Electrolytic Conduction. Conductivity experiment

1 Electrolytic Conduction Introduction Electrical conductors can be classified into two types: (1) electronic conductors and (2) electrolytic conductors. Solid and molten metals, semiconductors, and some salts are examples of electronic conductors. Conduction takes place in electronic conductors by direct migration of electrons through the conductor under the influence of an applied potential. Here the atoms or ions that compose the conductor remain stationary (except for vibrations about their equilibirum positions) and these conductors carry a current as electrons pass through the orbitals of the atoms or ions. (See pages 449-501 in your text.) Electrolytic conduction is observed in solutions of strong and weak electrolytes, in molten salts, and in some ionic solids. Conduction occurs in electrolytic conductors as both positive ions and negative ions migrate toward electrodes. In contrast to electronic conduction, electrolytic conduction involves a transport of ions from one part of the conductor to another. Further, the flow of current in an electrolytic conductor is accompanied by chemical changes at the electrodes. (Examples of such reactions can be found on pages 743 -747 of your text.) Electrolytic conduction plays an important role in the function of electrochemical cells, batteries, electrolysis, and electroplating. We know that solutions can be nonelectrolytes (nonconductors), weak electrolytes (poor conductors), or strong electrolytes (good conductors). In this experiment we will determine the conductivity of such solutions in a more quantitative manner by measuring their conductance. We will examine how the conductivity of various solutions changes as the concentration and the identity of ions change. We will also see how conductivity can be used to follow chemical reactions. Measuring Conductance The conductance of an electrolytic or electronic conductor is the reciprocal of its resistance in ohms. At one time the unit of conductance was called a mho (the reverse of ohm). This unit has been renamed siemens after W. Siemens, a noted German physical scientist who did extensive research into the behavior of electricity. Electrolyte solutions have conductances that are lower than those of metals. Their conductances are often reported in units of microsiemens (1 µS = 10−6 S). We will measure the conductance of several electrolytic conductors using a conductivity probe similar to that shown in the figure to the right. When the probe is in a solution with ions, an electrical circuit is completed across the electrodes which are on either side of the hole in the probe. A potential difference is applied to the two electrodes and a current results which is proportional to the conductance of the solution. This conductance is converted to a voltage that is read by an interface and displayed by the LabView program on a laboratory computer. Our conductivity probe uses an alternating current in order to prevent the 2 complete ion migration to the electrodes (a situation called polarization) and consumption of the solute by reactions at the electrodes. With each half-cycle of the alternating current the polarity (sign) of the electrodes is reversed. This reverses the direction of the ion flow and reverses any chemical reaction that may have occurred at the electrode in the previous half-cycle. Thus the solutions under study retain their identity and the electrodes are not contaminated by oxidationreduction reactions occurring on their surface. The Experiment Calibrating the Conductivity Probe 1. The computer must be interfaced with a conductivity probe. Make sure that your multimeter and computer are on and that the power supply for the probe is plugged in. Click on the conductivity titration icon on your computer. The multimeter should now say "remote" indicating a correct probe-computer connection. You are ready to calibrate the probe. 2. Select the calibration button S Calib . While the probe is in air, set the blank calibration µS reading to 0.00 (no conductance) and click read . The red light should now turn green. Then place the probe into the standard solution with a conductance of 10,000 µS. Be certain that no bubbles are trapped in the hole in the probe. After about 15 seconds the reading should settle down. Set the calibration standard µS reading to 10,000.0 and click read . The red light should turn green. Click on OK to return to the previous screen. Your probe is now calibrated for the remainder of the lab period. Rinse the probe several times with DI water and blot dry. Comparing Electrolytic Solutions 1. Get six large dry clean six-inch test tubes. Mark all 6 test tubes at the same level, roughly 1/3 full, with a Sharpie marker. Fill two test tubes to the line with 0.05 M HCl solution from the reagent bench. Fill the third test tube to the line with 0.05 M NaCl solution. Fill the fourth test tube to the line with 0.05 M (CH3CO2H) acetic acid solution. Fill the fifth test tube to the line with 0.05 M NaOH solution. Fill the sixth test tube to the line with 0.05 M NaCH3CO2 solution. 2. Now place the probe into each solution. (Watch out for bubbles.) After the reading stops changing record the reading that appears in the µS box into your notebook. Make sure to rinse the probe with plenty of DI water between readings and blot dry. Do not try to save any of these readings on the computer. 3. Measure the conductivity of samples of DI water and of tap water. 4. Mix the contents of the test tube containing NaCH3CO2 and one containing HCl, the contents of the tubes containing HCl and NaOH, and the contents of the tubes containing NaCl and CH3CO2H. Predict the conductivities of these mixtures. 3 5. Take new conductivity readings of the three mixtures. Rinse the probe between readings and after you are done. You may pour the solutions down the drain. A Conductivity Titration 1. In a clean, dry, labeled beaker obtain about 30 mL of the unknown (a mixture of HCl(aq) and acetic acid). Carefully rinse a 10.00 mL pipet with a small amount of this solution, repeat the rinse, then pipet 10.00 mL of the acidic mixture into a clean 250 mL beaker. Rinse the pipet with DI water for further use. Place a magnetic stir bar into the mixed acid solution. Using a graduated cylinder, add 90.0 mL of DI water in order to submerge the hole in the probe. 