OBJECTIVES

The Formati= on and Decomposition of Zinc Iodide

Compounds are chemical combinations of elements. Many chemical reactions of elements to form compounds are spectacular but must be run under special laboratory conditions because they are dangerous. The rea= ction of sodium and chlorine to form sodium chloride pictured at the beginning of Chapter 4 is a good example. If elements react spontaneously to form compou= nds, that is a good indication that the compound state is more stable than the f= ree element state. To break a stable compound down into its component elements, energy must be put into the compound. Electricity is often used as this ene= rgy source. The process of decomposing a compound into its component elements by electricity is called electrolysis.

OBJE= CTIVES

• Compare a compound with its component e= lements.

• Observe and monitor a chemical reaction= .

• Observe the decomposition of the compou= nd back to its elements.

HYPO= THESIS:

MATE= RIALS

= = = = = =

apron = &nb= sp; goggles

10 X 150-mL test tube test-tube rack

test-tube holder = 100-mL beaker

spatula = &nb= sp; plastic stirring rod

zinc = &nb= sp; = iodine crystals

distilled water

9-V battery with terminal clip and leads two 20-cm ins= ulated copper wires stripped at least 1 cm on each end

CAUTION: Iodine crystals are toxic and can s= tain the skin. Use care when using solid iodine. The reaction of zinc and iodine releases heat. Always use the test-tube holder to handle the reaction test tube.

PROC= EDURE

1. Obtain a test tube and a small beaker. Place= the test tube upright in a test-tube rack.

2. Carefully add approximately 1 g of zinc dust= and about 10 mL of distilled water to the test tube.

3. Carefully add about 1 g of iodine to the test tube. Record your observations in the data table.

4. Stir the contents of the test tube thoroughl= y with a plastic stirring rod until there is no more evidence of a reaction. Record any observations of physical or chemical changes.

5. Allow the reaction mixture to settle. Using a test-tube holder, carefully pick up the test tube and pour off the solution= phase into the small beaker.

6. Add water to the beaker to bring the volume = up to about 25 mL.

7. Obtain= a 9-V battery with wire leads and two pieces of copper wire. Attach the copper wi= res to the wire leads from the battery. Make sure that the wires are not touchi= ng each other.

8. Dip th= e wires into the solution and observe what takes place. Record your observations.

9. After = two minutes, remove the wires from the solution and examine the wires. Again record your observations.

DATA AND OBSERVATIONS

ST= EP	OB= SERVATIONS
3. Addition of iodine to zinc
4. Reaction of iodine and zinc
5. Electrolysis of solution<= /b>
9. Examination of wires<= /p>

ANAL= YZE AND CONCLUDE

1. O= bserve and Inferring: What evidence was there that a chemical reaction occurred?

2. C= omparing and Contrasting: How did you kn= ow the reaction was complete?

3. M= aking Inferences: What term is used to describe a reaction in which heat is given off?

How can you account for the heat given off in th= is reaction?

4. C= hecking Your Hypothesis: What evidence = do you have that the compound was decomposed by electrolysis?

5. D= rawing Conclusions: Why do you think t= he reaction between zinc and iodine stopped?

APPL= Y AND ASSESS

1. <= /span>What role did the water play in this reaction?

2. <= /span>Do you think zinc iodine is an ionic or covalent compound?

What evidence do you have to support your conclu= sion?

3. The fo= rmula of zinc iodine is ZnI₂. Use Lewis dot structures to show how it forms from its elements. Hint: Zinc atoms have two valence electrons.