Electrolytes and Nonelectrolytes

Transcription

1 Exploring Bond Types Through Conductivity Electrolytes and Nonelectrolytes OBJECTIVE Students will use a self-constructed conductivity apparatus made earlier to classify the solute in a series of solutions as strong electrolyte, weak electrolyte, or nonelectrolyte. Students will also relate the conductivity of solutions to the bond type of the solute. LEVEL Middle Grades: Chemistry NATIONAL STANDARDS UCP.2, UCP.3, A.1, A.2, B.2, G.1, G.2 CONNECTIONS TO AP AP Chemistry: I Structure of Matter B. Chemical Bonding 1. Binding forces a. Types: ionic II. States of Matter C. Solutions TIME FRAME 45 minutes MATERIALS (for a class of 28 students working individually) *each student needs 10 ml of solution, directions are given for making 500 ml of each solution 24-well microplate NaCl solid sucrose solid 5% NaCl solution dissolve 25 g in 500 ml of distilled water 5% sucrose solution dissolve 25 g in 500 ml of distilled water distilled water tap water 6 test solutions from the following list T E A C H E R Provide six solutions for testing from the following list: glacial acetic acid use only if you have a fume hood and keep under the hood throughout the period 6M acetic acid add 170 ml glacial acetic acid to 200 ml distilled water and dilute to 500 ml 6M ammonia add 200 ml concentrated NH 3 to 250 ml distilled water and dilute to 500 ml 1M ammonia add 83 ml of the above solution to 300 ml distilled water and dilute to 500 ml 1M HCl add 40 ml concentrated acid to 200 ml of distilled water and dilute to 500 ml 1M NaOH add 20.0 g NaOH to 350 ml of chilled distilled water while stirring; then dilute to 500 ml ethanol kerosene Copyright 2012 Laying the Foundation, Inc., Dallas, TX. All rights reserved. Visit us online at AP, Pre-AP, and Advanced Placement are registered trademarks of the College Entrance Examination Board. The College Board was not involved in the production of this product. 2

2 glycerin any other nitrate or chloride salts [except silver, mercury or lead] you have on hand make a 5% solution by taking 25 g of the salt and dissolving it in 500 ml of distilled water Safety Alert 1. Goggles and aprons must be worn at all times. 2. Use extreme caution when handling chemicals: Do not inhale any vapors during this laboratory and make sure there is adequate ventilation. 3. Clearly label all solutions. 4. Label the ethanol and kerosene CAUTION: Flammable. 5. Make sure there are no open flames or hot plates allowed in the lab room during this exercise. TEACHER NOTES This laboratory exercise should be performed after students have constructed their conductivity apparatus according to the procedure presented in the Laying the Foundations Light Up My Life. Alternatively, you may purchase a conductivity apparatus from a science supply company. The procedure is written to use 24-well microplates so either purchase devices that allow the electrodes to fit within that small well or modify the procedure to use larger cups or beakers to contain the solutions. It is also possible to conduct this exercise as a teacher demonstration if necessary. Dispensing solutions in small dropper bottles minimizes contamination. If you must dispense from open containers clearly label a disposable pipet for each solution and caution students against cross contamination. T E A C H E R Copyright 2012 Laying the Foundation, Inc., Dallas, TX. All rights reserved. Visit us online at 3

3 POSSIBLE ANSWERS TO THE CONCLUSION QUESTIONS AND SAMPLE DATA Data Table Substance tested: Classification: Predicted Bonding: Ions present in electrolytes NaCl solid NE ionic Na + and Cl - BUT locked in the solid lattice Sucrose solid NE covalent NaCl solution SE ionic Na + and Cl - Sucrose solution NE covalent distilled water NE covalent tap water WE covalent with ions [if the tap water in your area doesn t make the LED glow, you may have to «cheat» and add some salt to make this point!] kerosene NE covalent 1M HCl SE ionic H + and Cl - Mg 2+, Ca 2+, Cl -, F -, etc. 1M NaOH SE ionic Na + and OH - glacial acetic acid NE covalent 6M acetic acid WE ionic [water partially ionizes the acid] 6M NH 3 WE ionic [water partially ionizes the base] 1M NH 3 WE ionic [water partially ionizes the base] H + and C 2 H 3 O 2 - OH - and NH 4 + OH - and NH 4 + T E A C H E R ethanol NE covalent glycerine NE covalent any nitrate salt SE ionic cation and NO 3 - any chloride salt [except silver, mercury or lead] SE ionic cation and Cl -

