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EXPERIMENT 6: CONDUCTIVITY

Pre-lab Questions:

1. Write the formulas for the following acids:

   (a) phosphoric _________;

   (b) perchloric _________;

   (c) nitric _________;

   (d) sulfuric _________;

   (e) hydrochloric _________;

   (f) acetic _________.

2. Write the formulas for the following bases:

   (a) calcium hydroxide _________;

   (b) potassium hydroxide _________;

   (c) sodium hydroxide _________;

   (d) ammonia _________.

3. Write the formulas for the following salts:

   (a) potassium chromate _________;

   (b) potassium sulfate _________;

   (c) copper (II) nitrate _________;

   (d) calcium carbonate _________;

   (e) potassium iodide _________;

EXPERIMENT 6: CONDUCTIVITY

In this experiment (and again in Experiment 9) you will use a piece of equipment called a well plate shown in Figure I. This piece of equipment is made of clear plastic and contains wells used to hold solutions. Each well can be identified using a combination of a letter and a number. In the figure, well B3 (2nd row-3rd column) is labeled. A particular well will be indicated by a letter (row) and a number (column). In this experiment you will use a 96-well plate. The wells are small and only a few drops of reagent will be needed. Doing experiments on a 'microscale' is very economical and considerably safer than large scale experiments.

The conductivity apparatus used in this experiment consists of a 9-volt battery inside a 35 mm plastic film container. A light-emitting-diode (LED) has been wired to the battery. Two wires (electrodes) are attached to the LED so that if the electrodes are placed in a solution which conducts electricity the LED will glow at a particular intensity. The best way to observe the light intensity after immersing the electrodes into a conducting test solution is to view the LED from the top, not the side.

EXPERIMENT 6: CONDUCTIVITY  Top

EQUIPMENT:
beaker, 50 mL ........................ 1	petri dish .............................1
conductivity apparatus ............... 1	plate, 96-well........................1
dropper ................................ 1	spatula or microspatula.............2
paper towels........................... several	watch glass ..........................2

PART I: Introduction to Conductivity

Check the conductivity apparatus using a test solution. Place 4 drops of the test solution in H12 and insert the electrodes of the conductivity apparatus into the test solution. Check that the LED (light emitting diode) glows brightly. When viewing the LED, look down from above the LED rather than from the side. It may also help to slightly darken the room. If the conductivity apparatus works, remove the electrodes from the solution and wash the electrodes with deionized water and dry with a paper towel. It is important to wash the electrodes with deionized water following each measurement. Try not to immerse the electrodes so deep that solution leaks underneath the plastic sheath covering the wires. If this happens clean the electrodes carefully to prevent erroneous observations.

Using the 96-well plate and a dropper, fill A1 with tap water and introduce the electrodes to a depth of about 5 mm. Note whether the LED glows brilliantly, faintly, or not at all.

Obs. #1

Try immersing the electrodes more and more deeply into the tap water. Record your results.

Obs. #2

NOTE: For each of the tests that follow, you should immerse the electrodes to approximately the same depth.

Dry the electrodes. In A3, test deionized water using the conductivity apparatus. Does the LED glow brilliantly, moderately, faintly, or not at all?

Obs. #3

Test dry sucrose (C ₁₂H ₂₂O₁₁ ) with the conductivity apparatus. Use a microspatula to half- fill A7. Be careful not to spill sucrose in the wells surrounding A7 and be sure the electrodes are clean and dry before testing the sample.

Obs. #4

With the electrodes still in contact with the solid sucrose in A7, add a few drops of deionized water and observe LED. Record your observations.

Obs. #5

In A9, test dry sodium chloride (NaCl) with the conductivity apparatus. Use a dry microspatula to place the NaCl into the well. Be sure the electrodes are dry before testing the sample.

Obs. #6

With the electrodes still in contact with the dry sodium chloride in A9, add a few drops of deionized water. Record your observations.

Obs. #7

Clean and dry the conductivity apparatus electrodes. Place 4 drops of KI solution in A11 and then test the conductivity of KI. Record your observations.

