faster the ball will roll. If we decrease the steep- ness enough, the ball won’t roll at all. Now, let’s apply this analogy to electrical poten- tial. We replace the ball with electrically charged particles. Increasing the potential causes charge to move, and the greater the potential, the more charge moves. With electric potential, however, the potential exists through the wire and the salt bridge, which is why positive and negative charges move along them. It is important to know that unlike the gravitational potential that pulls the ball down the ramp, electric potential can be either positive or negative because it is induced by positive and negative charges. As a result, positive and negative charges move in opposite directions when exposed to the same potential. A potential that causes negatively charged particles to move left will cause positively charged particles to move right. In our electrochemical cell, however, the wire will only be conducting negative charges, and the salt bridge will only be conducting positive charges. The positively charged ions are too large to move through the wire, and the tiny, negatively charged electrons move more easily through the wire than they do through the salt bridge. PREDICTING A CELL’S POTENTIAL Amazingly, someone—the German chemist and physicist Walther Nernst (1864–1941)—was actu- German chemist and physicist ally able develop an equation that could predict Walther Nernst developed an the electric potential of a cell. This equation, called equation that could predict the the Nernst equation, is one of the most important electric potential of a cell. developments in electrochemistry:

(3.1) fied molal concentration. The molality, or molal concentration, is the moles of solute per kilograms In this equation: of solvent. In a lot of situations, the activity and m E is the potential across the cell. molar concentration of a solution are nearly identi- m E° is the standard electrode potential of the cal, and Equation 3.1 can be turned into: reaction. (3.2) m R is the universal gas constant, which is approx- imately equal to 8.314 J*K–1*mol–1. In Equation 3.2, b is the molar concentration. m F is Faraday’s constant, which is equal to In scenarios where the activity does not equal the 96,485 C*mol–1. molar concentration, the activity is necessary. In m T is the absolute temperature in Kelvins. these cases, the concentration alone is not enough to accurately solve the Nernst equation due to z is the number of electrons transferred in the m effects like ionic strength and ion size. While the reaction. molar concentration is easier to calculate when m ared is the activity of the reduced species. preparing a solution, the activity can be easily m aox is the activity of the oxidized species. determined experimentally using an ion selective electrode (but that’s a whole other subject in itself). The three most confusing variables in this equation tend to be E°, ared, and aox. If you haven’t had physi- Now, we are left to consider the standard electrode cal chemistry yet, the activities ared and aox might potential (E°). The standard electrode potential is seem like foreign concepts. Activity is just a modi- the difference in the standard reduction potentials

E RESOURCE GUIDE • REVISED PA USAD SCIENC GE 2015 • 2014– 31