CHEM 203 Harry S Truman Measuring Electrode Potentials of Voltaic Cells Discussion

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Exp. 8: Measuring Electrode Potentials of Galvanic Cells Chemistry 203 – General Chemistry II Dept. of Physical Sciences and Engineering Wilbur Wright College Experiment 8: Measuring Electrode Potentials of Voltaic Cells INTRODUCTION Voltaic (or Galvanic) cells convert chemical energy into electrical energy and form essential components of commercial batteries. In an oxidation–reduction reaction, electrons flow from the substance that is oxidized, which loses electrons, to the substance that is reduced, which gains electrons. In a voltaic cell, the flow of electrons accompanying a spontaneous oxidation–reduction reaction occurs via an external pathway, and an electric current is produced. The basic design of a galvanic or voltaic cell is shown in Figure 1. The substances involved in each half-reaction are separated into two compartments connected by an external wire and a salt bridge. Each half-reaction takes place at the surface of a metal plate or wire called an electrode. The electrode at which oxidation occurs is called the anode, while the electrode at which reduction occurs is called the cathode. Electrons flow spontaneously from the anode (the negative electrode) to the cathode (the positive electrode). Charge buildup at the electrodes is neutralized by connecting the half-cells internally by means of a salt bridge, a porous barrier containing an electrolyte. Dissolved ions flow from the salt bridge to either electrode, thus completing the electrical circuit. Figure 1. Voltaic Cell: Zn(s) ∣ Zn2+ (aq, 1M) ∣∣ Cu2+ (aq, 1M) ∣ Cu(s) The ability of a voltaic cell to produce an electric current is called the cell potential (Ecell ) and is measured in volts (V). If the cell potential is large, there is a large “electromotive force” pushing or pulling electrons through the circuit from the anode to the cathode. The cell potential for a spontaneous chemical reaction in ° a voltaic cell is always positive. The standard cell potential (Ecell ) is defined as the maximum potential difference between the electrodes of an electrochemical cell under standard conditions (25ºC, 1 M concentrations of ions, and 1 atm pressure for gases). It is impossible to directly measure the potential for a single electrode. However, the overall cell potential for an electrochemical cell may be expressed as the sum ° of the standard reduction potential (Ered ) for the cathode half-reaction and the standard oxidation potential ° (Eox ) for the anode half-reaction. ° ° ° Ecell = Ered + Eox When two half-cells are combined in a voltaic cell, the half-reaction that has a more positive standard ° reduction potential (Ered ) will occur as a reduction, while the half-reaction that has a less positive standard 1 Exp. 8: Measuring Electrode Potentials of Galvanic Cells Chemistry 203 – General Chemistry II Dept. of Physical Sciences and Engineering Wilbur Wright College reduction potential will be reversed and will take place as an oxidation. Since all standard half-cell potentials ° ° ° are listed as reduction potentials, the sign for Eox is reversed (in other words (Eox = -Ered ). ACTIVITY SERIES OF METALS: RELATIONSHIP TO HALF-REACTION POTENTIALS An active metal, Zn(s), can reduce another metal cation, Cu2+, in a solution and itself get oxidized. For example, in