035 Electrical Conductivity

Expt 035 -- Electrical Conductivity

The electrical conductivities of solutions of strong electrolytes, weak electrolytes, and nonelectrolytes are studied.

In order for an electric current to be conducted, some charged, mobile carriers such as free electrons or free ions must be present. Metals have mobile electrons to carry current. A solution which contains ions can conduct electricity.
A strong electrolyte contains a relatively large number of ions and is a good conductor; a weak electrolyte produces fewer ions and is a poor conductor. A non-electrolyte has very few ions; it is more of an insulator than a conductor.
In terms of test devices that make use of lamps or flashing diodes, solutions of strong electrolytes cause the devices to flash brightly and often, those of weak electrolytes flash dimly and infrequently, and those of nonelectrolytes do not flash.
Strong electrolytes are thought to dissociate completely into ions. Most salts, such as NaCl, and some acids, such as HCl and HNO₃, are examples of strong electrolytes. A few salts (e.g., CdI₂) and a few acids and bases (e.g., acetic acid, ammonia) are weak electrolytes. These substances are only partially dissociated into ions when dissolved in water. Non-electrolytes do not dissociate much at all. Ethanol is a non-electrolyte, for example.
To sustain a direct current, some electrode process of oxidation or reduction must occur. An alternating current reduces or avoids this problem. The 9-volt battery makes use of a direct current and, although safe and inexpensive, is limited for this reason.

Toluene and ethanol are flammable. Keep flames and other ignition sources 10 feet (3 m) away from the flammable chemicals; provide adequate ventilation. Toluene, ethanol, and ammonia are toxic. Other substances tested also may be toxic. Wear goggles and apron. Avoid ingesting or inhaling chemical substances. Wash spills immediately with large amounts of water. Wash hands after the experiment.

Procedure

Test the conductivity of each of the following liquids: distilled water; toluene; 0.1 M sodium chloride (sodium sulfate, potassium chloride); sugar; acetic acid; and ammonia.

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Make sure the distance between the electrodes is not changing while you make measurements. Wash thoroughly
Record the results in the table.
1. Water
  
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2. 0.1M NaCl
  
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3. 0.1M Na₂SO₄
  
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4. Toluene
  
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5. Sugar
  
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6. 5% acetic acid
  
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Use a plastic transfer pipet to remove the toluene from the well and discard it into the disposal jar placed under the hood. Rinse the remaining wells into the sink. Wash the strip with water.
Place some 1 M NaCl in the first well of a 12-well strip. Prepare a series of 1 to 5 dilutions from this well. Place 8 drops of distilled water in wells 2 through 12 using a plastic transfer pipet with a restricted tip as a dropper. Empty this dropper. Use the plastic pipet dropper to remove 2 drops from well 1 and place in the second well. Mix using the dropper. Remove 2 drops from this well and add to the third well. Mix. Continue in this fashion for all of the wells. Test the conductivity of each well. Wells tested will be 1M, 0.2M, 0.04M, 0.008M, etc. Note and record changes.

!!!Click here to See Movie. Movie corresponds to 1M.
!!!Click here to See Movie. Movie corresponds to 0.2M.
!!!Click here to See Movie. Movie corresponds to 0.008M.

Questions

Would copper metal be classified as a strong electrolyte, weak electrolyte, or nonelectrolyte. Explain.
Classify methanol as a strong electrolyte, weak electrolyte, or nonelectrolyte.
Explain why the distance between the electrodes in the conductivity probe is kept constant.

Handout Makeup

Name ___________________________ Class _______

Teacher __________________________

SmallScale 035 Electrical Conductivity

Watch the movies and record your observations below.

Classify each solution as a non-, strong, or weak electrolyte.

Water
0.1M NaCl
0.1M Na₂SO₄
Toluene
Sugar
5% acetic acid
1M NaCl
0.1M NaCl
0.008M NaCl

Note any other observations.

Answer the questions

Would copper metal be classified as a strong electrolyte, weak electrolyte, or nonelectrolyte. Explain.
Classify methanol as a strong electrolyte, weak electrolyte, or nonelectrolyte.
Explain why the distance between the electrodes in the conductivity probe is kept constant.

Curriculum-

Use when discussing solutions or when discussing types of bonds, since ionic materials using are strong electrolytes in solution.

Safety-

Time-

Teacher Preparation: 10 minutes if apparatus is being reused.

