Download fied molal concentration. The molality, or molal concentration, is the

faster the ball will roll. If we decrease the steepness enough, the ball won’t roll at all.
Now, let’s apply this analogy to electrical potential. We replace the ball with electrically charged
particles. Increasing the potential causes charge
to move, and the greater the potential, the more
charge moves. With electric potential, however,
the potential exists through the wire and the
salt bridge, which is why positive and negative
charges move along them. It is important to know
that unlike the gravitational potential that pulls
the ball down the ramp, electric potential can be
either positive or negative because it is induced by
positive and negative charges. As a result, positive
and negative charges move in opposite directions
when exposed to the same potential. A potential
that causes negatively charged particles to move
left will cause positively charged particles to move
right.
In our electrochemical cell, however, the wire will
only be conducting negative charges, and the salt
bridge will only be conducting positive charges.
The positively charged ions are too large to move
through the wire, and the tiny, negatively charged
electrons move more easily through the wire than
they do through the salt bridge.
PREDICTING A CELL’S POTENTIAL
Amazingly, someone—the German chemist and
physicist Walther Nernst (1864–1941)—was actually able develop an equation that could predict
the electric potential of a cell. This equation, called
the Nernst equation, is one of the most important
developments in electrochemistry:
(3.1)
equation:
In this
mE is the potential across the cell.
mE ° is the standard electrode potential of the
German chemist and physicist
Walther Nernst developed an
equation that could predict the
electric potential of a cell.
fied molal concentration. The molality, or molal
concentration, is the moles of solute per kilograms
of solvent. In a lot of situations, the activity and
molar concentration of a solution are nearly identical, and Equation 3.1 can be turned into:
reaction.
m
R is the universal gas constant, which is approximately equal to 8.314 J*K–1*mol –1.
mF
is Faraday’s constant, which is equal to
96,485 C*mol –1.
mT is the absolute temperature in Kelvins.
mz is the number of electrons transferred in the
reaction.
ma
ma
red
ox
is the activity of the reduced species.
is the activity of the oxidized species.
The three most confusing variables in this equation
tend to be E°, ared, and aox. If you haven’t had physical chemistry yet, the activities ared and aox might
seem like foreign concepts. Activity is just a modi-
2 0 15
2 0 14 –
(3.2)
In Equation
3.2, b is the molar concentration.
In scenarios where the activity does not equal the
molar concentration, the activity is necessary. In
these cases, the concentration alone is not enough
to accurately solve the Nernst equation due to
effects like ionic strength and ion size. While the
molar concentration is easier to calculate when
preparing a solution, the activity can be easily
determined experimentally using an ion selective
electrode (but that’s a whole other subject in itself).
Now, we are left to consider the standard electrode
potential (E ° ). The standard electrode potential is
the difference in the standard reduction potentials
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