the redox reaction Zn(s) + Cu2+ (aq) → Zn2+ (aq) + Cu(s) the metal Zn(s) is more active than Cu(s), and therefore it is a stronger reducing agent when compared to Cu(s). Consequently, Zn(s) readily gets oxidized because it has a more positive standard half-cell potential for its oxidation, relative to Cu(s). Zn(s) → Zn2+ (aq) + 2e− ° Eox = 0.76 V Cu(s) → Cu2+ (aq) + 2e− ° Eox = −0.34 V ° In this laboratory activity, you will measure the standard cell potentials (Ecell ) of voltaic cells using an ° electrochemical cell simulator. Upon measuring the standard cell potentials (Ecell ) of various voltaic cells in ° which Cu(s) serves as the cathode, the standard oxidation potentials (Eox ) for various anodes will be ° calculated. A comparison of the Eox values for each metal acting as an anode reflects the relative tendency of ° the metal to undergo oxidation (i.e. act as a reducing agent). A more positive the Eox value indicates the metal is: more easily oxidized, a stronger reducing agent, and a more active metal. Organizing the standard ° oxidation potentials (Eox ) in order of increasing value creates the activity series of metals. ° Standard Potentials: List of reduction half-reactions with corresponding Ered Literature values Select Electrode on the Left: Metal (assigned as the anode) that will connect to the black (-) voltmeter lead. Select Solution on the Left: Matching (left) metal nitrate solution. ° Measure Cell Voltage: Reveals voltmeter reading (i.e. Ecell ). Select Electrode on the Right: Metal (assigned as the cathode) that will connect to the red (+) voltmeter lead. Select Solution on the Right: Matching (right) metal nitrate solution. New Problem: Clears the electrochemical cell components. Level: Setting to access various features for problem-solving. 2 Exp. 8: Measuring Electrode Potentials of Galvanic Cells Chemistry 203 – General Chemistry II Dept. of Physical Sciences and Engineering Wilbur Wright College PROCEDURE Simulation – Data Collection 1. Locate the simulation using the following link: http://web.mst.edu/~gbert/Electro/Electrochem.html ° Table 1: Measurement of 𝐄𝐜𝐞𝐥𝐥 1. Construct the electrochemical cell indicated in the first row of Table 1. Select the LEFT: metal & ion solution, and select the RIGHT: metal & ion solution. Leave the solution concentrations at 1.00M and Level set to zero. ° 2. Select “Measure Cell Voltage”. Record the measured Ecell in Table 1. Select “New Problem”. 3. Repeat for all the electrochemical cells represented in the rows of Table 1. ° Table 2: Determination of 𝐄𝐨𝐱 and Activity Series of Metals 1. Construct the electrochemical cell indicated in each row of Table 2. Select the LEFT: metal & ion solution, and select the RIGHT: metal & ion solution. Leave the solution concentrations at 1.00M and Level set to zero. ° 2. Select “Measure Cell Voltage”. Record each measured Ecell in Table 2. ° ° ° 3. Using the measured Ecell values and given Ered = 0.34 V for copper, calculate Eox for the various ° ° ° anodes using the equation: Ecell = Ered (𝑐𝑎𝑡ℎ𝑜𝑑𝑒) + Eox (𝑎𝑛𝑜𝑑𝑒) ° Table 3: Determine the 𝐄𝐨𝐱 value of Whodatium (Wd)/Whodatium (II) Nitrate 1. On the lower right side of the simulation, find “Level” and select “2”. 