Experiment 34

ELECTROCHEMISTRY

Learning Objectives:

Upon completion of this experiment, students will be able to:

1. 1. 1. Identify Redox Reactions

      2. Write complete balanced net reaction

      3. Determine the cell potential and spontaneity of the net reaction

      4. Use the Nerst Equation to determine the equilibrium constant

Like acid-base reactions, redox reactions are a matched set, that is, there cannot be an oxidation reaction without a reduction reaction happening simultaneously. The oxidation alone and the reduction alone are each called a half-reaction, because two half-reactions always occur together to form a whole reaction. When writing half-reactions, the gained or lost electrons are typically included to allow the half-reactions to be balanced with respect to the transfer of the same number of electrons or charge.

Oxidation Reduction reactions (Redox) involve the movement of electrons from one species to another species.  In learning to balance these chemical reactions, the number of atoms and the number of electrons must be balanced in the chemical reaction.  One method of balancing redox reactions involves the use of oxidation and reduction half reactions.  A half reaction is the representation of a single movement of electrons to the species who has either lost electrons (oxidation) or the species who has gained electrons (reduction). 

Oxidation and reduction properly refer to a change in oxidation state — the actual transfer of electrons may never occur. Thus, oxidation is better defined as an increase in oxidation state, and reduction as a decrease in oxidation state. In practice, the transfer of electrons will always cause a change in oxidation state. 

A measure of the tendency for a reduction to occur is its reduction potential, E, measured in units of volts. At standard conditions, 25 °C and concentrations of 1.0 M for the aqueous ions, the measured voltage of the reduction half-reaction is defined as the standard reduction potential, E°. Standard reduction potentials have been measured for many half-reactions and they are listed in tables. A short list is also provided below.  For the reduction half-reactions in equations (1) and (3), the standard reduction potentials are +1.09 and +1.36 respectively.   The more positive (or less negative) the reduction potential, the greater is the tendency for the reduction to occur. So Cl2 has a greater tendency to be reduced than Br2. Furthermore, Br2 has a greater tendency to be oxidized than Cl2. The values of E° for the oxidation half-reactions are opposite in sign to the reduction potentials.  The cell potential can be determined by adding the reduction potential of the reduction half reaction to the oxidation potential of the oxidation half reaction.  

2 Br-1 (aq)  →  Br2 (aq) +  2e-  Eoox = - 1.09 V (note the change in sign)     

Cl2 (g)  +  2e-  →  2 Cl1- (aq)  Eored = + 1.36 V

Cl2 (g)  +  2 Br-1 (aq)   →  Br2 (aq)  +  2 Cl1- (aq) Eocell = 1.36 – 1.09 = 0.27 V

The positive voltage for Eocell indicates that at standard conditions the reaction is spontaneous. Recall that ΔGo = − nFEocell, so that a positive Eocell results in a negative ΔGo. Thus the redox reaction would produce an electric current when set up as a galvanic cell.   

When conditions are not standard, the Nernst equation is used to calculate the potential of a cell.

In the above equation, R is the gas constant (8.314 J / mole K ), T is the temperature (Kelvin), F  is Faraday's constant (96,485 coulombs/mole), n is the number or electrons transferred in the balanced oxidation/reduction reaction, and Q is the reaction quotient, or ([products]/[reactants]).

Remember, solids and pure liquids are not included in the Q expression.  Theoretically, E°cell   for the above reaction is 1.10 V.  Thus, the value for Ecell can be calculated, knowing [Zn2+] and [Cu2+].  A Flash based  simulation, which is no longer supported, was used to determine the cell voltage of the Copper/Zinc cell at various concentrations:

This is an older version of the above simulation.   Using the video of me performing the simulation below,  determine the cell voltage of the Copper/Zinc cell at various concentrations:

galvanic/voltaic cells to change electrodes - https://teachchemistry.org/classroom-resources/voltaic-cells   online virtual lab assignment https://www.mrpalermo.com/virtual-lab-electrochemical-cells.html 

daniel cell simulation: https://javalab.org/en/chemical_cell_en/  not interactive
standard reduction potentials:  https://javalab.org/en/standard_reduction_potentials_en/ 

https://www.edumedia-sciences.com/en/media/711-galvanic-cell 

https://chemcollective.org/vlab/106  VIRTUAL LAB: Exploring Oxidation-Reduction Reactions