Leftover Aluminum Wire – A Stoichiometry Lab®Flinn Scientific Kit # AP 4678
Introduction
The redox experiment between copper (II) chloride and aluminum metal consistently gives impressive results in just a matter of minutes!
The progress of the reaction is easily followed by observing the disappearance of the green – blue copper color in the solution and the formation of
solid copper on the aluminum wire. Use the balance equation and Stoichiometry calculations to predict the amount of aluminum that should react
with the copper (II) chloride and compare this to the actual amount of “leftover” aluminum wire.
Chemical Concepts
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Stoichiometry
Limiting reactants
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Percent Yield Calculations
Oxidation – Reductions (REDOX) reactions
Background
Stoichiometry is the branch of chemistry that deals with the numerical relationships and mathematical proportions of reactants and
products in chemical reactions. Chemical reactions are represented by balanced chemical equations. Proper interpretation of an equation provides
a great deal of information about the reaction it represents and about the substances involved in the reaction. For example, the coefficients in a
balance chemical equation indicate the number of moles of each substance in the reaction. Therefore, the ratio of moles of one substance to moles
of any other substance in the reaction can be easily determined.
Consider the following unbalanced chemical equation representing the reaction that will take place in this lab:
CuCl2 (aq) + Al (s)  AlCl3 (aq) + Cu (s)
In the experiment, aluminum wire will be added to an aqueous solution of copper (II) chloride causing a single replacement, Redox reaction to take
place. The oxidation of aluminum metal to aluminum (III) [Alo to Al3+] will occur, which is apparent by dissolving the aluminum wire to form
aluminum chloride (AlCl3). The simultaneous reduction of copper (II) ions to copper metal [Cu2+ to Cuo] will occur and solid copper metal will
precipitate from solution. As the copper (II) ions are reduced, the green-blue color will fade until the solution is completely colorless – the
indication that the reaction is complete.
When performing an experiment such as this, the Stoichiometry of the reaction and the actual experimental procedure should be examined to 1st
determine which material is the limiting reactant. The limiting reactant (LR) is the reagent that is used up in the reaction and on which the overall
yield of product depends. The quantities of copper (II) chloride and aluminum used in this lab are such that the aluminum is in excess and the
copper (II) chloride is the limiting reagent in the reaction. The limiting reactant in any reaction, however, can be determined by calculating the
starting number of moles of each reactant. The balanced equation is then used to determine which starting material will ‘run out’ first or, in other
words, limit the amount of product formed. Consider the reaction between hydrogen gas and oxygen gas to produce water. The balanced equation
is as follows:
2H2 (g) + O2 2H2O (l)
If 10.0 g of H2 are mixed with 10.0g of O2, which one will ‘run out’ first and act as the limiting reactant? We must first determine the number of
moles of each reactant that we are starting with:
10.0 g H2 x 1 mole H2/2.02 g H2 = 4.95 mole H2 to start
10.0 g O2 x 1 mole O2/32.0 g O2 = 0.313 mole O2 to start
Then we must determine which of these reactants limits the amount of product formed. Consider H 2 first. The 4.95 moles of H2 could theoretically
produce 4.95 moles of H2O. This is determined by looking at the balance equation to determine the stoichiometric ratio, which is in this case is 2:2.
For every two moles of hydrogen that react, two moles of water can be produced.
Now consider O2. The 0.313 mole of O2 could theoretically produce 0.626 mole of H2O. This is determined from the stoichiometric ration, which in
this case is 1:2. That is, for every one mole of oxygen that reacts, two moles of water can be generated.
Therefore, if all the H2 reacted, 4.95 moles of H2O could theoretically form while only 0.626 moles of H2O could form from the available O2. The O2
is therefore, the limiting reactant in this example sine O2 limits the amount of H2O produced. The O2 will ‘run out’ first while some of the H2 will
remain in excess.
Theoretical yield then, is the maximum number of grams of product expected from the stoichiometric reaction when the limiting reagent
(LR) is completely consumed, with side reactions, reversibility, losses, and the like ignored. The theoretical yield
Theoretical yield = (moles of LR x moles of product/moles of LR) x (# g product/moles of product) = grams of product expected.