2. In a clean, dry beaker obtain about 70 mL of the standard NaOH(aq) solution. Record the molarity. Carefully rinse the buret with small portions of the solution and then fill the buret with the basic solution. Let a small amount run through the tip until the buret is on a whole number mark. You will need to use 25 mL of base in each titration. 3. Rinse the conductivity probe with DI water. Place the probe in the unknown acid solution making sure that the stir bar will not hit it and that the NaOH solution will not drip on it. Note that the hole in the probe must be completely submerged without an air bubble trapped in it. 2. Perform a scout titration. Record the µS reading with 0.00 mL of base added as your initial titration data point by clicking on the record button. Now add 1.00 mL of base, change the current buret reading to 1.00 mL, and record the reading. Make sure this data point is saved on your graph. Continue adding base in this manner 1.00 mL at a time until 25.00 mL of base have been added. When your titration is finished, print out the graph by clicking Print. You may use the View menu to view the first derivative of the titration curve. 3. Save your data to your floppy disk by clicking on Save Data. You will want to save both the titration data and its derivative. After both of these are selected, click on Save. Give a name to your file and save to your disk in drive A:/. Now look at your titration curve and locate the area(s) where the slope changes. Try to estimate at exactly what volume the slopes are changing. The derivative plots will aid you in this task. These volumes correspond with endpoint(s) of your titration. 4. Caution: This step will erase all of your titration data. When your data has been saved, your graph has been printed, and you have identified the location of the endpoint(s), you may prepare for another titration by clicking on Clear. The solution in your beaker may be washed down the drain. Clean the beaker and stir bar for the next titration. 5. Now perform a more accurate titration to clearly identify the endpoint(s). Prepare the mixed acid solution and buret as before. Make sure the probe has been rinsed. Once again record the µS reading with 0.00 mL base added for your first titration data point. 6. Add 1.00 mL of base and record the reading. Continue adding 1.00 mL of base until you are within 1.0 mL of the first endpoint located from your scout titration. Now you will need to 4 add the base in one drop increments until about 1.0 mL after the first endpoint. After this, you may add the base in 1.00 mL increments again. When you are within 1.0 mL from the second endpoint, begin to add the base in one drop increments again until 1.0 mL after the second endpoint. After this point, you may add the base in 1.00 increments until 25.00 mL base has been added. 7. Print the titration curve and save your data to your disk as before. 8. Rinse all solutions down the drain. Rinse all your glassware with several portions of DI water. Return the buret, stir bar, magnetic stirring apparatus, and pipet to your TA. Make sure to clean your bench area. The Laboratory Report Your group laboratory report will be due at the start of the next laboratory period. This is not a formal lab report. Divide your report into the following sections: 1. Title Page: Use a title page with the title of the experiment, the names of each member of the group, course number, section number, and your TA's name. 2. Statement of Purpose: State the purpose of the experiment in 150 words or less. 3. Data Section: This section should include your conductance measurements used in your comparison of electrolytic solutions, a print out of your conductivity titration measurements and copies of your graphs of conductivity vs mL of added base. 4. Calculations: Show each calculation. 5. Results and Discussion: Summarize your results and conclusions. Address the points raised below in your discussion. Calculations 1. Calculate the concentration of both the HCl and of the acetic acid in the unknown solution before it is diluted. 2. Calculate the concentrations of all species in the HCl/acetic acid mixture after it is diluted for titration but before the titration begins, the concentrations at the first equivalence point, and the concentrations at the second equivalence point. You will need these values for the discussion. Discussion 1. Compare the conductance values of the 0.05 M HCl, NaOH, CH3CO2H, NaCH3CO2 and NaCl solutions; DI water; and tap water. Write chemical equations for the dissociation of the compounds. Why are the conductances of HCl and NaCl different? Why are the 5 conductances of NaCl and NaOH different? Why are the conductances of HCl and CH3CO2H different? 2. Explain the changes observed upon mixing the contents of the test tube containing NaCH3CO2 and one containing HCl, the contents of the tubes containing HCl and NaOH, and the contents of the tubes containing NaCl and CH3CO2H. 3. Conductivity is related to concentrations of ions and the speed with which ions diffuse through solution. Faster-moving ions giver higher conductivities Order the rate of diffusion of the ions present in the 0.05 M HCl, NaOH, CH3CO2H, NaCH3CO2 and NaCl solutions using "greater than" or "about equal". Explain your reasons. (It may not be possible to include all ions in the same series.) 4. List any ions and/or molecules (you can omit water) present in solution in order of decreasing concentration at the following points in the titration: At the start of the titration before any base is added. Half way to the first equivalence point. At the first equivalence point. Halfway between the first and second equivalence points. At the second equivalence point. Past the second equivalence point after two milliliters of NaOH solution have been added. What are the concentrations of the ions after dilution but before titration, and at the two equivalence points? 5. Look at your graph of conductivity versus added milliliters of NaOH solution. Locate the regions with different slopes. What do each of these regions represent? Write balanced equations for the reaction (if any) occurring in each of these regions. 6. Discuss your titration curve in detail providing explanations for the changes in conductivity in terms of the ions present and their relative concentrations. Use balanced equations in your discussion. 7. What conclusions can you draw about the relationship of conductivity and concentration of ions? Explain the reasons for your conclusions. 8. Predict what a titration curve would look like if a strong acid were titrated with a weak base.