Chemistry 121
IONIC AND COVALENT COMPOUNDS:
ELECTRICAL CONDUCTANCE AND ELECTROLYTES
Substances that conduct an electric current when dissolved in water or when molten are called electrolytes. Compounds which do not conduct a current under these conditions are called non-electrolytes. In order for conduction to occur, ions must be present and free to move. When the electrodes from a battery are placed in an electrolyte (see figure 6.9, page 193 in your text), the mobile anions travel to the positive electrode (the anode) and the mobile cations travel to the negative electrode (the cathode). In doing so, the the circuit is complete and conductance occurs.
When a pure ionic compound is dissolved in water or is molten, the ions are free to move and hence, to carry an electric current. Pure covalent compounds can't conduct an electric current; however, some covalent compounds form ions when dissolved in water and it is these aqueous (water) solutions which are now capable of conducting an electric current. Any aqueous solution of a substance which ionizes completely is called a strong electrolyte. Any aqueous solution which ionizes to a lesser extent is called a weak electrolyte.
In today's lab, you will be determining whether certain substances and/or their aqueous solutions are strong, weak, or non-electrolytes. You will also be testing the conductance of the reactants and the products of a chemical reaction. Finally, the instructor will test the conductance of a molten ionic compound.
(over) EXPERIMENTAL PROCEDURES:
Your conductance apparatus consists of a 9 volt battery connected to a circuit board which has a small light bulb and two metal electrodes. If a conducting material connects these two electrodes, the bulb will light up. A strong electrolyte (one which conducts electricity well) causes the bulb to burn brightly and to blink. A weak electrolyte (one which conducts electricity poorly) causes the bulb to burn dully without blinking. A non-electrolyte (one which does not conduct electricity at all) results in the bulb not burning at all.
Using a clean well-plate, you will be testing samples of different compounds or their aqueous solutions. Make sure there is no mixing of your compounds in the well- plate. Also make sure that you rinse and dry the electrodes of your conductance apparatus completely with distilled water between each measurement.
Part I: The conductance of some compounds and solutions.
A: Test and record the conductance of the following compounds: distilled water, solid sugar or sucrose (C12H22O11), isopropyl/rubbing alcohol (C3H8O), solid salt (NaCl), and solid sodium acetate (NaCH3COO or NaC2H3O2).
B: Test and record the conductance of the following prepared 0.1 M aqueous solutions: tap water, hydrochloric acid solution (HCl dissolved in water), acetic acid solution (CH3COOH, dissolved in water), sugar solution, salt solution, sodium acetate solution, and ammonium hydroxide solution (ammonia, NH3, when dissolved in water forms some NH4OH).
Part II: Conduction of reactants and products.
In a beaker, mix 20 drops (about 1 mL) of 1.0 M acetic acid solution with 20 drops of 1.0 M ammonium hydroxide solution. Test and record the conductance of this mixture.
Part III: Conductance of a molten ionic compound.
Your instructor will melt some sodium acetate and test its conductance. Record the results of this test. REPORT SHEET: CONDUCTANCE AND ELECTROLYTES
Name: Lab Partner: Section:
Part I : Complete the following table with your observations and conclusions. compound conductance type of electrolyte (strong, weak, or none) (strong, weak, or none) distilled water solid sugar rubbing alcohol solid salt solid sodium acetate tap water
HCl solution vinegar sugar solution salt solution sodium acetate solution ammonium hydroxide
Part II : mixture of vinegar and ammonium hydroxide Part III : molten sodium acetate Part I :
1) a) Is sodium chloride an ionic or covalent compound?
b) Explain why there should be a difference in the conductance of solid sodium chloride versus a sodium chloride solution.
2) a) Is sugar an ionic or covalent compound?
b) Is hydrogen chloride an ionic or covalent compound?
c) Explain why there should be a difference in the conductance of a sugar solution versus a hydrogen chloride solution. Use a reaction equation as part of your explanation.
d) Utilizing your answers to ‘1a’ and ‘2a’, explain why there should be a difference in the conductance of a sugar solution versus a sodium chloride solution.
e) Explain why there should be no difference in the conductance of a hydrogen chloride solution versus a sodium chloride solution. Use a reaction equation as part of your explanation. Part II :
1) When an aqueous solution of acetic acid (vinegar) is mixed with ammonium hydroxide, the products are ammonium acetate and water. Write a balanced equation for this reaction.
2) From your conductance result Part II and from your results in Part I, classify each reactant and product from Part II as either a strong, weak, or non-electrolyte.
Part III :
1) Is sodium acetate an ionic or covalent compound?
2) Explain why there should be differences or similarities in the conductance of a sodium acetate when solid, in solution and molten.