The theoretical yield is the maximum amount of product that can be produced from the quantities of reactants used. However, the amount of
product predicted by the theoretical yield is seldom actually obtained due to side reactions, losses or other complications. The actual yield of
product is often given as a percentage of the theoretical yield. This is called the percent yield which describes the efficiency of the reaction and is
calculated from the expression:
Percent Yield = (actual yield/theoretical yield) x 100%
The percent error for the reaction can be calculated as an indication of accuracy using the expression:
Percent error = (|actual yield – theoretical yield|/theoretical yield) x 100%
Materials (each group should try to perform two trails)
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Aluminum wire, 16-18 cm (~6 “)
Copper (II) chloride, 0.5g each
trial
Steel wool
Water, distilled or deionized, 10
mL each trial
Water, tap, 300 mL (Water bath)
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Balance
Beaker, 400 mL
Drying oven (optional)
Evaporating dish
Graduated cylinder, 10 mL
Hot plate
Spatula
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Stirring rod
Test tube holder
Test tube rack
Test tube
Forceps
Wire cutters
Safety precautions
Copper (II) chloride solution is TOXIC by ingestion and is a body tissue irritant; avoid contact with body tissues. Wear chemical safety goggles, chemical-resistant gloves
and a chemical resistant apron. Wash hands thoroughly with soap and water before leaving the lab.
Procedure – Repeat steps 1-12 if two trials are run.
1. Set up a hot water bath in a 400 mL beaker using approximately 300 mL of tap water. Bring the water to a boil
using a hot plate while performing steps 1-5.
2. Obtain approximately 0.50 g of CuCl2 crystals. Use the scale to determine the exact mass to the nearest
hundredth of a gram. Record the mass in the data table.
3. Place the CuCl2 crystals in the test tube and add 10 mL of distilled water. Stir to dissolve the crystals.
4. Cut a 16-18 cm piece of Al wire with the wire cutters. Use a small piece of steel wool to shine the wire and to
remove the surface impurities.
5. Measure and record the exact mass of the aluminum wire to the nearest hundredth of a gram in the data table.
6. Use forceps to place the Al wire into the test tube containing CuCl2 solution. The wire will extend beyond the
height of the tube. Place the tube into the boiling water bath for approximately 5-10 min. The reaction is
complete when the solution becomes colorless.
7. Use the forceps to remove the wire from the test tube. Don’t knock off any of the copper on the wire.
8. Measure the mass of a clean, dry evaporating dish and record the mass in the data table.
9. Use the spatula to scrape as much solid copper as possible from the aluminum wire into the evaporating dish.
10. Allow the solid Cu in the dish to dry overnight. (Note: the Cu will slightly oxidize & change color to green or dark brown.)
11. Once dry, measure the mass of the Cu and evaporating dish. Record this mass in the data table.
12. Rinse the aluminum wire with tap water to remove impurities and dry wire with a paper towel. Once dry,
measure & record the actual ‘leftover’ mass of the Al wire in the data table.
Name:
Partners:
Per.
Leftover Aluminum Wire Worksheet
Data Table
MEASUREMENTS (in grams to the hundredths decimal place)
Initial mass of CuCl2.2H2O
Initial mass of Al wire
Mass of ‘leftover’ Al wire
Actual mass of aluminum reacted
Mass of evaporating dish + Cu
Mass of evaporating dish
Actual mass of recovered dry Cu
TRIAL 1
TRIAL 2
TRIAL 1
TRIAL 2
Results Table
CALCULATIONS (to the hundredths decimal place)
a. Moles of CuCl2.2H2O
b. Moles of Al that should react (theoretical)
c. Mass of Al that should react (theoretical)
d. Percent error for reacted Al
e. Predicted mass of recovered Cu (theoretical)
f. Percent error for recovered Cu
g. Percent yield of recovered Cu
Post Lab Questions and Calculations
1. Write the balanced chemical equation for the reaction that occurred between the aluminum wire and the
copper (II) chloride solution.
2. Which starting material in the reaction is the limiting reactant and which material is present in excess? Show
calculations and all work for your determination of this.
3. Complete the Data and Results Table. Show all calculations, clearly indication where each result came from for
each box ‘a’ through ‘g’.
4. What physical evidence do you have that shows the copper (II) chloride is, indeed, the limiting reactant?
5. Discuss reasonable and potential sources of error in this experiment.
6. Discuss potential reasons why the percent yield of recovered copper may be greater than 100%.
7. Why do you think that scientists add excess of one or more chemicals when performing a reaction rather than
combine the exact stoichiometric ratio?
8. What factors do you think may lead scientists (in industry, for instance) to decide to use a certain starting
material as the limiting reactant and another as the excess chemical?