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9.1: Voltaic Cells - Experiment

General Chemistry II Lab Manual · LibreTexts

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    SAFETY PRECAUTIONS
    • Wear chemical splash goggles and a protective lab coat throughout this experiment.
    • Metal salt solutions (Cu(NO3)2, Zn(NO3)2, and unknown metal nitrates) are toxic and irritating to skin and eyes. Avoid contact.
    • Dispose of all metal-containing solutions and salt-bridge strings in the designated HEAVY METAL CHEMICAL WASTE container.

    EQUIPMENT AND CHEMICALS NEEDED

    Table \(\PageIndex{1}\): Equipment and Chemical Supplies
    Hardware & Equipment Chemicals & Electrodes
    Vernier LabQuest & Voltage Probe leads 0.10 M Cu(NO3)2 solution & copper strips
    24-well test plate 0.10 M Zn(NO3)2 solution & zinc strips
    Cotton string (for salt bridges) 1.0 M KNO3 (salt bridge solution)
    Steel wool pads (for electrode polishing) 0.10 M Unknown X solution & Unknown X strips
    Disposable transfer pipets & 10-mL graduated cylinder 0.10 M Unknown Y solution & Unknown Y strips
    100-mL beaker (for salt bridge preparation) 1.0 M CuSO4 stock solution
    Four small test tubes (for serial dilutions) Deionized water

    EXPERIMENTAL PROCEDURE

    Schematic setup of a galvanic cell with two half-cells connected by a salt bridge and voltmeter probe leads
    Figure \(\PageIndex{1}\): Experimental setup for measuring semi-microscale cell potentials.

    Part A: Determine the Cell Potential (Ecell) for a Cu/Zn Voltaic Cell

    1. Obtain a clean 24-well test plate. Pipet approximately 1 mL of 0.10 M Cu(NO3)2 into Well 1 and 1 mL of 0.10 M Zn(NO3)2 into adjacent Well 2.
    2. Obtain copper and zinc metal strips. Thoroughly polish both strips with steel wool until bright and free of oxide coating.
      VERNIER TECH BOX: MEASURING VOLTAGE

      1. Hardware Connection

      • Connect the Voltage Probe leads to CH 1 on the LabQuest interface.

      2. Lead Polarity Rules

      • RED Lead (+): Connect to the electrode expected to be the Cathode (reduction site / higher reduction potential).
      • BLACK Lead (−): Connect to the electrode expected to be the Anode (oxidation site / lower reduction potential).

      3. Potential Sign Interpretation

      • Positive Potential (+): The assumed cathode is correct; electrons flow spontaneously from anode to cathode.
      • Negative Potential (−): Reversed polarity. Swap the lead positions to record a positive potential and correctly identify the cathode.
    3. Prepare a salt bridge by soaking a short length of cotton string in 1.0 M KNO3 solution. Place the string bridge so that one end dips into Well 1 and the other dips into Well 2.
    4. Connect the voltage probe leads (Red to Cu, Black to Zn) and insert the metal electrodes into their respective wells.
    5. Record the stabilized cell potential (Ecell) to the nearest 0.001 V.
    6. Remove, clean, and re-polish both metal strips with steel wool. Re-insert and record a second potential reading. Repeat once more for a third trial. Calculate the average cell potential.

    Part B: Cell Potentials of Copper paired with Unknown Metals X and Y

    1. Pipet 1 mL of 0.10 M Unknown X solution into Well 3 adjacent to a well containing 0.10 M Cu(NO3)2.
    2. Polish an Unknown X metal strip with steel wool. Connect a new KNO3-soaked salt bridge between the two wells.
    3. Measure the cell potential (X/Cu) three times with cleaning/polishing between trials. Note which electrode acts as the cathode (Red lead). Record all data and the average potential.
    4. Repeat the complete procedure for Unknown Y pairing 0.10 M Unknown Y solution and an Unknown Y metal strip against 0.10 M Cu(NO3)2. Record all data and the average potential.
    DATA SKILL: THE LOGIC MATRIX

    Identifying Unknown Metals:

    • If Red Lead is on Copper: Copper is the cathode (E°Cu = +0.34 V). The unknown is the anode with a lower reduction potential (E°unknown = +0.34 V − Ecell).
    • If Red Lead is on Unknown: Unknown is the cathode with a higher reduction potential (E°unknown = +0.34 V + Ecell).

    Part C: Serial Dilution and Nernst Plot of Copper Concentration Cells

    1. Prepare Serial Dilutions: Label four small test tubes as 0.10 M, 0.010 M, 0.0010 M, and 0.00010 M.
      • 0.10 M: Pipet 1.0 mL of 1.0 M CuSO4 stock solution into the first test tube, add 9.0 mL of deionized water using a 10-mL graduated cylinder, and mix thoroughly.
      • 0.010 M: Pipet 1.0 mL of the newly prepared 0.10 M solution into the second test tube, add 9.0 mL of deionized water, and mix.
      • 0.0010 M: Pipet 1.0 mL of the 0.010 M solution into the third test tube, add 9.0 mL of deionized water, and mix.
      • 0.00010 M: Pipet 1.0 mL of the 0.0010 M solution into the fourth test tube, add 9.0 mL of deionized water, and mix.
    2. Load the Well Plate: Pipet 1 mL of 1.0 M CuSO4 stock solution into Well C1 (this serves as the constant 1.0 M concentrated reference cathode). Pipet 1 mL of each prepared dilution (0.10 M, 0.010 M, 0.0010 M, and 0.00010 M) into adjacent Wells C2, C3, C4, and C5, respectively.
    3. Measure Cell Potentials: Insert a freshly polished copper strip into Well C1 and connect the Red (+) lead. Insert a second polished copper strip into Well C2 (0.10 M) and connect the Black (−) lead. Place a fresh KNO3-soaked string salt bridge connecting Well C1 and Well C2. Record the stabilized voltage (Ecell) to the nearest 0.001 V.
    4. Measure Dilution Series: Remove the salt bridge and the anode copper strip from Well C2. Rinse and polish the copper strip, then move it sequentially to Wells C3 (0.010 M), C4 (0.0010 M), and C5 (0.00010 M). For each half-cell pairing against the 1.0 M reference cathode in Well C1, use a fresh KNO3 salt bridge and record the stabilized cell potential.
    5. Replicate Measurements: Clean and re-polish both copper electrodes, replace the 1.0 M reference well solution with fresh 1.0 M CuSO4, and repeat the full potential series for a second trial. Calculate the average Ecell for each concentration.
    DATA SKILL: NERNST PLOT REGRESSION

    Constructing the Linear Plot:

    • For each concentration cell, calculate reaction quotient \( Q = \frac{[\text{dilute}]}{[\text{concentrated}]} = \frac{[\text{dilute}]}{1.0\text{ M}} \).
    • Plot Ecell (y-axis) vs. log Q (x-axis) in Microsoft Excel. Fit a linear trendline (\( y = mx + b \)).
    • Compare the experimental slope (\( m_{\text{exp}} \)) to the theoretical Nernst slope (\( -0.0592 / n = -0.0296\text{ V/decade} \)) to solve for the experimental electron transfer number (\( n \)).
    CHEMICAL DISPOSAL
    • Transfer all waste solutions from the well plate and test tubes into the designated HEAVY METAL CHEMICAL WASTE bottle.
    • Rinse the 24-well plate and test tubes thoroughly with tap water followed by deionized water and air-dry.

    This page titled 9.1: Voltaic Cells - Experiment was last modified on Wed, 28 Oct 2026 18:16:14 GMT and is shared under a CC BY 4.0 license and was authored, remixed, and/or curated by Vince Hradil.