Unit 8: Electrochemistry Applications Lesson 2: Standard Reduction Potentials

Measuring Voltage Voltage, or electrical potential, is the work done per unit of charge (made up of electrons) transferred. We can’t measure the voltage of a lone half-cell (no e- flow!), but we can measure the voltage between two half-cells. The hydrogen half-cell is arbitrarily defined as the zero point, and all other half-cell reactions are compared to it.

Standard Reduction Potential E° is the standard reduction potential in volts. Standard conditions are defined to be: 25°C All gases at 1 atm All elements in their standard states 1 M solutions

Calculating E°cell When we add together two half-reactions, we can also add together their voltages. Note that when you reverse a half-reaction, you must reverse the sign of its E° value. If the potential of the cell (E°cell) is positive, then the reaction will be spontaneous. If E°cell is negative, the reaction is non-spontaneous.

Examples What is the potential of the following cell? Ni2+ + Fe → Ni + Fe2+ Your turn! Calculate the potential of each cell. a) Ni + Fe2+ → Ni2+ + Fe b) 3 Ag+ + Al → 3 Ag + Al3+ *Note: since voltage is work done per unit charge, it does not need to be multiplied when you multiply a half-reaction to cancel electrons. 25 min

Brain Break! Double Shutter

The Big Picture You must consider all species present in the cell. If a reaction occurs in an acidic solution, the reduction of H+ is a possible reaction. If a reaction occurs in a neutral solution, the reduction of neutral water is a possible reaction.

Cells and Equilibrium As a cell operates, the [reactant ion] decreases and the [product ion] increases. The solutions do not remain at 1 M! Because of the changes in concentration, the E of the reduction reaction decreases, and the E of the oxidation reaction increases, until the two values are equal but opposite, and Ecell is zero. The cell is now at equilibrium.

Practice Pg. 224 #36 5 min