4 1. What are the characteristics of a strong electrolyte solution? strong electrolytes must first be soluble in water strong electrolytes must be ionically bonded in order to release ions into solution the number of ions is also important; strong electrolytes dissociate completely in solution 2. For each strong or weak electrolyte write the symbols of the ions that are present in a solution of the substance. See the table above for these answers. NOTE that students may classify ammonia and acetic acid as ionically bonded. Acetic acid is a weak acid and ammonia is a weak base which means that they exist as molecules most of the time, but do dissociate to some degree in solution. A 6.0 molar solution is used so that enough dissociation takes place to give a reading. Review the electronegativities of the bonds involved with students to clear up any misconceptions. 3. Compare the conductivity of solid sodium chloride to a solution of sodium chloride. Explain any differences observed. Solid sodium chloride did NOT conduct and electric current even though ions exist. The ions are not free to move about; they are fixed in a crystal lattice. When sodium chloride is dissolved in water, its ions are free to move and thus an electric current is conducted. 4. Compare and contrast the bonding of electrolytes and nonelectrolytes. Be sure and include the characteristics of each in your discussion. Electrolytes are often ionically bonded and water soluble. Upon dissolving, they release charged ions that conduct an electric current. Nonelectrolytes are covalently bonded. When they dissolve in water they remain in solution as neutral molecules and thus do not conduct an electric current. T E A C H E R

5 Exploring Bond Types Through Conductivity Electrolytes and Nonelectrolytes Do you know why electrical devices caution against using them near water? If an electrical device falls into water in which you are standing, you could receive a severe or even fatal shock. Rain and tap water contain small amounts of dissolved ions that allow the solution to conduct electricity. A perfectly pure sample of water, containing only H 2 O molecules, conducts only a very minute electric current and poses no harm. Only about 2 per billion water molecules spontaneously separate into H + ions and OH - ions and therefore do not produce enough charged particles to carry a current that is measurable in most school science laboratories. When ionic substances dissolve in water, ions separate from each other. The charged particles released conduct an electric current through the water. If the substance ionizes almost completely in water, it is a good conductor of electricity and is classified as a strong electrolyte. If a substance ionizes only partially, it is a poor conductor of electricity and is classified as a weak electrolyte. Most of the solutes of both types of electrolytes are ionically bonded. Substances that dissolve in water without conducting an electric current are classified as nonelectrolytes. Nonelectrolytes usually contain covalent bonds and do not dissociate into ions when they dissolve in water. We call these solutes molecular solutes. PURPOSE To use either the conductivity apparatus you have constructed or one provided by your teacher to classify the solute in a series of solutions as strong electrolytes, weak electrolytes, or nonelectrolytes. You will also relate the conductivity of these solutions to the bond type of the solute. MATERIALS conductivity apparatus 24-well microplate NaCl solid sucrose solid solutions provided by your teacher Safety Alert 1. Goggles and aprons must be worn at all times. 2. Use extreme caution when handling chemicals: Do not inhale any vapors during this laboratory and make sure there is adequate ventilation. 3. Clearly label all solutions. 4. Label the ethanol and kerosene CAUTION: Flammable. 5. Make sure there are no open flames or hot plates allowed in the lab room during this exercise. Copyright 2012 Laying the Foundation, Inc., Dallas, TX. All rights reserved. Visit us online at 4

6 PROCEDURE 1. Formulate a hypothesis in using the if-then format which relates a solute s conductivity to the type of bonding it exhibits. 2. Make a diagram of your 24-well microplate on your student answer page. 3. List the solutions to be tested in the data table on your student answer page. 4. Fill separate wells half-full with each solid to be tested. Clearly label the wells on your diagram. 5. Place 10 drops of each test solution into a separate well. Clearly label the wells on your diagram. 6. Insert both of the electrodes of the conductivity tester into the first well. The electrodes must NOT touch each other. 7. Note the relative conductivity of the solids and solutions by monitoring the brightness of the LED. Record your observations in the data table on your student answer page using the following code: SE = strong electrolyte [LED glows bright] WE = weak electrolyte [LED glows dim] NE = nonelectrolytes [LED produces no light] 8. Predict the bond type for each substance tested and record you results in the data table on your student answer pages. Copyright 2012 Laying the Foundation, Inc., Dallas, TX. All rights reserved. Visit us online at 5

7 Exploring Bond Types Through Conductivity Electrolytes and Nonelectrolytes HYPOTHESIS DATA AND OBSERVATIONS Make a diagram of your 24-well microplate in the space provided below: Copyright 2012 Laying the Foundation, Inc., Dallas, TX. All rights reserved. Visit us online at 6

8 ANALYSIS Substance being tested NaCl solid Sucrose solid NaCl solution Sucrose solution distilled water tap water Data Table Electrolyte Classification Predicted Bonding CONCLUSION QUESTIONS 1. What are the characteristics of a strong electrolyte solution? 2. For each strong or weak electrolyte write the symbols of the ions that are present in a solution of the substance. 3. Compare the conductivity of solid sodium chloride to a solution of sodium chloride. Explain any differences observed. Copyright 2012 Laying the Foundation, Inc., Dallas, TX. All rights reserved. Visit us online at 7

9 4. Compare and contrast the bonding of electrolytes and nonelectrolytes. Be sure and include the characteristics of each in your discussion. Copyright 2012 Laying the Foundation, Inc., Dallas, TX. All rights reserved. Visit us online at 8