Class Time: 30 minutes

Materials-

1 mL of toluene
1 mL of 95% ethanol (or absolute ethanol)
1 mL of distilled water
1 mL of 0.1 M sucrose -- (Dissolve 3.423 g sucrose (table sugar, C₁₂H₂₂O₁₁) in enough water to make 100 mL solution.)
1 mL of 0.1 M Na₂SO₄ -- (Dissolve 1.420 g sodium sulfate (Na₂SO₄) in enough water to make 100 mL solution.)
1 mL of 0.5 % acetic acid -- (Add 10 mL vinegar to 90 mL of distilled water)
1 mL of 0.1 M NH₃ -- (Add 3.3 mL 3 M ammonia (NH₃) to enough water to make 100 mL solution.)
2 mL of 1 M NaCl -- (Dissolve 5.84 g sodium chloride (NaCl) in enough water to make 100 mL solution.)
plastic wash bottle filled with distilled water
electrical conductivity apparatus (see Lab Hints below)

Disposal-

Store the toluene in a bottle specifically labeled for reuse in this experiment.
Select nontoxic substances for testing. Dispose of these substances at the sink.
Remove the 9-volt battery and use for other experiments; it will not retain energy under long term storage. Save all of the apparatus for future use.

Lab Hints-

during storage to minimize shorting-out problems.
After many years of trying different devices, we have reached the conclusion that the simplest device works best. When tested over excellent conductors, the diode may burn out. This has not happened often in classroom experience, however. Construct an apparatus using a 9-volt battery and a blinking LED (light emitting diode). (See a schematic diagram below.)
To build the electrical conductivity apparatus suitable for use by students (Radio Shack^® catalog numbers are provided):
- Blinking LED 276-030 (or 276-036)
- 4 cm length black 7-mm i.d. rubber tubing
- 9-volt battery clip 270-325
- 9-volt battery
- 40-cm length of two-conductor, 24-gauge, ribbon wire 278-755
- (soldering iron, solder, razor or sharp knife, wire cutters, wire strippers)
1. Construct the electrodes from a 10-cm length of ribbon wire. Use a razor or sharp knife to remove about 10 mm of insulation from both sides of the end the length of ribbon wire.
  
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2. The insulation is removed to expose the wires so that they can come into direct contact with the solutions. Insulation is left between the wires to keep them from touching one another thereby preventing a "short circuit." For reproducible measurements the distance and area of the exposed wires must be constant.
  
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3. Solder the red wire from the battery clip to the long lead of the LED.
  
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4. Solder the short lead of the LED to one wire of the electrode ribbon. Solder the black wire from the battery clip to the other wire in the ribbon/electrode assembly.
  
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5. Place a short length of black rubber tubing over the LED to improve visibility when faint flashes are emitted.
6. Note the increased visibility in a light room with the tubing in place.
  
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Answers-

Q1. Would copper metal be classified as a strong electrolyte, weak electrolyte, or nonelectrolyte. Explain. A1. Metallic conductors usually are not characterized using these terms. These terms are reserved for electrolytic conductors that use ions in solution as the charge carriers.
Q2. Classify methanol as a strong electrolyte, weak electrolyte, or nonelectrolyte. A2. Methanol, like ethanol, is a nonelectrolyte.
Q3. Explain why the distance between the electrodes in the conductivity probe is kept constant. A3. The longer the distance traveled by the current, the higher the resistance. The space between electrode is kept constant to control for this distance factor.

Handout Ans.-

Record the results in the table.

Water- nonelectrolyte
0.1M NaCl- strong electrolyte
0.1M Na₂SO₄- strong electrolyte
Toluene- nonelectrolyte
Sugar- nonelectrolyte
5% acetic acid- weak electrolyte
1M NaCl- strongest electrolyte
0.1M NaCl- strong electrolyte

Others not shown in movies

CoopLearn-

Assigning large numbers of nontoxic substances to different students or student groups and pooling testing data is appropriate.

Reference-

The small scale version of this lesson was developed by Steve Wignall, Seward, NE. It was developed at an EESA workshop at Lincoln East HS, Lincoln, NE, in 1988.

Key Words 1-

electrolyte, strong electrolyte, weak electrolyte, non-electrolyte, electrode, conductor, insulator, conductivity

Elements-

C N O H Na Cl S