Related concepts Kohlrausch s law, equivalent conductivity, temperature dependence of conductivity, Ostwald s dilution law.

Transcription

1 Conductivity of strong and weak electrolytes TEC Related concepts Kohlrausch s law, equivalent conductivity, temperature dependence of conductivity, Ostwald s dilution law. Principle It is possible to differentiate between strong and weak electrolytes by measuring their electrical conductance. Strong electrolytes follow Kohlrausch s law, whereas weak electrolytes are described by Ostwald s dilution law. The examination of the concentration dependence of the conductivity allows the molar conductivities of infinitely diluted electrolytes to be determined, and facilitates the calculation of the degree of dissociation and the dissociation constants of weak electrolytes. Equipment 1 Cobra4 Wireless Manager Cobra4 Wireless-Link Cobra4 Sensor Unit Conductivity Software measure Cobra Holder for Cobra4 with support rod Magnetic stirrer, mini Magnetic stirrer bar, l = 15 mm Retort stand, h = 750 mm Right angle clamp Spring balance holder Support rod with hole, l = 100 mm Glass beaker, 250 ml, tall Glass beaker, 150 ml, tall Volumetric flask, 250 ml Volumetric flask, 500 ml Volumetric flask, 1000 ml Funnel, glass, d o = 80 mm Volumetric pipette, 1 ml Volumetric pipette, 5 ml Volumetric pipette, 100 ml Pipettor Pipette dish Pasteur pipettes Rubber bulbs Set balance Sartorius CPA 623S and software, 230 V, 620 g / g 1 Weighing dishes, 80 x 50 x 14 mm Spoon Cristallizing dish, 320 ml Wash bottle, 500 ml Desiccator Porcelain plate for desiccators Silicone grease, 50 g Silica gel, orange, granulated, 500 g Acetic acid, 1 M, 1000 ml Standard solution, 1413 S / cm,460 ml Potassium chloride, 250 g Water, distilled, 5 l Additional equipment PC with USB interface, Windows XP 1 or higher Fig. 1: Experimental set up. P PHYWE Systeme GmbH & Co. KG All rights reserved 1

2 TEC Conductivity of strong and weak electrolytes Safety instructions When handling chemicals, you should wear suitable protective gloves, safety goggles, and suitable clothing. Please refer to the appendix for detailed safety instructions. Tasks 1. Determine the concentration dependence of the electrical conductivity of potassium chloride and acetic acid. 2. Calculate the molar conductivity using data from the measurements taken and determine the molar conductivity at infinite dilution by extrapolation. 3. Determine the dissociation constant of acetic acid. Set-up and procedure - Prepare the solutions required for the experiment as follows: molar KCl solution: Weigh g of dried potassium chloride into a 1000 ml volumetric flask, add some distilled water to dissolve it, and then make up to the calibration mark with distilled water molar KCl solution: Weigh g of dried potassium chloride into a 1000 ml volumetric flask, add some distilled water to dissolve it, and then make up to the calibration mark with distilled water molar KCl solution: Pipette 25 ml of the 0.1 molar KCl solution into a 250 ml volumetric molar KCl solution: Pipette 25 ml of the 0.05 molar KCl solution into a 250 ml volumetric molar KCl solution: Pipette 5 ml of the 0.1 molar KCl solution into a 500 ml volumetric molar KCl solution: Pipette 5 ml of the 0.05 molar KCl solution into a 500 ml volumetric molar KCl solution: Pipette 1 ml of the 0.1 molar KCl solution into a 1000 ml volumetric molar CH 3 COOH solution: Pipette 100 ml of the 1 molar acetic acid solution into a 1000 ml volumetric molar CH 3 COOH solution: Pipette 50 ml of the 1 molar acetic acid solution into a 1000 ml volumetric molar CH 3 COOH solution: Pipette 25 ml of the 0.1 molar acetic acid solution into a 250 ml volumetric molar CH 3 COOH solution: Pipette 25 ml of the 0.05 molar acetic acid solution into a 250 ml volumetric molar CH 3 COOH solution: Pipette 5 ml of the 0.1 molar acetic acid solution into a 500 ml volumetric molar CH 3 COOH solution: Pipette 5 ml of the 0.05 molar acetic acid solution into a 500 ml volumetric molar CH 3 COOH solution: Pipette 1 ml of the 0.1 molar acetic acid solution into a 1000 ml volumetric - Set up the experiment as shown in Fig Connect the conductivity probe to the Cobra4 Sensor-Unit Conductivity +. 2 PHYWE Systeme GmbH & Co. KG All rights reserved P

