Overview
The voltage of an electrochemical cell is related to the
concentration of the solutions involved in the cell. A change in
concentration of cell solutions results in a change in voltage that
can be quantitatively described by the Nernst equation.
Background
The voltage of an electrochemical cell involves an oxidation reaction and a reduction reaction. In the case of a copper-zinc cell the two half reactions are written as reductions, and their standard cell potentials are:
Cu2+(aq) + 2e- ---> Cu(s) E° reduction = +0.34 V
Zn2+(aq) + 2e- ---> Zn(s) E° reduction = -0.76 V
Copper will be the reduced species in the copper-zinc cell due it's larger standard reduction potential. The voltage then for a Zn, Zn2+||Cu2+, Cu cell in which the copper ion and zinc ion concentrations are equal is found as follows:
Zn(s) + Cu2+(aq) ---> Cu(s) + Zn2+(aq)
equation 1
ex. Cu2+(aq) + 2e- ---> Cu(s) E° reduction = +0.34 V
Zn(s) ---> Zn2+(aq) + 2e- E° oxidation = -0.76 V
Ecell = Ereduction - Eoxidation
+1.10V = +0.34 V - (-0.76 V)
The voltage of any electrochemical cell is a function of the molar concentrations of the compounds involved in the cell, as described by the Nernst equation:
aA(s) + bB+(aq) ---> cC(s) + dD+(aq)
equation 2
Ecell = E° - (0.06/n) log {[D+]d/[B+]b}
equation 3.
Using equation 1 as the balanced chemical equation for the Zn,
Zn2+||Cu2+, Cu cell. Equation 3 becomes:
Ecell = E° - (0.03) log
[Zn2+]/[Cu2+]
As the Cu2+ concentration decreases the voltage of the
cell changes. The Cu2+ concentration can change due to a
dilution, or due to the formation of a complex ion,
Cu(NH3)42+, upon addition of
NH3.
Materials
0.1 M Zn(NO3)2
1-10 cm Zn strip
0.1 M Cu(NO3)2
1-10 cm Cu strip
3 M NH3
1 M KNO3
Filter paper
2-250 mL beakers
Procedure
1. Construct a two-beaker salt bridge using filter paper as instructed by your teacher.
2. Soak the salt bridges in 1 M KNO3 until it is completely saturated. Let the bridges dry on a paper towel until it is needed.
3. Fill one of the beakers with 100 mL of 0.1 M Zn(NO3)2 solution.
4. Fill the other beaker with 100 mL of 0.1 M Cu(NO3)2 solution.
5. Place a piece of copper wire in the copper solution, and a zinc strip in the zinc solution.
6. Place the salt bridge across the beakers creating an electrochemical cell.
7. Set up the CBL and the TI-8X system for the following:
1 voltage probe
Using the STORED calibration
Apply the CBL voltage probe leads to the metal pieces in the solution using the red lead on the copper metal. Measure the voltages for the Cu, Cu2+||Zn2+, Zn cell you have just created using the Monitor Input option under the Collect Data screen.
Set the TI-8X/CBL system up to collect data by the Trigger/Prompt method. You will add 3 M NH3 drop wise to the Cu, Cu2+ side of the electrochemical cell. You should collect data and create a graph of voltage to drop number of NH3.
2. Add one drop 3 M NH3 to the Cu(NO3)2 solution and stir. Record observations and the new voltage.
3. Continue adding 3 M NH3 drop wise until the measured voltage remains constant.
4. Graph the data.
Sketch the graph of voltage vs. drop number in the space below. You may need to use the Select( feature on the TI-83 in order to zoom on the important part of your data. Consult your teacher for this skill.
Use statistical analysis with the calculator and determine the mathematical model, and line of best fits.
Findings
1) Explain the appearance of your graph.
2) Explain the effects on the copper side of the battery due to the addition of ammonia.
3) If the voltage of the cell is determined based upon the Nernst equation, explain the voltage vs. drops NH3 graph using the Nernst equation.
4) How would the voltage of a Cu, Cu2+||Zn2+, Zn cell using equal 1 M concentrations of copper ion and zinc ion compare to the voltage recorded from a cell using 0.1 M solutions of copper and zinc ions. Explain the difference.
5) A student measures the potential of a cell made up with a 1 M CuSO4 in one solution and 1 M AgNO3 in the other. There is a Cu electrode in the CuSO4 and an Ag electrode in the AgNO3. She finds that the potential, or voltage, of the cell is 0.45V, and that the Cu electrode is negative.
a. At which electrode is oxidation occurring?
b. Write the equation for the oxidation reaction.
c. Write the equation for the reduction reaction.
d. If the potential of the silver, silver ion electrode is taken to be 0.000V in oxidation or reduction, what is the potential for the Cu electrode?
e. If the reduction potential for the Ag electrode equals 0.80V, as in standard tables of electrode potentials, what is the value of the potential for the oxidation reaction of copper.