Cracking the AP Chemistry Exam

Part IV

Content Review for the AP Chemistry Exam

Chapter 5

Big Idea #3: Chemical Reactions, Energy Changes, and Redox Reactions

GALVANIC CELLS

In a galvanic cell (also called a voltaic cell), a favored redox reaction is used to generate a flow of current.

Look at the following spontaneous redox reaction:

In a galvanic cell, the two half-reactions take place in separate chambers, and the electrons that are released by the oxidation reaction pass through a wire to the chamber where they are consumed in the reduction reaction. That’s how the current is created. Current is defined as the flow of electrons from one place to another.

In any electric cell (either a galvanic cell or an electrolytic cell, which we’ll discuss a little later) oxidation takes place at the electrode called the anode. Reduction takes place at the electrode called the cathode.

There’s a mnemonic device to remember that.

AN OX
RED CAT

The salt bridge maintains the electrical neutrality in the cell. At the cathode, where reduction occurs and the solution is becoming less positively charged, the positive cations from the salt bridge solution flow into the half-cell. At the anode, where oxidation occurs and the solution is becoming more positively charged, the negative anions from the salt bridge solution flow into the half-cell. If the salt bridge were to be removed, the solutions in the half-cells would become electrically imbalanced, and the voltage of the cell would drop to 0.

Under standard conditions (when all concentrations are 1 M), the voltage of the cell is the same as the total voltage of the redox reaction. Under nonstandard conditions, Le Châtelier’s principle can be applied to determine the voltage change. If the concentration of the products in a voltaic cell increases, the voltage decreases. If the concentration of the reactants increases, the voltage increases. This will be examined in greater detail when equilibrium is studied in Chapter 8.