Chemistry 101 Lab: Experiment #1 — Conductivity of Solutions Introduction: A chemical compound is made up of two or more elements. Compounds can be categorized as either ionic or molecular. An Ionic compound is formed from the electrical attraction between anions and cations, typically a metal with a non-metal (one exception being hydrogen). When an ionic compound forms, the anion transfers an electron to the cation which creates an electrostatic bond and an electrically neutral compound. Other characteristics of Ionic compounds are that they readily dissolve in aqueous solutions and that they are good conductors of heat and electricity. Further, ionic compounds can be classified as either strong conductors or weak conductors. In contrast, molecular compounds are formed from non-metals. Molecular compounds share electrons between atoms forming a covalent bond. In general, molecular compounds do not dissolve readily in solution and are poor conductors of heat and electricity. They are classified as non-electrolyte compounds. The factors that determine the electrical conductivity of a given compound in solution include the degree of its solubility in that solvent, the total ionic molar concentration, and the concentration of the compound in the solution. Objective: The purpose of the experiment was to demonstrate the differences in electrical conductivity between ionic compounds and molecular compounds, both qualitatively and quantitatively. Each compound tested was also categorized as either a weak electrolyte, a strong electrolyte, or a non-electrolyte (ie. molecular compound). Compounds were compared based upon their degree of conductivity. Methods: In the first experiment, the conductivity of various solutions of the same concentration were determined and compared. First, the conductivity of distilled water and tap water were measured as comparative controls. Then, the conductivity of 0.05M solutions of CH3OH, C2H6O2, H3BO3, CH3COOH, KBr, and HCl were measured. The conductivity values were then used to classify each as a non-electrolyte, a weak electrolyte, or a molecular compound. In the second experiment, the effect of a compound’s concentration on its conductivity was determined by varying the concentration of each. First, the conductivity of a 1.0M solution of each compound was measured. Then, the conductivity of increasingly more concentrated solutions of were measured. Nine data points in total were recorded. This procedure was performed with solutions of CH3OH, AlCl3, CaCl2, and NaCl. The conductivity of each compound at varying concentrations were compared, as were the individual compounds compared to each other. Linear regression analysis was performed to determine the best fit slope for each graph. The greater the slope for a given compound, the better the conductor.
Experimental Data and Graphs:
Experiment A: Data Analysis (in decreasing order of conductivity): Compound 0.05M HCl 0.05M KBr Tap Water 0.05M CH3COOH Distilled Water 0.05M CH3OH 0.05M C2H6O2 0.05M H3BO3
Electrical Conductivity 19613 (µS/cm) 7323 (µS/cm) 796 (µS/cm) 446 (µS/cm) 67 (µS/cm) 54 (µS/cm) 38 (µS/cm) 38 (µS/cm)
Experiment B: AlCl3 Number of Drops 0 1 2 3 4 5 6 7 8
Conductivit y 20.18679171 73 307.1444428 25 472.7150541 4 737.0995500 25 970.1643051 33 1301.028996 37 1596.753716 84 1889.446833 88 2079.905270 6
2
CaCl2 Number of Drops 0 1 2 3 4 5 6 7 8
Conductivit y 13.33495830 33 220.4876976 61 374.0752819 29 576.1992466 88 804.1942595 84 1009.667326 77 1145.567143 77 1249.051336 32 1417.356535 85
3
NaCl
Number of Drops 0 1 2 3 4 5
Conductivit y 8.664650326 13 121.6123616 7 234.4781377 85 318.2569080 78 406.5421158 93 517.5028979 65
4
Number of Drops 6 7 8
Conductivit y 596.9288592 88 681.2299666 57 764.6912379 43
CH3OH Number of Drops 0 1 2 3 4 5 6 7
Conductivit y 9.453276892 46 8.019410408 23 7.947717084 02 7.845298049 43 7.957958987 47 7.957958987 47 7.927233277 1 8.029652311 69
5
Number of Drops 8
Conductivit y 8.009168504 77
Combined Data Number of Drops Conductivit y AlCl3 0 1 2 3 4 5 6 7 8
20.18679171 73 307.1444428 25 472.7150541 4 737.0995500 25 970.1643051 33 1301.028996 37 1596.753716 84 1889.446833 88 2079.905270 6
CaCl2
CH3OH
NaCl
13.33495830 33 220.4876976 61 374.0752819 29 576.1992466 88 804.1942595 84 1009.667326 77 1145.567143 77 1249.051336 32 1417.356535 85
9.453276892 46 8.019410408 23 7.947717084 02 7.845298049 43 7.957958987 47 7.957958987 47 7.927233277 1 8.029652311 69 8.009168504 77
