In-Class Demonstration

In-Class Demonstration

Conductivity Measurements: Do Strong and Weak Electrolytes Exist?

Student Introduction

Electrolytes are defined as chemical compounds that ionize when dissolved in water. Strong electrolytes ionize completely while weak electrolytes only partially ionize in solution. Strong electrolytes are typically soluble salts (such as NaCl), strong acids (such as HCl), or strong bases (such as NaOH). Weak electrolytes are very slightly soluble salts, weak bases, or weak acids

9such as acetic acid, vinegar).

Solutions of electrolytes conduct electricity. It is reasonable to postulate that strong electrolytes are better conductors of electricity than weak electrolytes. In this demonstration, we will investigate the conductivity properties of various solutions of both strong and weak electrolytes. Carefully observe as your instructor performs this experiment and record important findings and observations in your notebook.

Questions

1.  Explain the relationship between the brightness of the bulb and the degree of ionization of the electrolyte.

2.  Write the balanced chemical equation for the ionization of NaCl.

3.  Based on your observations, explain the conductivity results obtained for deionized water.

Instructor’s Resource Guide

Materials required:

1 conductivity apparatus consisting of a light bulb and connecting wires

4 1-l beakers

2 l deionized water

20 g of NaCl

42 ml concentrated HCl

29 ml concentrated glacial acetic acid

1 magnetic stir plate

4 magnetic stir bars

Procedure: (Note: Acid dilutions should be prepared in a fume hood prior to the demonstration.)

1.  Add 500 ml of deionized water and one magnetic stir bar to each of the 4 1-l beakers.

2.  Label one of the beakers “Deionized Water.”

3.  Carefully add 20 g of NaCl to a second beaker. Label the beaker “NaCl” and stir to dissolve the NaCl.

4.  Slowly add 42 ml of concentrated HCl to a third beaker. Label the beaker “HCl” and stir to mix the solution.

5.  Slowly add 29 ml of concentrated glacial acetic acid to the fourth beaker. Label the beaker “Acetic Acid” and stir to mix the solution.

6.  Determine the conductivity of each of the four solutions by placing the electrodes of the conductivity apparatus into each of the four beakers. In order to prevent contamination, the deionized water should be measured first, and the electrodes should be rinsed with deionized water between measurements. (Note: Unplug the conductivity apparatus during each set up and cleaning operation.)

Time required: 20 minutes

Safety: Instructors should assess the risks associated with the chemicals used in this demonstration. Concentrated acids should be handled with extreme care. Always add the concentrated acid SLOWLY to the water. NEVER add water to concentrated acid. Dilutions should be prepared in a fume hood prior to the demonstration. Protective clothing and eye goggles should be used. Dispose of chemicals properly.

Reference: Public domain

Answers to Questions

1.  The brighter the bulb, the greater the conductivity of the solution. Because solution conductivity is directly related to the degree of ionization, the more ionized a compound, the greater its conductivity in water and the brighter the bulb.

2.  NaCl → Na+ + Cl−

3.  By definition, deionized water is devoid of ions. Because there are no electrolytes present, there is no conductivity and the bulb does not light.

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