothetical pathway. Any real pathway wastes energy. If we could discharge the battery infinitely slowly by an infinitesimal current flow, we would achieve the maximum useful work. Also, if we could then recharge the battery using an inflnitesimally small... 

Thus enthalpy is a potential for (reversible) work in a constant S, P process, and will always decrease in spontaneous processes in such systems, only PAV work being allowed. Again, the fact that H is a potential for (isobaric, reversible) work is almost self-evident from looking at equation (5.27), since PAV is the maximum pressure-volume work available and AU is tlie rest of the energy change available. In fact, the restriction of constant entropy is not necessary for H to be a work potential. If AS is not zero, the amount of energy available for work will be reduced but still maximized in a reversible process. 

With the exception of entropy, each of these functions reaches a minimum at equilibrium, and each decreases during spontaneous processes (at constant values of the subscripted variables). Thus each serves as a thermodynamic potential, or as a measure of the capacity for a system described by the subscripted variables to undergo some change or process. These thermodynamic potentials also provide a measure of the maximum work or heat output available from a process ... 

From the discussion of heat engines, the second law of thermodynamics states that it is impossible to achieve heat, taken from a reservoir, and convert it into work without simultaneous delivery of heat from the higher temperature to the lower temperature (Lord Kelvin). It also states that some work should be converted to heat in order to make heat flow from a lower to a higher temperature (Principle of Clausius). These statements acknowledge that the efficiency of heat engines could never be 100% and that heat flow from high temperatures to low temperatures is not totally spontaneous. Simply, the second law states that natural processes occur spontaneously toward the direction in which less available work can be used. 

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