8.664650326 13 121.6123616 7 234.4781377 85 318.2569080 78 406.5421158 93 517.5028979 65 596.9288592 88 681.2299666 57 764.6912379 43
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Data Analysis (in decreasing order of conductivity): AlCl3 > CaCl2 > NaCl > CH3OH
Conclusions and Discussion: In the first experiment, the electrical conductivity of identical concentrations of several solutions both ionic and molecular were determined. In addition, the conductivity of tap water and distilled water were measured as comparison controls. The data show that based upon each compound’s conductivity KBr and HCl can be considered strong electrolyte compounds; tap water and CH3COOH can be considered weak electrolyte compounds; finally, distilled water, CH3OH, C2H6O2, and H3BO3 can be considered non-electrolyte compounds. Interestingly, there is a marked difference in conductivity between tap water and distilled water. This difference likely arises from the various metallic elements, as well as fluoride, that are found in very low concentrations in tap water. These elements form weak ionic bonds with the very small concentration of dissociated hydrogen. The measured conductivity of H3BO3 (38 µS/cm) is interesting as this compound is an acid (boric acid), albeit a weak one, and acids are usually considered electrolyte compounds. However, boric acid’s electrical conductivity classifies it as a non-electrolyte compound. Boric acid’s weak conductivity is due in part to it’s poor solubility in aqueous solutions.
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In the second experiment, the effect of varying the concentration of four different solutions (CH3OH, AlCl3, CaCl2, and NaCl) was determined. The concentration of each solution was gradually increased and nine conductivity readings were obtained for each. As expected, the three electrolyte compounds exhibited greater conductivity than the molecular compound CH3OH. In addition, the conductivity of each electrolyte compound showed a near linear relationship to its concentration, increasing in a step-wise fashion as the concentration was increased. In contrast, the conductivity of CH3OH remained essentially the same regardless of concentration, as was expected given it’s poor solubility in water and that it is an electrically neutral compound. Of the three electrolyte compounds, AlCl3 exhibited the greatest conductivity while NaCl exhibited the least. This can be explained by the number of ions generated by each compound when dissociated in an aqueous solution. As the ionic “concentration” of any given solution increases, its electrical conductivity likewise increases. AlCl3 dissociates into 4 separate ions - one Al3+ cation and three Cl- anions. Whereas, the two other compounds - CaCl2 and NaCl - dissociate into three ions and two ions, respectively. Therefore, it would be expected that AlCl3 would be a better conductor than CaCl2 and that CaCl2 would be a better conductor than NaCl. Of course, this only holds true if each compound has the same molar concentration as the others. In conclusion, chemical compounds can be classified as either ionic or molecular. The type of bond between its constituent elements determines the classification. Ionic compounds have an electrostatic bond, while molecular compounds have a covalent bond. Ionic compounds can further be classified into weak compounds and strong compounds based upon their degree of electrical conductivity. In the first experiment, the electrical conductivity of several compounds of identical molar concentration dissolved in an aqueous solution was determined, and each compound was classified according to its conductivity. As expected, the ionic compounds exhibited greater conductivity than the molecular compounds. Then, in the second experiment, the effect of varying the concentration of several solutions was determined. The conductivity of equal molar concentrations of each compound were compared as the concentration of each solution was gradually increased. As expected, the ionic compounds exhibited greater conductivity for any given concentration compared to the molecular compound. Further, the degree of conductivity was strongly affected by the number of ions that each compound dissociates into when in solution. The greater the molar concentration of ions, the greater the conductivity.
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