280
Electrochemistry I: Batteries and Free Energy
Therefore,
electrical energy
, ,. w .
c , t
J
coulombs
\ / calories \
= (volts)(moles of electrons)! — : — j— : - — : - : - -]
\ mole of electrons/ \ volt coulomb/
r
96,487
= 23,060 x nE cal
(17-8)
The cell voltages we have been talking about in this chapter are all maximal
voltages, voltages measured with a potentiometer that just matches the voltage
of the cell without actually draining any current from the cell, or with a
vacuum-tube voltmeter whose resistance is so high that the result is essentially
the same. If a battery is used to do work, the voltage will be less; not all the
energy that is produced can be employed usefully because of partial dissipation
as heat. In fact, all the electrical energy could be wasted if you so wished.
Nevertheless, our potentiometrically-measured (maximal) voltages can be
used for the calculation of the maximal amount of available electrical energy
that can be obtained from a chemical reaction, regardless of whether it actually is
used for work. For a given chemical reaction, we can equate maximal available
electrical energy with maximal available work, and write
maximal available work = maximal available electrical energy
= 23,060 x nE cal
(17-9)
THE CONCEPT OF FREE ENERGY
The preceding statements imply that every substance has an amount of energy,
called its free energy (G), that could be used for useful work. If, as a result of a
chemical reaction that occurs at constant temperature and pressure, the sum of
the products ("state 2") possesses an amount of free energy G 2 whereas the
sum of the reactants ("state 1") possesses an amount of free energy G 1? then
the change in free energy (AG) for the reaction will be
AG = G2 — GI = Gproduets ~~ Greactants
(17-10)
If the sign of AG is negative, it means that the products have less free energy
than the reactants. The magnitude of the change is the maximal amount of work
that might have been obtained from the reaction. The maximal obtainable work
is associated with a decrease in free energy (-AG)—that is, the work is obtained at the expense of the chemical system (Figure 17-3). In equation form,
we would write
maximal available work = -AG
(17-11)
Electrochemistry I: Batteries and Free Energy
Therefore,
electrical energy
, ,. w .
c , t
J
coulombs
\ / calories \
= (volts)(moles of electrons)! — : — j— : - — : - : - -]
\ mole of electrons/ \ volt coulomb/
r
96,487
= 23,060 x nE cal
(17-8)
The cell voltages we have been talking about in this chapter are all maximal
voltages, voltages measured with a potentiometer that just matches the voltage
of the cell without actually draining any current from the cell, or with a
vacuum-tube voltmeter whose resistance is so high that the result is essentially
the same. If a battery is used to do work, the voltage will be less; not all the
energy that is produced can be employed usefully because of partial dissipation
as heat. In fact, all the electrical energy could be wasted if you so wished.
Nevertheless, our potentiometrically-measured (maximal) voltages can be
used for the calculation of the maximal amount of available electrical energy
that can be obtained from a chemical reaction, regardless of whether it actually is
used for work. For a given chemical reaction, we can equate maximal available
electrical energy with maximal available work, and write
maximal available work = maximal available electrical energy
= 23,060 x nE cal
(17-9)
THE CONCEPT OF FREE ENERGY
The preceding statements imply that every substance has an amount of energy,
called its free energy (G), that could be used for useful work. If, as a result of a
chemical reaction that occurs at constant temperature and pressure, the sum of
the products ("state 2") possesses an amount of free energy G 2 whereas the
sum of the reactants ("state 1") possesses an amount of free energy G 1? then
the change in free energy (AG) for the reaction will be
AG = G2 — GI = Gproduets ~~ Greactants
(17-10)
If the sign of AG is negative, it means that the products have less free energy
than the reactants. The magnitude of the change is the maximal amount of work
that might have been obtained from the reaction. The maximal obtainable work
is associated with a decrease in free energy (-AG)—that is, the work is obtained at the expense of the chemical system (Figure 17-3). In equation form,
we would write
maximal available work = -AG
(17-11)