Standard Reduction Potentials

The reaction in a galvanic cell is always a redox reaction that can be broken down into two half-reactions.  We can determine the cell potential by assigning a potential to each half-reaction and summing them.
Standard reduction potentials are the E° values corresponding to reduction half-reactions with all solutes at 1 M and all gases at 1 atm.
Combining the half-reactions to get a balanced redox reaction often requires one of the reactions to be reversed (in order to cancel out the electron transfers and since redox reactions must involve a substance being oxidized and a substance being reduced).  Also, the half-reaction with the largest possible potential (the reduction) will not be the reaction that is reversed.  The other half-reaction will be forced to run in reverse (the oxidation).  During this manipulation, the net potential of the cell will be the difference between the two.  (cathode - anode)  Also, because subtraction means "change the sign and add", we will change the sign of the oxidation (anode) reaction when we reverse it and add it to the reduction (cathode) reaction.
Another possible manipulation is multiplying the half-reaction by integers to make the number of electrons lost equal the number gained.  However, when this is done, the value of  E° is not changed.  Since standard reduction potential is an intensive property, the potential is not multiplied by the integer.
Another question is which reaction must run in reverse.  This can be answered by considering the sign of the potential of a working cell.  A cell will always run spontaneously in the direction that produces a positive cell potential.

A complete description of a galvanic cell usually contains these four items:
-the cell potential
-the direction of electron flow (obtained by inspecting the half-reactions and using the direction that gives a positive Ecell)
-designation of the anode and cathode
-the nature of each electrode and the ions present in each compartment

Galvanic Cells

The reactions most commonly involved in electrochemistry are oxidation-reduction reactions, which involve a transfer of electrons from the reducing agent to the oxidizing agent.  The key to harnessing the chemical energy that comes from these reactions is separating the oxidizing agent from the reducing agent, which requires the electron transfer to occur through a wire.  The current produced in the wire by the electron flow from the redox reactions can then be directed into a device such as a motor to provide work.
However, sometimes the current stops flowing because of charge buildups in the two compartments of an apparatus.  The solutions then must be connected so that ions can flow to keep the net charge in each compartment zero.  A salt bridge or a porous disk is required to allow ions to flow without extensive mixing of the solutions.
The electrode compartment in which oxidation occurs is called the anode and the electrode compartment in which reduction occurs is called the cathode.
These are the characteristics of a galvanic cell, which is defined as a device in which chemical energy is changed to electrical energy.
Cell potential (unit=volt) is the driving force on the electrons that occurs when the oxidizing agent in one compartment of the cell pulls electrons through a wire from the reducing agent.  Cell potential can be measured by a voltmeter or a more accurate potentiometer.  Potentiometers are better at representing maximum cell potential, in which no energy is wasted.

What is electrochemistry?

Electrochemistry is defined as the study of the interchange of chemical and electrical energy.  It primarily involves oxidation-reduction reactions and two major processes: the generation of an electric current from a spontaneous chemical reaction and the use of a current to produce chemical change (the opposite process).

The breakdown of topics presented in this blog will be:

I. Galvanic Cells
II. Standard Reduction Potentials
III. Cell Potential, Electrical Work, and Free Energy
IV. Dependence of Cell Potential on Concentration
V. Batteries
VI. Corrosion
VII. Electrolysis
VIII. Commercial Electrolytic Processes

Welcome!

Hello everybody!  My goal in creating this site is to attempt to demystify the complex subject of electrochemistry.  Since the class doesn't get to spend much time on it, perhaps a blog that covers the basics will help.  Since our class this year self-taught this chapter from the textbook, this blog will be based heavily on the information presented in the book.  Enjoy!