2. Construct the electrochemical cell indicated in Table 3. Select the LEFT: metal & ion solution, and select the RIGHT: metal & ion solution. Leave the solution concentrations at 1.00M. ° 3. Select “Measure Cell Voltage”. Record the measured Ecell in Table 3. ° ° ° 4. Using the measured Ecell value and given Ered = 0.34 V for copper, calculate Eox for the anode using ° ° ° the equation: Ecell = Ered (𝑐𝑎𝑡ℎ𝑜𝑑𝑒) + Eox (𝑎𝑛𝑜𝑑𝑒) IMPORTANT – READ BEFORE STARTING THE EXPERIMENT 1. Each student will submit individual lab report. 2. The answers to all parts of the lab report can be typed or inserted into this Word document. 3. When completed, save as PDF file, and submit to Brightspace > Assignments > Lab 8, by indicated due date. 4. File name should include student’s name and report submitted. Example “R. Todorovic-Lab 8”. 3 Exp. 8: Measuring Electrode Potentials of Galvanic Cells Chemistry 203 – General Chemistry II Dept. of Physical Sciences and Engineering Wilbur Wright College PRE-LAB QUESTIONS A student measures the potential of a cell made up with 1.0 M CuSO4 in one solution and 1.0 M AgNO3 in the other. There is a Cu electrode in the CuSO4 solution and an Ag electrode in the AgNO3 solution. The salt bridge connects the two half-cells. The student finds that the potential, or voltage of the cell is 0.46 V and that the Cu electrode is negative. a) At which electrode is the oxidation occurring? b) Write the balanced equation for the oxidation reaction. c) Write the balanced equation for the reduction reaction. d) Write the balanced redox reaction. DATA AND CONCLUSIONS ° Constructing Voltaic Cells & Measuring 𝐄𝐜𝐞𝐥𝐥 ° Table 1: Measurement of Ecell Left (s/aq) Right (s/aq) Zn, Zn2+ Pb, Pb2+ Ni, Ni2+ Cu, Cu2+ Mg, Mg2+ Cd, Cd2+ Fe, Fe2+ Ag, Ag+ Zn, Zn2+ Cu, Cu2+ Mg, Mg2+ Zn, Zn2+ Anode Metal Cathode Metal ° Ecell (V) Measured 1. Answer the following questions for the following cell. Fe (s) | Fe2+ (1.0 M) || Ag+ (1.0 M) | Ag (s) a) Write the balanced oxidation half-reaction. b) Write the balance reduction half-reaction. c) Write the balance redox reaction. d) How many moles of electrons are transferred between the half-reactions? ° e) Calculate Ecell (Literature) using standard reduction potentials.(Show calculation) 4 Exp. 8: Measuring Electrode Potentials of Galvanic Cells Chemistry 203 – General Chemistry II Dept. of Physical Sciences and Engineering Wilbur Wright College ° Table 2: Determination of 𝐄𝐨𝐱 and Activity Series of Metals Left Right (s/aq) (s/aq) ° Ecell (V) Anode Half-Rxn Measured ° ° Eox (𝑎𝑛𝑜𝑑𝑒) (V) Ered (𝑐𝑎𝑡ℎ𝑜𝑑𝑒) (V) Calculated Literature Ag, Ag+ Cu, Cu2+ 0.340 Pb, Pb2+ Cu, Cu2+ 0.340 Ni, Ni2+ Cu, Cu2+ 0.340 Cd, Cd2+ Cu, Cu2+ 0.340 Fe, Fe2+ Cu, Cu2+ 0.340 Zn, Zn2+ Cu, Cu2+ 0.340 Mg, Mg2+ Cu, Cu2+ 0.340 1. Answer the following questions while referencing Table 2. a) Which metal is most easily oxidized? b) Which ion is most easily reduced? c) Which metal is the strongest reducing agent? d) Which metal is the weakest reducing agent? e) Write the activity series of metals showing increasing order of activity. Exploration of Whodatium (Wd)/Whodatium (II) Nitrate ° Table 3: Determine the 𝐄𝐨𝐱 value of Whodatium (Wd)/Whodatium (II) Nitrate Left Right (s/aq) (s/aq) ° Ecell (V) Anode Half-Rxn Measured Wd, Wd2+ Cu, Cu2+ ° ° Eox (𝑎𝑛𝑜𝑑𝑒) (V) Ered (𝑐𝑎𝑡ℎ𝑜𝑑𝑒) (V) Calculated Literature 0.340 1. Show this new metal’s location relative to the other metals in the activity series listed above. 2. Insert an image of the Wd,Wd2+ and Cu, Cu2+ electrochemical cell. 5
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Exp. 8: Measuring Electrode Potentials of Galvanic Cells

