Unit the Fifth: Electrochemistry
Stuff to know:
Faraday’s Constant: F = 96,500 Coulombs per mole (of electrons)
Q = [C]c[D]d for the reaction a A + b B  c C + d D
[A]a[B]b
Ecell = E0cell – (RT/nF) *ln(Q)
= E0cell – (0.0592/n) *log(Q) @250C
R = 8.31 volt*coulomb/(mol*K)
Gibbs Free NRG= -nFEcell
log K = nE0 _
0.0592
Oxidation-Reduction reactions:
One reactant gains an electron (gets
reduced), one reactant loses an electron (gets oxidized)
The reactant that gets reduced is the oxidizing agent, the one that gets oxidized is
the reducing agent.
Oxidation numbers: An atom's oxidation number is its apparent electric charge within its
compound.
Determining oxidation numbers:
-Pure elements' atoms have an oxidation number of zero. (Al, Cl2, S8 = 0)
-Single-atom ions have an oxidation number equal to the overall charge. (K+ = +1)
-Fluorine is always -1 in compounds with other elements.
-Cl, Br, and I are -1 unless they are with oxygen or fluorine.
-H is usually +1 and O is usually -2.
-The sum of the oxidation numbers in any compound equal the overall charge of the
compound.
Balancing Redox reactions: (Example: X + Y  Xn+ + Yn-)
-Split the reaction into 2 half reactions: X  Xn+ + ne- and ne- + Y  Yn-Add H+ and H2O for balancing in acid or OH- and H2O for balancing in a base.
-Balance half reactions and multiply them by constants so the number of electrons in
products equal the number of reactant electrons.
-Add the two half reactions, with the electrons cancelling out.
Electrochemical Cells
They generate a current by immersing metals with differing reduction potentials into a
solution of their ions. A salt bridge containing a negative ion connects the two solutions.
Example: Zinc(II) and Copper(II) have reduction potentials of -0.763 V and 0.337
V, respectively. This means that the Cu half-reaction experiences a reduction since its
reduction potential is higher. Reduction occurs at the cathode, the “destination” of the
electron flow through the wire. E0 (standard electric potential) of the cell is equal to the
higher reduction potential minus the lower potential (0.337 - -0.763 = 1.10 volts).
The zinc electrode is:
The copper electrode is:
-An anode
-Part of an oxidation
-Losing mass
-Sending electrons through
the wire
-A cathode
-Part of a reduction
-Gaining mass
-Receiving electrons
from the wire