Conductivity of Electrolytes in Solution Introduction: Electrical current can be thought of as the movement of electrons or ionic charges from an area of high potential to an area of low potential. Materials which conduct current via the movement of electrons are called primary conductors. This class of conductors contains the metallic elements and substances like graphite (an allotrope of carbon). Primary conductors are usually solid but they may also be liquids, such as mercury or molten metals. Materials which conduct current via the movement of ions are called secondary conductors. Anions move toward the area of positive potential while cations move towards the area of negative potential. Therefore, it is required that the ions be mobile. There are two general situations when ions are relatively mobile: (1) when an ionic compound is molten and (2) when a substance dissolves in a media which supports the presence of ion pairs (a cation and an anion). Solutions which conduct electricity are called electrolytic solutions. During the process of passing current through an electrolytic solution, chemical changes occur at the interface between the primary conductor (called the electrode) and the secondary conductor. Cations move towards the cathode where reduction occurs while anions move towards the anode where oxidation occurs. The diagram below illustrates the process: H + Cl
2 The electrolytic cell contains an aqueous solution of hydrogen chloride (hydrochloric acid) that has ionized to form hydrogen ions, H +, and chloride ions, Cl. The cathode and anode are primary conductors such as graphite or a metal which is chemically inert towards HCl. The electrodes are connected by a metallic wire to the battery which supplies the electrical potential causing current to flow. The equations below show the chemical processes occurring at the electrodes: At the anode: 2 Cl Cl2 + 2 e At the cathode: 2 H + + 2 e H2 Substances which form secondary conductors are termed electrolytes. Soluble salts such as NaCl or NH4NO3, and inorganic acids like H2SO4, HCl, or HNO3 are examples of good electrolytes. Substances which do not dissolve or do not ionize in solution are termed nonelectrolytes. Common examples of non-electrolytes are sugar and alcohol. Other strong and weak electrolytes are listed below: Strong electrolytes Most salts HCl H2SO4 HNO3 NaOH KOH Ba(OH)2 Ca(OH)2 Weak electrolytes HC2H3O2 H2SO3 HNO2 H2CO3 H2S H2C2O4 H3PO4 NH3 It is important to recognize the roll of the solvent in forming electrolytic solutions. The solvent must support the formation of the separation of substances into ion pairs. Good solvents for this process are polar such as water. Poor solvents would be non-polar such as benzene or chloroform. A simple apparatus to determine conductance is shown below: Small light Bare Wires
3 When the two wire electrodes are connected either by immersion in an electrolytic solution or by direct contact through a primary conductor, the light bulb shines brightly. If the connection between the two electrodes is poor, then the light may shine dimly. And if no connection between the electrodes is made that will conduct electricity, the bulb doesn t shine at all. Procedure: Rinse the testing apparatus with distilled water and blot dry with a small piece of paper. Make sure no metal objects are in direct contact with the electrodes. Conductance in Solids. Obtain a beaker containing one of the solids listed in the data section. Bring the testing apparatus in contact with the solid the apparatus and record your observations. Test each solid listed in the data section. Conductance in Aqueous Solutions. Obtain a bottle containing one of the solutions listed in the data section. Insert the testing apparatus electrodes in the solution. If the light bulb glows at all, then classify the solution as an electrolyte. If the bulb does not appear to glow, then classify the solution as a non-electrolyte. Clean the electrodes with a squirt bottle holding the apparatus over the sink or a large beaker. Dry the electrodes with a Kimwipe or paper towel. Repeat the procedure for each of the aqueous solutions listed in the data section. Use the solutions prepared. Do not mix any. Conductance in Non-Aqueous Solutions. Repeat the procedure described above using the non-aqueous chemicals listed in the data section. Ionic Reactions. Prepare a solution by mixing 10 ml of dilute NH3 and 10 ml dilute HC2H3O2. Test the conductivity of the mixture. Record your results in the data section. Pour the mixture down the sink when finished. Prepare a solution by mixing 10 ml of dilute H2SO4 and 10 ml dilute Ba(OH)2. Test the conductivity of this mixture. Record your results in the data section. Dump this mixture down the sink as well.
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5 Name Lab Day Report for Conductivity Experiment: Data: Conductance in Solids: Material Copper Rubber Iron Sodium Chloride Sugar Magnesium Copper(II) Sulfate Graphite Aluminum Zinc Test Result Conductance in Aqueous Solutions: Solution Tap water NaCl Test Result (classification) NH3 HC2H3O2 HCl H2SO4 C12H22O11 (sugar) CuSO4 Ba(OH)2
6 Conductance in Non-Aqueous Solutions: Solution Pure Cyclohexane Pure Ethyl Alcohol HCl in Cyclohexane HCl in Ethyl Alcohol NaCl in Cyclohexane NaCl in Ethyl Alcohol Test Result (classification) Ionic Reactions: 1. What was the conductivity of the mixture of dilute NH3 and dilute HC2H3O2? 2. What was the appearance of the mixture? 3. What was the conductivity of the mixture of dilute H2SO4 and dilute Ba(OH)2? 4. What was the appearance of the mixture?
7 Questions: Conductance in Solids: 1. List the solids that were conductors. 2. List the solids that were non-conductors. 3. How is current conducted through solid conductors (how does it move)? 4. How are these conductors classified? Conductance in Aqueous Solutions: 5. List the conductors and the non-conductors. 6. How is current conducted through solutions (what moves)? 7. Why does sugar form a non-electrolyte solution?
8 8. Why is HCl a good conductor and NH3 or HC2H3O2 are not? Conductance in Non-Aqueous Solutions: 9. Why are aqueous solutions of HCl electrolytic while cyclohexane solutions of HCl are non-electrolytic? 10. Explain the difference between HCl in cyclohexane and HCl in ethyl alcohol. 11. Arrange the solvents: water, cyclohexane, and ethyl alcohol according to polarity. Hi Polar Lo Polar 12. Explain how polarity of the solvent affects the ionization of acids.