Obs. #8

Clean and dry the conductivity apparatus electrodes. Test a sample of Pb(NO₃)₂ solution in C1. Record your observations.

Obs. #9

Place a small sample of solid lead nitrate on a watch glass. On a second watch glass, place a sample of solid potassium iodide. Briefly describe the initial appearance of the dry potassium iodide and lead nitrate solids.

Obs. #10

Clean a Petri dish. Cover the bottom of the dish with a thin layer of deionized water. Use a spatula to carefully place a few crystals of lead nitrate into the water close to one side of the Petri dish. Try not to agitate the water when adding the solid. Use a clean spatula to carefully add a few crystals of potassium iodide to the opposite side of the Petri dish. Do not bump the Petri dish. It is important that the water not be agitated during the experiment. Watch what happens.

Draw the arrangement of the Petri dish and the samples of lead nitrate and potassium iodide in the space below. Draw a second picture showing what happened as time passed.

Picture #1

Briefly describe your observations of what happened after you placed the solids in the dish.

Obs. #11

Do not continue experimenting until your instructor has completed the group discussion to clarify the experiment just performed. Following the discussion explain your observations. In your explanation include the use of the following terms: anion, cation, electrolyte, precipitate, soluble, hydration, and electrical conductivity.

Expl. #1

Explain your observation of the conductivities of deionized water and tap water.

Expl. #2

Explain your observations of the conductivities of solid sucrose and of sucrose solution.

Expl. #3

Explain your observations of the conductivities of solid sodium chloride and of sodium chloride solution.

Expl. #4

Explain the difference in the conductivities of the sodium chloride solution and sucrose solution.

Expl. #5

EXPERIMENT 6: CONDUCTIVITY

EQUIPMENT:

beaker, 50 mL ........................ 1
conductivity apparatus ............... 1
dropper ................................ 1
paper towels........................... several
plate, 96-well ......................... 1

PART II: Strong and Weak Electrolyes

Place four drops of 0.1 M hydrochloric acid (HCl) in C3, four drops of 0.1 M acetic acid (HC₂H₃O₂) in C5, and four drops of 0.1 M sulfuric acid (H₂SO₄) in C7. Compare the conductivities of the three solutions by observing the intensity of the LED, for example, no glow (non conductor), faint (poor conductor) or brilliant (good conductor). Be sure to clean and dry the elecrodes after each use. Classify each acid as either a nonelectrolyte, weak electrolyte or strong electrolyte. Identify the ions present in each solution which account for the conductivity.

Obs. #12

Place four drops of 0.1 M sodium hydroxide (NaOH) in C9 and four drops of 0.1 M ammonia (NH₃) intensity of the LED. Be sure to clean and dry the electrodes after each use. Classify each in C11. Compare the conductivities of the two solutions by observing the

base solution as either a nonelectrolyte, weak electrolyte or strong electrolyte. Identify the ions that are present in each solution which would account for the conductivity.

Obs. #13

Place four drops of 0.1 M sodium acetate (NaC₂H₃O₂) in E1, four drops of 0.1 M sodium chloride (NaCl) in E3, four drops of 0.1 M ammonium acetate (NH₄C₂H₃O₂) in E5, four drops of 0.1 M ammonium chloride (NH₄Cl) in E7. Use the conductivity apparatus to check each solution. Be sure to clean and dry the electrodes after each test. Record your results. Classify each solution as either a nonelectrolyte, weak electrolyte or strong electrolyte. Identify the ions that are present in each solution which would account for the conductivity.

Obs. #14

Place four drops of methanol (CH₃OH) in E9 and four drops of ethanol (C₂H₅OH) in E11. Compare the conductivities of the two solutions by observing the intensities of the LED, for example, no glow (non conductor), faint (poor conductor) or brilliant (good conductors). Be sure to clean and dry the electrodes after each use. Classify each compound as either a nonelectrolyte, weak electrolyte or strong electrolyte. Identify the ions that are present in each solution which would account for the conductivity.

