Experiment 2 : Daniell Cell 210-512-DW: Electrochemistry Objectives • To build a complete galvanic cell (Daniell cell). • To find the EMF of this system under standard conditions. • To verify the Nernst equation. • To find by potentiometry the copper(II) ion concentration (a) in Cu2+ saturated solution and (b) in an unknown Cu2+ solution. Theory The Daniell Cell in 1836 was the starting point of modern electrochemistry because, for the first time, a reliable source of current at a precise voltage was available. It also was the first practical cell that does not generate a gas when operating. This cell was used as the first standard to defined the unit “Volt”. Zn(s) + Cu2+(aq) Zn2+(aq) + Cu(s) with Eo = 1.10 V However, when the concentration of either Cu2+(aq) or Zn2+(aq) changes in the solution, the EMF of the cell (or potential) also changes according to the Nernst equation. RT [products] [Zn2+ ] E = Eo − lnQ where Q = therefore: E = 1.10 V − 0.0296 log at 25 °C. nF [reactants] [Cu2+] with R = 8.314 J.K−1.mol−1 , T = 298 K, n = 2 (electron exchanged), F = 96485 C.mol−1. The purpose of this lab is to use the Daniell cell to corroborate the Nernst law (potential vs. concentration). It will also be used to find the concentrations (activity) of copper in a Cu 2+(sat.) reference electrode as well as in an unknown sample of Cu2+. Material Chemical Tools and devices CuSO4·5H2O (249.68 g/mol) two 50 mL beakers ZnSO4 (161.45 g/mol) two 100 mL beakers CuSO4(aq) 0.100 M (for reference electrode). seven 100.0 mL volumetric flasks CuSO4(aq) unknown solution. one 10.00 mL pipette with rubber bulb 1 copper wire (10 cm x 1 mm diameter) 50 mL burette 1 zinc metallic plate (2 cm x 10 cm) pH meter used as a voltmeter 1 zinc wire (diameter = 1 mm) one coaxial cable ending with two alligator clips 6 M or concentrated HNO3 Filter paper used as a salt bridge (1.5 M KNO3) 1.5 M KNO3(aq) copper reference electrode (from exp. 1). Distilled water for all the solutions. emery paper (sandpaper) analytical scale (laboratory balance) - 1 - Laboratory setup oxidation (left) reduction (right) Fig. 2-1. Daniell cell assembly and connections. Two 50 mL beakers are used as half-cell. The salt bridge is a rolled filter paper having both extremities in contact with each solutions together. Some 1.5 M KNO3 drops are added to wet the paper at the center to ensure ionic conductivity. A pH meter is used to measure the EMF of the cell. The device set to “mV” position. It is essential to use a pH meter to read the voltage since this instrument does not require any current from the circuit to perform its measurement (high impedance). A coaxial cable ending with two alligator clips is used (ask teacher). The black cable (low potential) is connected to the electrode performing an oxidation here the zinc. The connection are made according to cell representation (electrons from the left to th right). (left = black cable) Zn | Zn2+(0.10 M) | Cu2+(0.10 M) | Cu (red cable = right) Since the oxidation reaction always happens at the left electrode, a spontaneous reaction (galvanic cell) will give a positive EMF. - 2 - Procedure 1. Make 100.00 mL of 0.10 M ZnSO4(aq). (Use a concentration as close as possible to this value and record the actual one). 2. Make 100.00 mL of 0.30 M CuSO4(aq). (Use a concentration as close as possible to this value and record the actual one). The water used should be acidic, therefore, add 5 drops of conc. HNO 3 to the distilled water to prevent any Cu(OH)2(s) formation. 3. From the 0.30 M CuSO4(aq) solution, prepare a 100.00 mL of both the following diluted copper(II) sulfate solutions: 3.0x10−2 M, 3.00x10−3 M. Record the precise concentrations. hint: use serial dilution to perform those solutions. 4. With a burette, transfer 33.3 mL of the 0.30 M CuSO4(aq) solution into a 100.0 mL volumetric flask to make a 0.10 M CuSO4(aq) solution. 5. From the 0.10 M CuSO4(aq) solution, prepare 100.00 mL of both the following diluted copper(II) sulfate solutions: 1.0x10−2 M, 1.00x10−3 M. Record these precise concentrations. 6. Put together the setup described in Fig. 2-1. 