3 Conductivity of strong and weak electrolytes TEC - Combine the Cobra4 Sensor Unit Conductivity + with the Cobra4 Wireless-Link. - Start the PC and connect the Cobra4 Wireless Manager with a USB socket of the computer. - After the Cobra4 Wireless-Link has been switched on, the sensor is automatically recognized. An ID number (01) is allocated to the sensor, which is indicated in the display of the Cobra4 Wireless- Link. - Call up the Measure programme and boot the experiment Conductivity of strong and weak electrolytes: potassium chloride (experiment > open experiment). The measurement parameters for this experiment are loaded now. - For calibration: Pour some standard solution into a beaker and immerse the well-rinsed probe into the solution (Advice: Both platinum electrodes of the probe have to be covered completely with the solution). - In the Cobra4 Navigator under Devices double-click the Conductivity symbol. Now you can change some measurement parameters. - Enter the value for the conductivity at a given temperature under the menu point Calibration. You can find this value on the label of the standard solution (at 25 C C = 1413 µs / cm, see Fig. 2). Click the Apply button and finish the calibration with OK. - Procedure: When measuring the conductivity, always begin with the solution having the lowest concentration in each series of measurements. Before each new measurement, thoroughly rinse the probe, the glass beaker and the magnetic stirrer bar, first with distilled water and then with the solution to be subsequently measured. - Determine the conductivity of the distilled water that is used for the dilution of the solutions and note the result. This is to enable the conductivity of the water used to be taken into consideration in the evaluation. - Place a glass beaker containing a magnetic stirrer bar on the magnetic stirrer. Pour the first potassium chloride solution to be measured into the glass beaker, and immerse the previously well-rinsed conductivity cell to a depth of approximately 5 cm in the solution. - Adjust the magnetic stirrer bar to a medium stirring speed (Note: The stirring bar must not touch the measurement cell). - Start the measurement with. Record the first value by pressing. - Subsequently determine the respective conductivities of the other solutions by pressing, whereby in each case the solution with the next higher concentration is measured. - While recording the measuring series, pay strict attention to cleanliness as even the slightest trace of contaminants (e.g. by carrying over some of one solution into another) would result in the registration of erroneous data. - Stop the measurement by pressing. - Send all data to measure (see Fig. 3). - In measure click on to get the graph for conductivity. Fig. 2: Settings for the calibration mode of the sensor. - Open the display options with. - Change the setting as shown in fig. 4. Fig. 3: Window which appears after measurement. P PHYWE Systeme GmbH & Co. KG All rights reserved 3

4 TEC Conductivity of strong and weak electrolytes - Open the data table with. - Exchange the x values manually with the given concentration values in the right order. - Save the measurement (File > Save meausrement as ). - Fig. 5 shows the graph as it is now presented by the programme. Fig. 4: Settings for the conductivity of KCl solutions as functions of the concentration. Fig. 5: Conductivity of potassium chloride solutions as a function of the concentration. - Call up the Measure programme and boot the experiment Conductivity of strong and weak electrolytes: acetic acid (experiment > open experiment). The measurement parameters for this experiment are loaded now. - Carry out the measurements and the alterations using the same procedure as above. - Fig. 6 shows the graph as it is presented by the programme. Fig. 6: Conductivity of acetic acid solutions as a function of the concentration. 4 PHYWE Systeme GmbH & Co. KG All rights reserved P

