electrochem
more fun facts and concepts
Spontaneity and Electrochemistry
• For those wondering how to determine spontaneity in
redox reactions, this is how to do it:
• We know that ΔG° determines spontaneity and we can
manipulate the equation used to determine it to
incorporate E°.
• The equation we can use is ΔG°=-nFE°, where n is the
mols of electrons exchanged, F is Faraday’s constant
(96500 C/mol of e-), and E° is the standard potential.
• In case E is given instead of E°, we can find ΔG instead
with ΔG=-nFE.
Spontaneity and Electrochemistry
• Suppose we want to find E° for a cell, but one
of the two reactants is an unknown
compound. However, let’s say that these two
reactants are at their standard state.
• If we know their equilibrium concentrations at
the standard state we can find E°.
Spontaneity and Electrochemistry
• To do this, we must manipulate the equation
ΔG°= -nFE°.
• Think back to entropy. ΔG° also equals –RTlnK.
• So if ΔG°= -RTlnK and ΔG°= -nFE°, than we can
say that -RTlnK = -nFE°.
• From that we can derive that E° = RTlnK/-nF
• As you can see, K is the only real unknown
here and solving for it is easy enough given
the concentrations at equilibrium.
Spontaneity and Electrochemistry
• A useful picture to have when talking about
ΔG° and E° is:
More Manipluations!
• Suppose we have reactants that are not at their
standard states. We can’t use standard reduction
potentials to find E° of the cell. Instead, we must
find E of the cell.
• In this case, we’ll have to think back to ΔG and
relate it to ΔG°. The equation for that is
ΔG = ΔG° + RT ln Q.
• We know that ΔG°= -nFE° and ΔG=-nFE, therefore
we can change ΔG = ΔG° + RT ln Q to
nFE = -nFE° + RT ln Q.
More Manipulations!
• If we divide both sides of that equation by
-nF, we can get the equation E = E° - RTlnQ/nF.
• Since we are using E°, the temperature will be
25°C or 298K.
• This is a very famous equation called the
Nernst Equation, which was made by a guy
named Walther Nernst.
Electrolysis
• What is electrolysis?
• Electrolysis is the process in which electrical
energy is used to cause a nonspontaneous
chemical reaction to occur.
• An example of electrolysis is running a current
through water in order to produce hydrogen
gas.
Electrolysis
• Generally you will have to know electrolysis
when a question asks how many mols of
grams of something is produced when
subjected to a current.
• The current most commonly used is Amperes,
which are coulombs per second.
• Along with the information that
1 mole e- = 96,500 C, we can easily apply the
idea of electrolysis.
Electrolysis
• Let say we have a solution of NaCl and we, for
some reason, want Na(s). Suppose we run a
current of 15 Amperes through the solution for 2
minutes. How many grams of Na(s) will we get?
• First of all, 2 minutes is really 120 seconds and 15
Amperes is really 15 Coulombs/sec. Thus, in 120
seconds, we ran a total of 120 * 15 coulombs or
1800 coulombs through the solution.
Electrolysis
• Given that 1 mole e- = 96,500 C, we can now
easily find out how many mols of e- we ran
through the solution.
• 1800C/96500C/mols e- = .018653 mols e- .
• Since you only need one mol of electrons to
reduce Na+, there are .018653 mols Na(s).
• To find grams of Na(s), just divide the mols by
the molar mass, or
.018653/22.99 = 8.11E-4g Na(s)
So that’s that.
We blame Apple for our aesthetically pleasing, yet still really boring looking
PowerPoint.
We also blame G because he made us teach this to you kids.