Hydrogen Fuel Cells
Direct production of electricity from fuels occurs in a fuel cell.
An example is a hydrogen fuel cell.
At the cathode:
2H2O(l) + O2(g) + 4e 4OH(aq)
At the anode:
2H2(g) + 4OH(aq) 4H2O(l) + 4e
This cell is known as a PEM fuel cell
Balancing Equations by the Method of Half-Reactions
Consider the titration of an acidic solution of Na2C2O4 (sodium
oxalate, colorless) with KMnO4 (deep purple).
MnO4 is reduced to Mn2+ (pale pink), while the C2O42 is
oxidized to CO2.
The equivalence p
Balancing Equations for Reactions Occurring in Basic
The same method as above is used, but OH is added to
neutralize the H+ used.
The equation must again be simplified by canceling like terms
on both sides of the equation.
A Molecular View of the Electrode Process
Rules of voltaic cells:
At the anode electrons are products.
Oxidation occurs at the anode.
At the cathode electrons are reagents.
Reduction occurs at the cathode.
The flow of electrons from anode to cathode
Standard Reduction (Half-Cell) Potentials
We can conveniently tabulate electrochemical data.
Standard reduction potentials, E are measured relative to
The emf of a cell can be calculated from standard reduction
E = (cathode) E(a
Strengths of Oxidizing and Reducing Agents
Consider a table of standard reduction potentials.
We can use this table to determine the relative strengths of
reducing (and oxidizing) agents.
The more positive the E, the stronger the oxidizing agent
EMF and G
We can show that:
G = nFE
where G is the change in free energy, n is the number of
moles of electrons transferred, F is Faraday's constant, and
E is the emf of the cell.
We define a faraday (F) as:
Since n and F are positive, if G < 0 then E
A concentration cell is one whose emf is generated solely
because of a concentration difference.
Example: Consider a cell with two compartments, each with a
Ni(s) electrode but with different concentrations of Ni2+(aq).
One cell has
The most common nonrechargeable battery is the alkaline
Powdered zinc metal is immobilized in a gel in contact with a
concentrated solution of KOH.
Thus, these batteries are alkaline.
The reaction at the anode is:
Zn(s) + 2OH
Chemical reactions in which the oxidation state of one or more
substances change are called oxidation-reduction reactions
Oxidation involves loss of electrons (OIL).
Reduction involves gain of electrons (RIG