7. Zinc electrode is cleaned with some diluted HNO3 acid (≈ 0.1 M). The copper wire is refreshed by scrubbing the surface with an emery paper. Both electrodes are rinse with distilled water prior to use and placed in their respective beakers. 8. Pour about 30 mL of 0.10 M ZnSO4(aq) in the beaker with the zinc electrode; the volume of liquid does not have be precise. 9. In the same beaker, place your copper reference electrode (Lab 1) filled with Cu2+(aq, 0.10 M). 10. Connect both electrode with the alligator clips in such a way that you get a positive voltage reading (black connector = zinc electrode). Record the value. 11. Repeat this process by replacing your reference electrode by the one provided by the teacher. Ref: Cu with Cu2+(sat.). After the reading, this electrode could be rinse and stored. −3 12. Pour roughly 30 mL of 1.0x10 M CuSO4(aq) in the other beaker (copper wire electrode). 13. Place the salt bridge between your two beakers (strip of filter paper) to ensure the ionic conductivity between the two half cells. The two extremities of the paper are in contact with both solutions. Use some 1.5 M KNO3 to wet the paper at the center. Record the EMF of this cell (in volt). −3 14. While keeping the same zinc solution throughout the experiment, discard the 1.0x10 M CuSO4(aq) solution. Rinse and fill the beaker and the electrode with the next most diluted Cu2+ solution (3.0x10−3 M), and record its potential. 15. Repeat the step number 14 with the four Cu2+ solutions remaining (see data sheet 15 a to 15 d.) in the order of the least concentrated to the most concentrated. 16. Finally, repeat the step number 14 with the unknown solution of Cu2+(aq) provided by the teacher. 17. Put a thermometer in the zinc solution and record its temperature. 18. Put a zinc wire in the copper solution for 10 seconds and note your observations. 19. Put a copper wire in the zinc solution for 10 seconds and note your observations. - 3 - Experiment 2 : Daniel cell data sheet Potential Note: The electrode on the left in the cell notation is the anode. It is the negative one (black cable) in a spontaneous galvanic cell. It is important to follow the cell notation when measuring voltages. step System (cell notation) Potential / mV 10 Zn | Zn2+(0.10 M) | Cu2+(0.1 M) reference electrode 11 Zn | Zn2+(0.10 M) | Cu2+(sat.) reference electrode 13 Zn | Zn2+(0.10 M) | Cu2+(1.0x10−3 M) | Cu 14 Zn | Zn2+(0.10 M) | Cu2+(3.0x10−3 M) | Cu 15a Zn | Zn2+(0.10 M) | Cu2+(0.010 M) | Cu 15b Zn | Zn2+(0.10 M) | Cu2+(0.030 M) | Cu 15c Zn | Zn2+(0.10 M) | Cu2+(0.10 M) | Cu 15d Zn | Zn2+(0.10 M) | Cu2+(0.30 M) | Cu 16 Zn | Zn2+(0.10 M) | Cu2+(unknown solution) | Cu Other readings and observations 17 Temperature / °C : _______________ 18 zinc wire in the copper solution: 19 copper wire in the zinc solution: - 4 - Experiment 2 : Daniell cell Lab report Calculations 1. Plot a graph of: potential vs. ln([Zn2+]/[Cu2+]), with “V” on the y-scale, for the six values of Cu2+(aq). Label your axes with units (if any) and provide a meaningful title. 2. Use the linear part of the graph and calculate the slope. Provide the slope equation with correlation. 3. Use this slope to calculate the number of electrons exchanged in this process (slope = −RT/nF with R in joule.mol−1.K−1). Comment on any discrepancy. 4. Use this slope as a calibration curve to find the concentration of the Cu2+ unknown. 5. Use the Nernst law to calculate the “effective” Cu2+(sat) concentration in the reference electrode. Questions 1. In the Daniell cell, is the ion Cu2+ a product or a reactant? 2. Is the Nernst law respected over the complete range of concentration? In the case of a negative answer, provide a reason? 3. Indicate how the le Châtelier's principle applies to your electrochemical cell in this experiment. 4. The EMF value of the potential recorded in 15c should be the one of the standard potential since [Cu2+(aq)] = [Zn2+(aq)]. Is it the case? If not, How can you explain this difference? 5. Instead of using copper(II) sulfate for this experiment, what will be the effect of using instead: a) copper(II) nitrate b) copper(I) nitrate 6. Would it be possible to make this experiment without the use of a salt bridge? (using a solution containing both Cu2+ and Zn2+ together.) Comment. - 5 -.