Chemistry 203 – General Chemistry II
Dept. of Physical Sciences and Engineering
Wilbur Wright College

Experiment 8: Measuring Electrode Potentials of Voltaic Cells
INTRODUCTION
Voltaic (or Galvanic) cells convert chemical energy into electrical energy and form essential components of
commercial batteries. In an oxidation–reduction reaction, electrons flow from the substance that is oxidized,
which loses electrons, to the substance that is reduced, which gains electrons. In a voltaic cell, the flow of
electrons accompanying a spontaneous oxidation–reduction reaction occurs via an external pathway, and an
electric current is produced. The basic design of a galvanic or voltaic cell is shown in Figure 1. The substances
involved in each half-reaction are separated into two compartments connected by an external wire and a salt
bridge. Each half-reaction takes place at the surface of a metal plate or wire called an electrode. The electrode
at which oxidation occurs is called the anode, while the electrode at which reduction occurs is called the
cathode. Electrons flow spontaneously from the anode (the negative electrode) to the cathode (the positive
electrode). Charge buildup at the electrodes is neutralized by connecting the half-cells internally by means
of a salt bridge, a porous barrier containing an electrolyte. Dissolved ions flow from the salt bridge to either
electrode, thus completing the electrical circuit.

Figure 1. Voltaic Cell: Zn(s) ∣ Zn2+ (aq, 1M) ∣∣ Cu2+ (aq, 1M) ∣ Cu(s)
The ability of a voltaic cell to produce an electric current is called the cell potential (Ecell ) and is measured
in volts (V). If the cell potential is large, there is a large “electromotive force” pushing or pulling electrons
through the circuit from the anode to the cathode. The cell potential for a spontaneous chemical reaction in
°
a voltaic cell is always positive. The standard cell potential (Ecell
) is defined as the maximum potential
difference between the electrodes of an electrochemical cell under standard conditions (25ºC, 1 M
concentrations of ions, and 1 atm pressure for gases). It is impossible to directly measure the potential for a
single electrode. However, the overall cell potential for an electrochemical cell may be expressed as the sum
°
of the standard reduction potential (Ered
) for the cathode half-reaction and the standard oxidation potential
°
(Eox ) for the anode half-reaction.
°
°
°
Ecell
= Ered
+ Eox

When two half-cells are combined in a voltaic cell, the half-reaction that has a more positive standard
°
reduction potential (Ered
) will occur as a reduction, while the half-reaction that has a less positive standard
1

Exp. 8: Measuring Electrode Potentials of Galvanic Cells

Chemistry 203 – General Chemistry II
Dept. of Physical Sciences and Engineering
Wilbur Wright College

reduction potential will be reversed and will take place as an oxidation. Since all standard half-cell potentials
°
°
°
are listed as reduction potentials, the sign for Eox
is reversed (in other words (Eox
= -Ered
).
ACTIVITY SERIES OF METALS: RELATIONSHIP TO HALF-REACTION POTENTIALS
An active metal, Zn(s), can reduce another metal cation, Cu2+, in a solution and itself get oxidized. For
example, in the redox reaction
Zn(s) + Cu2+ (aq) → Zn2+ (aq) + Cu(s)
the metal Zn(s) is more active than Cu(s), and therefore it is a stronger reducing agent when compared to
Cu(s). Consequently, Zn(s) readily gets oxidized because it has a more positive standard half-cell potential
for its oxidation, relative to Cu(s).
Zn(s) → Zn2+ (aq) + 2e−

°
Eox
= 0.76 V

Cu(s) → Cu2+ (aq) + 2e−

°
Eox
= −0.34 V

°
In this laboratory activity, you will measure the standard cell potentials (Ecell
) of voltaic cells using an
°
electrochemical cell simulator. Upon measuring the standard cell potentials (Ecell
) of various voltaic cells in
°
which Cu(s) serves as the cathode, the standard oxidation potentials (Eox ) for various anodes will be
°
calculated. A comparison of the Eox
values for each metal acting as an anode reflects the relative tendency of
°
the metal to undergo oxidation (i.e. act as a reducing agent). A more positive the Eox
value indicates the metal
is: more easily oxidized, a stronger reducing agent, and a more active metal. Organizing the standard
°
oxidation potentials (Eox
) in order of increasing value creates the activity series of metals.

°
Standard Potentials: List of reduction half-reactions with corresponding Ered
Literature values
Select Electrode on the Left: Metal (assigned as the anode) that will connect to the black (-) voltmeter lead.
Select Solution on the Left: Matching (left) metal nitrate solution.
°
Measure Cell Voltage: Reveals voltmeter reading (i.e. Ecell
).
Select Electrode on the Right: Metal (assigned as the cathode) that will connect to the red (+) volt...


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