Electrochemistry Lab - Reduction Potentials

Reduction Potentials Experiment PURPOSE The purpose of this laboratory activity is to establish the reduction potentials of several known metals relative to an arbitrarily chosen metal. This will be done by measuring the voltage, or potential difference, between various pairs of halfcells. THEORY A voltaic cell (also known as a galvanic or electrochemical cell) uses a spontaneous oxidation-reduction reaction to produce electrical energy. Half-cells are normally produced by placing a piece of metal into a solution containing a cation 2+ of the metal (e.g., copper (Cu) metal in a solution of copper nitrate (Cu(NO 3)or Cu )). The two half-reactions are normally separated by a salt bridge, an aqueous solution of inert ions. The salt bridge completes the circuit, and allows the charge in each solution to remain electrically balanced. In this web simulation of a voltaic cell, the half-cell will be an electrode made of a piece of metal placed into a beaker of corresponding cation solution. Here, the salt bridge will be a 2.0M solution of aqueous sodium nitrate (NaNO3) placed in a “U-tube” with semipermeable membranes linking the two half cells. Using the computer as a voltmeter, the red (+) lead of the voltage sensor makes contact with one metal and the black (-) lead of the voltage sensor with another. By comparing the voltage values obtained for several pairs of half-cells, and by recording which metal made contact with the red (+) and black (-) leads, you can establish the reduction potential sequence for the metals in this lab. PROCEDURE For this activity, the simulated voltmeter is used to measure the difference in electric potential of multiple voltaic half-cell cells. Open the following website to access the simulation: Part I - Measuring Cell Voltages Attached the black lead (left hand half-cell) from the volt meter to Pt/H2 electrode and fill the corresponding beaker with 1.00M HNO3. Use the matching 1.00M solution of the cation for each possible metal electrode in the other half cell. Cell pairing with Pt/H2 Cd Cu Fe Pb Mg Ni Ag Zn Measured cell voltage (V) Part II - Developing a Reduction Potentials Table If the reaction 2H+(aq) + 2e-  H2(g) is assigned a value of 0.00 V, the cell voltages measured in Part I of this procedure can be used to construct a set of reduction potentials for the other electrodes. Because the solution concentrations were all 1.00M, these are called the “standard reduction potentials” or E0. Complete the following table: Half Reactions Ag+(aq) + e- Reduction Potential vs. Pt/H+ Electrode (V)  Ag(s) Does a negative reduction potential mean that metal ion does not “pull” on electrons? Cu2+(aq) + 2e-  Cu(s) 2H+(aq) + 2e-  H2(g) 0.00 Pb2+(aq) + 2e-  Pb(s) Ni2+(aq) Which metal ion “pulls the hardest” on electrons? + 2e-  Ni(s) Cd2+(aq) + 2e-  Cd(s) Fe2+(aq) + 2e-  Fe(s) Which pulls the least? Zn2+(aq) + 2e-  Zn(s) Mg2+(aq) + 2e-  Mg(s) Using your completed table above and the equation E0cell = E0cathode – E0anode , calculate the potentials (voltages) of the following electrochemical cells. Cu/Zn _____ V Cu/Cd _____ V Cu/Ag _____ V Cu/Mg _____ V Pb/Zn _____ V Pb/Cd _____ V Pb/Ag _____ V Pb/Mg _____ V Mg/Zn _____ V Mg/Cd _____ V Ag/Zn ______V Ag/Cd _____ V Cu/Pb _____ V Mg/Ag _____ V Cd/Zn ______ V Part III – Comparison of Predicted Potentials to Measured Potentials. Construct and measure the cell voltages (potentials) for at least five of the following cells on the website. The red lead (+) goes to the metal with more positive potential (see the above table). Use only 1.00M solutions for the matching cation solutions. (Note: if the measured voltage for the cell is a negative number, you’ve constructed it backwards!) Cu/Zn _____ V Cu/Cd _____ V Cu/Ag _____ V Cu/Mg _____ V Pb/Zn _____ V Pb/Cd _____ V Pb/Ag _____ V Pb/Mg _____ V Mg/Zn _____ V Mg/Cd _____ V Ag/Zn ______V Ag/Cd _____ V Cd/Zn ______ V Mg/Ag _____ V Cu/Pb _____ V How do the measured and calculated cell reduction potentials compare? Part IV – The Effect of Solution Concentration on Cell Potentials. Write the balanced equation for the spontaneous process taking place in the following cell: Mg | Mg2+|| Cu2+ | Cu → In this reaction, Mg2+ is a ________________ and Cu2+ is a __________________ . Construct an electrochemical cell with a copper electrode and a magnesium electrode. Start with 1.00M solutions of the matching cation in each half cell. Then, once solution at a time, change the solution concentration as indicated in the table below and measure the new cell voltage. [Mg2+] [Cu2+] 0.10 1.00 0.50 1.00 1.00 1.00 2.00 1.00 1.00 2.00 1.00 0.50 1.00 0.10 measured cell voltage (V) Based on your results, in general: As [reactants] increases, cell voltage __________ As [products] increases, cell voltage __________ So… When recharging a battery, [reactants] ________, [products] ___________, and the cell voltage ____________ When using (discharging) a battery, [reactants] ___________,[products] ___________, and the cell voltage ____________ . Part V – The Reduction potential of a mystery metal. A new metal has been discovered and named “whodatium”. Devise a plan to determine its standard cell potential. Describe your procedure below. Be detailed enough that next hour could follow your directions and successfully determine the value requested.