5 Conductivity of strong and weak electrolytes TEC Theory and evaluation The resistance of a conductor having a uniform cross section is proportional to the length l an inversively proportional to the cross sectional area A of the conductor. The substance constant ρ is known as the specific resistance; its reciprocal κ as the specific conductivity, and the reciprocal of the resistance as the conductance L. It is usual to use ρ for metallic conductors and κ for electrolytes. The conductivity for an electrolytic solution results in the following: having the dimension Ω -1. cm -1. If the conductivity of a solution is to be measured, then the measurements of the cell (length an area) must be known. Therefore, the cell is usually calibrated with a solution with a known conductivity. The ratio of the measured to the tabulated conductivity of a calibration solution directly provides the ratio of the length to the cross section. This ratio is also known as the cell constant. Usually it can be found in the accompanying test certificate. As a result of the strong concentration dependency, the conductivity is not appropriate for comparing electrolytes. For these purposes it is better to determine the molar conductivity Λ. This is calculated from the specific conductivity κ and the concentration c (in mol. l -1 ) of the substance in the electrolyte solution: When the concentration dependence of the conductivity in electrolytes is examined, one finds that the conductivity basically increases with the concentration because the number of the charge carriers (ions) increases. The plot of molar conductivity versus concentration can be calculated with by setting the parameters as given in Fig. 7. In this operation you can also substract the conductivity of the distilled water. The two diagrams for potassium chloride and acetic acid are shown in Fig. 8. Fig. 7: Parameters of channel modification. P PHYWE Systeme GmbH & Co. KG All rights reserved 5

6 TEC Conductivity of strong and weak electrolytes Fig. 8: Molar conductivities of aqueous potassium chloride and acetic acid solutions as functions of the concentration. The molar conductivity approaches a limit Λ with increasing dilution. This is the conductivity at infinite dilution. Kohlrausch found the following conformity to natural law for the concentration dependency of the molar conductivity for strong electrolytes: According to Kohlrausch s law, plotting the molar conductivity of KCl against the square root of the concentration should result in a straight line. This line s intersection with the ordinate is the molar conductivity at infinite dilution. Weak electrolytes do not dissociate completely and have a lower conductivity than strong electrolytes. As the concentration increases, the dissociation equilibrium shifts in the direction of non-dissociated molecules. The degree of dissociation α of weak electrolytes is the quotient of the molar conductivity divided by the molar conductivity at infinite dilution. Ostwald s dilution law is valid for weak electrolytes. It enables dissociation constants to be calculated: The limiting value of the molar conductivity of weak electrolytes at infinite dilution is first reached at extremely low concentrations; therefore, exact measurements in this are no longer possible. Consequently Λ cannot be obtained by extrapolating Λ m / c 0.5 -curves for weak electrolytes. The following equation is derived by transforming Ostwald s law of dilution: 6 PHYWE Systeme GmbH & Co. KG All rights reserved P

7 Conductivity of strong and weak electrolytes TEC From this equation it can be seen that a linear relationship exists between the reciprocal of the conductivity and the product of the molar conductivity and the concentration of weak electrolytes. Furthermore, Ostwald s law of dilution shows that the molar conductivity at infinite dilution can be obtained from the line s point of intersection of the line with the ordinate 1 / Λ m over c. Λ m. Disposal The diluted solutions of potassium chloride and acetic acid can be disposed by rinsing into the drain. Appendix Hazard symbol, signal word Potassium chloride Hazard statements Precautionary statements Acetic acid Danger H226: Flammable liquid and vapour. H314: Causes severe skin burns and eye damage. P280: Wear protective gloves/protective clothing/eye protection/face protection. P : IF IN EYES: Rinse cautiously with water for several minutes. Remove contact lenses if present and easy to do. continue rinsing. P310: Immediately call a POISON CENTER or doctor/physician. P PHYWE Systeme GmbH & Co. KG All rights reserved 7

8 TEC Conductivity of strong and weak electrolytes Room for notes 8 PHYWE Systeme GmbH & Co. KG All rights reserved P