Electrochemistry studies how chemical
and electrical energy are converted.
Oxidation numbers & Redox reactions
Oxidation numbers are the charge on an atom if the electrons
involved in the bond are assigned to the more electronegative atom
in the bond. When oxidation numbers change during a chemical
reaction, it is a redox reaction.
Oxidation number method of balancing
The oxidation number method is used for balancing simple redox
reactions that cannot be easily balanced by the inspection method.
It includes:
- Determine the oxidation numbers of each atom.
- Determine the net change in charge. Use the net change
to determine the ratio of atoms that would cancel out the
net charge change.
- Use the ratio as coefficients in the simplest compounds
containing those elements.
- Finish balancing by the inspection method.
Half-reaction method of balancing
The half-reaction method is for the most difficult redox reactions:
- Use oxidation numbers to determine what�s oxidized and
what�s reduced.
- Write two half-reactions, one for reduction and one for
oxidation
- Balance all elements except H and O using inspection
method.
- For an acid redox reaction: Balance the
O�s by adding H2O to the side needing more O. For a
base redox reaction: Balance O by adding twice as
many OH- to the side needing more O.
- For a acid redox reaction: Balance the
H�s by adding H+ to the side needing more H�s. For a base
redox reaction: Balance H�s by adding H2O to the side
needing more H�s.
- Determine the charge of each side of each reaction.
Balance the charges by adding electrons to the side with the
more positive charge for each reaction.
- Multiply the half-reactions by factors that will allow
the electrons to cancel out.
- Add the two half-reactions back together.
Voltaic cells
A voltaic cell separates the reduction and oxidation reaction and
forces the electrons to flow over a wire (producing electricity)
from the oxidation reaction (at the anode) to the reduction reaction
(at the cathode). The cell consists of the two separate half
reaction, metal electrodes and a wire for conducting the electrons,
and a salt bridge for balancing the charge build-up to extend the
time the cell will operate. Line notation is a short-hand way of
describing a cell:
- Anode written first
- Reactants written 1st on each side
- Anode & Cathode separated with ║
- Different states of matter within same side separated
with │
- Same states of matter within same side separated with a
comma
Cell potentials
The cell potential (or electromotive force) of a voltaic cell is due
to the potential energy difference of the electrons before the
transfer and after the transfer. A standard reduction potential is
the potential that would be produced between a given half-reaction
and hydrogen (hydrogen�s standard reduction potential has been
defined as 0). The standard reduction potentials can be used to
calculate the cell potential: EMF = cathode � anode. Positive EMF
values indicate a spontaneous process.
Electrolytic cells
An electrolytic cell is the opposite of a voltaic cell. An
electrolytic cell converts electrical energy into chemical energy by
forcing a reaction to proceed in the non-spontaneous direction by
putting electricity in. The voltage need to force the reaction in
the opposite direction is at least that produced by the spontaneous
process.
