EXPERIMENT 7

[ions.lab]

OBJECTIVES:

1. Be able to test liquids for conductivity.

2. Be able to identify a liquid that contains ions.

3. Be able to classify common substances as being non-electrolytes, strong electrolytes or weak electrolytes.

DISCUSSION:

Our conductivity apparatus consists of a simple electric circuit with a light bulb. The light goes on if the substance conducts electricity. [Note: it does make a difference which kind of bulb is used. With a lower wattage, measurement is more sensitive. For our purpose, a 40 watt bulb is best. For comparison, the Instructor will demonstrate a 15 watt bulb and a 1 watt neon bulb.] We will test the conductivity by immersing the wires about 3 or 4 cm into the material.

Substances will be tested pure (as solids or liquids) and also in solution with water. Water solutions are marked "aq", which stands for aqueous.

- no light = there are only molecules or else the ions are locked in place. This is a NON-ELECTROLYTE. **

- bright light = many ions are present and free to move around. No molecules remain. This is a STRONG ELECTROLYTE. Ionic compounds.

- weak or dim light = some ions are present but MOST of the substance remains in molecular form. This is a WEAK ELECTROLYTE. Often polar covalent compounds.

** Non-electrolytes typically have covalent bonding. However, if the substance is a metal/non-metal ionic compound, it may have such strong bonds that the substance cannot dissolve in water and the ions cannot get free. An example of this is BaSO₄.

When substances react with each other, the product may be a different kind of electrolyte than either of the reactants. We will be using three types of substances: ACIDS, BASES and SALTS.

- Formulas beginning with "H__" are acids. They are usually weak electrolytes. Among the common acids, only HCl, HNO₃ and H₂SO₄ are strong electrolytes.

- Formulas ending with "__OH" are bases. Most bases are strong electrolytes. You will meet one exception during the experiment.

- Substances made with a metal ion and a non-metal ion are salts.

EXPERIMENTAL PROCEDURE: (Working in pairs is suggested to save time.)

Part 1 - Test each substance listed. Pure substances are given by their formula (ie: C₂H₅OH). Water solutions are shown by "aq" or the molarity (0.1 M). [This indicates a very dilute solution.] Clean the wires between each test, unplugging the apparatus first !!

Record conductivity as S (strong), W (weak) or N (non-).

Under "Species" write IONS or MOLECULES or BOTH.

Part 2 - Fusion (melting) of KClO₃. This will be demonstrated.

Part 3 - We will use 0.1 M solutions of acids and bases. [0.1 M means that there is 0.1 mole of the substance in each liter of water. These are very dilute solutions.]

a. Put about 10 mL of the ACID into a 100 mL beaker.

b. Put 15-20 mL of the BASE into another beaker.

c. Test each for conductivity if you have not already done so. Record as Strong, Weak or Non-electrolytes. Add 1-2 drops of phenolphthalein (PP) to the acid. PP is an indicator and is colorless in acids and red in bases.

d. Now add the base slowly to the acid. You can pour it, but a dropper is recommended when you get near the neutralization point. Stir as you add the base. You will see a pink color that disappears when you stir. Keep on adding the base until a LIGHT pink color appears and remains. It is very important that the color be LIGHT pink. This means the acid and base are equivalent to each other and the mixture is neutral. Test the mixture for conductivity when the mixture is neutral.

EXP 7 -- LAB RECORD Name _______________ Sec ____

Partner ______________

ELECTROLYTES and NONELECTROLYTES

Part 1 CONDUCTIVITY SPECIES

Distilled H₂O ________________ ___________________

Tap H₂O ________________ ___________________

NaCl_(s) ________________ ___________________

NaCl_(aq) ________________ ___________________

C₂H₅OH_(l) ________________ ___________________

C₂H₅OH_(aq) ________________ ___________________

HC₂H₃O_2(l) ________________ ___________________

HC₂H₃O₂ (0.1M) ________________ ___________________

C₁₂H₂₂O_11(s) ________________ ___________________

C₁₂H₂₂O_11(aq) ________________ ___________________

CuSO_4(aq) ________________ ___________________

H₂SO₄ (0.1M) ________________ ___________________

HCl (0.1M) ________________ ___________________

NaOH (0.1M) ________________ ___________________

NH₄OH (0.1M) ________________ ___________________

Ba(OH)₂ (0.1M) ________________ ___________________

Part 2 - Explain your observation of the EFFECT OF FUSION on the conductivity of KClO₃.

Part 3 - TYPICAL IONIC REACTIONS

(a) 0.1 M HCl with 0.1 M NaOH.

The relative conductivities of the solutions tested are as follows:

0.1 M HCl ________ 0.1 M NaOH ________ Mixture ________

If the conductivity of the mixture is different from the individual solutions, explain why.

(b) 0.1 M HC₂H₃O₂ with 0.1 M NH₄OH.

The relative conductivities of the solution are as follows:

0.1 M HC₂H₃O₂ ________ 0.1 M NH₄OH ________ Mixture ________

If the conductivity of the mixture is different from the individual solutions, explain why.

(c) 0.1 M H₂SO₄ with 0.1 M Ba(OH)₂.

The relative conductivities of the solutions tested are as follows:

0.1 M H₂SO₄ ________ 0.1 M Ba(OH)₂ ________ Mixture ________

If the conductivity of the mixture is different from the individual solutions, explain why.