Obs. #15

Consider the following sample set of observations and the equation that results. Jennifer Redleg, an aspiring chemistry student, tested the conductivity of a 0.1 M nitric acid solution (HNO₃) and found that the LED glowed brightly. Jennifer concluded that the HNO₃ is a strong electrolyte. To demonstrate her knowledge of the ions formed in the

solution Jennifer wrote the following equation:

HNO₃(aq) H⁺(aq) + NO₃^-(aq) .

The equation can be interpreted the following way: An aqueous solution of HNO₃ contains the hydrated ions H⁺ and NO ^-₃ . Jennifer realized that she could write HNO₃(aq) or H⁺(aq) and NO₃^-(aq) to indicate an aqueous nitric acid solution. Write a similar chemical equation which indicates the species that are in solution for each of the compounds whose conductivities were measured.

Reactions

You have now collected data on a variety of substances. You have classified each solution as a nonelectrolyte, weak electrolyte or strong electrolyte. The terms strong electrolyte, weak electrolyte, or nonelectrolyte are used to summarize the experimental observations and refer to the ability of the compound to conduct electricity. In the case of the strong and weak electrolytes you identified the ions in solution that were responsible for the solutions observed ability to conduct electricity.

List the solutions that are strong electrolytes.

List the solutions that are weak electrolytes:

List the solutions that are nonelectrolytes:

Based on your observations of the above 14 solutions, what classes of compounds are strong electrolytes when dissolved in water? Weak electrolytes? Nonelectrolytes? (Note: See the preparatory questions you answered at the beginning of the experiment .)

Expl. #6

EXPERIMENT 6: CONDUCTIVITY

EQUIPMENT:
laboratory balance .................... 1	paper towels .........................3
beaker, 50 mL ........................ 2	plate, 96-well........................1
capillary tube.......................... 1	ring stand ............................2
conductivity apparatus ............... 1	spatula................................1
dropper ................................ 1	stopper, 1-hole #6 ..................1
flask, Erlenmeyer, 250 mL.......... 1	trough or pan ........................1
glass bend ............................. 1	tubing,1' rubber.....................1
glass tubing ........................... 1	utility clamps ........................2
graduated cylinder, 50 mL .......... 1	weighing paper......................3

PART III: Electrolyte Strength and Reaction Rate

You will assemble an apparatus such as that shown in Figure 1. Start by filling the trough and the graduated cylinder with deionized water. Invert the graduated cylinder into the trough. After you have inverted the graduated cylinder be sure that it remains full of water. Fit the Erlenmeyer flask with a 1-holed rubber stopper. Your instructor will demonstrate how to insert glass tubing into a rubber stopper. Follow these instructions carefully. Please ask if you have questions. The procedure is also demonstrated on the pre-lab video tape for Experiment #1. Set up your apparatus as shown in Figure I. Complete your setup and be sure that all parts fit snugly so that no gas can escape. Have your instructor check your apparatus before you begin the experiment.

Apparatus Checked ________________

Figure I.

Measure approximately 2 grams of powerded calcium carbonate (CaCO₃) onto a small piece of paper. Be sure the sample texture and particle size is uniform. Obtain 30 mL of the 1 M HCl in a small beaker. After the instructor has checked your apparatus, and you are ready, add the acid to the Erlenmeyer flask. Then add the calcium carbonate to the acid in the Erlenmeyer flask, quickly stopper the flask and collect the escaping gas in the graduated cylinder. Note the time required to collect 20 mL of gas. The acid may react with CaCO₃ very rapidly, generating the 20 mL of gas quickly. You may wish to indicate the time as less than a second. Clean and rinse the flask and repeat the experiment for the other two acids (be sure to use 1 M HC₂H₃O₂ and 0.5 M H₂SO₄). It is not necessary to try to time these reactions. Simply compare and note the relative rate of evolution of gas.

(Hint: Two of these should be about the same and the third noticeably different.)

Rate of evolution of 20 mL of gas

1 M HCl....1 M H(C₂H₃O₂)....0.5 M H₂SO₄

____________..__________..__________

Record your observations for each acid.

Obs. #16