AP CHEMISTRY LAB
THE EMPIRICAL FORMULA FOR COPPER IODIDE
(B)
THEORY:
Copper is the most abundant and active member of the Nobel Metals; copper,
silver and gold. It is used in many applications in our society the most notably being the
use as an electrical conductor; your home, cell phone, calculator, etc. are wired with
copper! But, Copper compounds can cause problems! “In animals, inhalation of copper
dust has caused hemolysis of the red blood cells, deposition of hemofuscin in the liver
and pancreas, and injury to the lung cells; injection of the dust has caused cirrhosis of the
liver and pancreas, and a condition closely resembling hemochromatosis or bronzed
diabetes. (Sax, Dangerous Properties of Industrial Materials, eighth edition) that means
that copper is like other heavy metals, it can be bad for your health if you aren’t careful!
Iodine is, on the other hand, is the least haziest of the halogens. “It is a solid at
room temperature. It exists in bluish-black plates with a rhombic shape, metallic luster,
and peculiar order. Iodine is a non-metallic element, existing in sea-water and sea-plants.
When it is dissolved in alcohol, it is called a tincture. That solution can be used as a very
effective disinfectant. But, since it does kill things, if it is used as a disinfectant, it will
cause scaring. There are other problems too.
Copper iodide is used in the production of some organic compounds, as a cloud
seeding compound, and to help stabilize nylon used in high temperature applications.
Gee, this is a lot of good to know info, isn’t it? Well let’s get back to the lab and think
what we are going to do today!
At times, we are fortunate enough to observe a chemical reaction that is easy to
control and involves chemicals that can be easily handled. We can use this type of
reaction to collect quantitatively data to confirm concepts that are under consideration in
the high school setting. One such reaction is the reaction between copper and iodine.
You will remember that during your introductory chemistry experience, you submerged a
copper wire into a solution of iodine. The color of the copper changed indicating a
chemical reaction. If we are careful and measure the mass of copper before the reaction,
the mass of the copper with the product on it, and then dissolve the product and re-mass
the copper, we should be able to determine the formula of the copper-iodine compound.
Since copper has two oxidation states (charges), you will be able to determine if the
compound is CuI or CuI2; we will be able to determine the empirical formula of copper
iodide. Let’s do some chemistry!
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LEARNING OBJECTIVES:
Determine the formula of a copper-iodine compound through gravimetric methods.
Apply stoichiometric calculations to analyze gravimetric data to arrive at the
formula for the copper-iodine compound.
Observe two elements, the reaction that takes place between them, and the
compound that results from the reaction.
To comprehend again, that the properties of a compound are different from the
properties of the starting elements.
To minimize possible errors that can cause the mole ratios of atoms to differ from
whole numbers.
Practice using the analytical balance.
To continue to use safe and effective lab procedures.
PROCEDURE:
(One way to speed the drying process is to warm an object with a hair dryer. You will
also remember that you should not find the mass of a hot object on the balances in room
610. Hot objects will mass light!)
1) Thoroughly clean a strip of copper. This can be accomplished by submerging it in a 6
M HCl solution and or polishing it with emery cloth.
2) Rinse the copper with distilled water, let it air dry, and then finally find its mass on the
analytical balance. (To speed the process, you may want to dip the copper in acetone.)
3) Place enough of the iodine solution in a 15X150 mm test tube so that it almost will
submerge the copper strip.
4) After you have massed the copper, place it into the iodine solution and stopper the test
tube. Allow the test tube to stand undisturbed for at least 30 minutes. Over night
would be better!
5) Carefully decant the solution, rinse the copper strip at least 3 times with distilled
water, and then finally with acetone.
6) Find the mass of a clean and dry weight boat, transfer the copper strip to the boat, and
allow the copper to air dry. Finally, take the necessary readings so that you can
calculate the mass of the copper/copper iodine compound.
7) Place the copper strip into a test tube that is ~ half full of 0.50 M sodium thiosulfate,
Na2S2O3. What is happening to the copper compound in that solution?
8) When the copper strip is clean, rinse it again with distilled water and then acetone.
Allow it to air dry and find its mass again.
9) Clean up your work area and your glassware. Make sure you dispose of the solutions
and copper as instructed by RT.
10) RT will demonstrate the sublimation of I2. Make sure you record your observations.
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QUESTIONS AND CALCULATIONS:
1) In the reaction, copper is oxidized and iodine is reduced. Define:
A) Oxidation.
B) Reduction.
C) Sublimation.
You will need to use either your text or the internet to find the necessary equations
for questions #2, 3, & 5!
2) Copper can have two charges. Look up a reaction that will show the oxidation of
copper to those two states.
3) Iodine will generally acquire a -1 charge. However, there are times when it can have a
positive oxidation number. Find at least two equations, one that will show iodine
developing a -1 charge, and another reaction (at least) in which iodine will have some
other oxidation state.
4) What was the purpose of the acid rinse for the copper at the first of the lab? Why did
we use HCl and not HNO3? (Talk to your classmates who are running the Tin Oxide
lab for some help here.)
5) Copper will show a different reaction with dilute nitric acid as compared to the one we
saw with concentrated nitric acid. Find and copy both reactions.
6) Calculate:
(Remember your calculations need to have labels and units!)
A) The ∆ mass of Cu.
B) The moles of Cu used.
C) The mass of iodine (I not I2) that collected on the wire.
D) The moles of I on the wire.
E) The ratio of the moles of I to the moles of Cu.
(You did show your work didn’t you?)
7) Now, plug the number of moles of Cu and I into the formula CuxIy. The value for X is
the moles you found in 6B; Y is the moles of I you found in 6D.
8) Now, divide the subscripts in CuxIy by the lower of the two values, X or Y.
9) If necessary now multiply your new values for CuxIy that you found in #8 by a factor
so that both X and Y are whole numbers.
10) Use your formula that you developed in #9 and complete the synthesis reaction:
Cu + I2 →
11) Naphthalene (moth flakes) is another substance that sublimes.
A) What evidence do you have that naphthalene does that?
B) Gertrude bought moth flakes at Wally World. She opened the container and
sprinkled some on the wool sweaters she was storing over the summer. She then set
the open container with the remaining naphthalene on a shelf so she would have the
rest of the flakes to use the next spring. Well... when she came back in a year, all of
her moth flakes were gone! Explain to her why she didn’t have any of the flakes
remaining. Remember, she is not in AP Chemistry, so you have to do a complete job
with your explanation!
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12) A) Write the equation for the reaction that takes place between iron (III) chloride and
sodium hydroxide.
B) Is the reaction in 12A a redox reaction?
C) State your reasons for thinking that the reaction in 12A is or is not a redox reaction.
13) Compare and contrast the reaction between copper and iodine and the reaction
between iron (III) chloride and sodium hydroxide.
A) How do you know that they are reactions?
B) What kind of “checks’ would you have to run on each reaction to determine if it is
a redox reaction?
C) How do the properties of the products compare to the reactants?
D) Etc.
14) List at least 3 sources for error in the lab and propose ways to minimize those errors.
Be specific!
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