Lab 14 Aluminum Leftovers – A Quantitative Analysis
Honors Chemistry
Introduction:
Stoichiometry deals with the 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 the balanced chemical equation indicate the relative 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.
When the amount of product that will be formed is important, the stoichiometry of the reaction and the actual
experimental procedure should be examined to determine which material is the limiting reactant. The limiting reactant
(LR) is the starting material that will “run out” first, and thus limit the amount of product formed. The limiting reactant
in any reaction can be determined from the amount of the starting materials and the balanced chemical equation. That
aluminum is in excess and the cupric chloride is the limiting reagent in the reaction is suggested by the title of the lab.
Prelab questions
1: Write and balance the reaction of aqueous copper (II) chloride reacts with solid aluminum to form aqueous aluminum
chloride and solid copper.
2: What type of chemical reaction is referenced in question 1?
3. Write the balanced equation for the reaction between hydrogen gas and oxygen gas to produce water
4. This is one long problem:
If 10 g of H2 are mixed with 10.0 grams 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.
a. starting with 10.0 g of H2 how many moles of water do we have?
b. starting with 10.0 g of O2, how many moles of O2 do we have?
c. Then we must determine which of these reactants limits the amount of product formed. Consider H2 first. If you
have 4.95 moles of H2 you could theoretically produce 4.95 moles of H2O. Show that this is a true statement by
looking at the balanced chemical equation to determine the stoichiometric ration.
Use stoichiometry, crossing of your labels to prove that in this example 4.95 moles of H2 = 4.95 mole H2O
d. Now consider O2. Using your answer from b, calculate the number of moles of water can theoretically be produced
from the number of moles of O2 you have.
e. Therefore, if all of the H2 reacted, ______ moles of H2O could theoretically form while only _______ moles of H2O
could form from the available O2 .
f. Based on the results stated in part d, which is the limiting reactant, H2O or O2?
g. Complete the statement: _____ is therefore the limiting reactant in this example since ____ limits the amount of
____ produced. The ___ will run out first while some of the ____ will remain in excess.
Theoretical yield 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.
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 form the quantities of reactants used.
However, the amount of product predicted by the theoretical yield is seldom actually obtained due to side reactions,
loses, 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
x 100%
Theoretical yield
The percent error of the reaction can be calculated as an indication of accuracy using the expression:
Percent error = l actual yield - theoretical yield l x 100%
Theoretical yield
Purpose:
In this experiment, you will compare the actual mass of aluminum reacted and the actual mass of copper
produced with that predicted from the balanced chemical equation using stoichiometric calculations.
Materials/Equipment:
Hot plate
10-mL graduated cylinder
250-mL beaker
1 g cupric chloride, CuCl22H2O
tap water
20-23 cm, aluminum wire
metal scoupula
Distilled water
wire cutters
test tube (16x125mm)
Crucible tongs
test tube holder
steel wool ( if needed)
stirring rod
analytical balance ( to 0.001)
Watch glass
Safety Considerations:
 Always wear goggles in the lab.
 Cupric chloride is TOXIC by ingestion and is a BODY TISSUE IRRITANT; avoid contact with body tissues.
Overview:
Dissolve a small (~1/2 g) weighed sample of CuCl22H2O in water.
Allow the solution to react with a known amount of aluminum wire.
[The reaction proceeds more rapidly at elevated temperatures.]
Collect and weigh the Cu metal produced; reweigh the aluminum wire.
Procedure:
1. Set up a hot water bath in a beaker using approximately 200 mL of tap water. Bring the water to a slow boil.
2. Place approximately 0.50 grams cupric chloride crystals in a tared test tube. Use a balance to determine the exact
mass of the crystals. Record the mass to the 0.001 place in the data table.
3. Add about 10 mL of distilled water to the test tube, and stir to dissolve the crystals.
4. Obtain a 20-23 cm piece of Al wire. Use a small piece of steel wool to shine the wire and to remove any surface
impurities that may be present on the wire (if needed).
5. Measure and record the exact mass of the clean aluminum wire.
6. Create a spiral out of your Al wire by winding it around a pencil. Lower the wire into the test tube containing the
cupric chloride solution.
7. Place the test tube in the boiling water bath for about 5-10 minutes. The reaction is complete when the solution
becomes colorless.
8. Obtain a clean, dry watch glass and write your group members’ names on it with the tape provided.
9. Measure the mass of the empty watch glass with tape and record the mass in the data table.
10. Use a scoupula remove the wire from the test tube. Use caution so as not to disturb the copper on the wire.
11. Use the spatula to scrape as much solid copper as possible from the Al wire on to the watch glass.
12. Stretch the coil of Al out so that the remaining copper may be scraped off.
13. Rinse the Al wire with tap water to remove any impurities, and dry the wire with a paper towel.
14. Measure and record the remaining mass of the dry Al wire in the data table.
15. Allow the solid copper to dry in the oven. [Note: overheating can cause the copper to oxidize which changes the
color to green or dark brown.]
16. Once dry, measure the mass of the copper and the watch glass. Record these mass in the data table.
17. Clean up.
Calculations and Results:
The balanced chemical equation for the reaction that occurred between the Al wire and the cupric chloride solution is
required to answer the post lab questions. (see prelab question 1)Complete the Data Table and Results Table. Show all
calculations either on the table or in your lab book, clearly indicating where you got each result.
Post lab questions:
1. Which starting material is the limiting reactant and which material is present in excess? Show calculations and all
work for your determination of this answer.
2. What physical evidence do you have that shows that cupric chloride is the limiting reactant?
3. Why is it unnecessary to carefully measure the amount of water used to dissolve the CuCl2?
4. Discuss reasonable potential sources of error in this experiment.
5. Discuss potential reasons why the percent yield of recovered copper may be greater than 100%.
6. Why do you think scientists add excess of one or more chemicals when performing a reaction rather than
combining the exact stoichiometric ratio?
7. What factors do you think may lead scientists (in industry, for example) to choose one certain starting material as
the limiting reactant and another as the excess chemical?
Lab rubric Left over Aluminum wire 80 pts
____ 2 pts
____ 2 pts
____ 5 pts
____ 8 pts
____ 4 pts
____ 14 pts
____ 7 pts
____ 14 pts
____ 5 pts
____ 2 pts
____ 2 pts
____ 2 pts
____ 2 pts
____ 2 pts
____ 2 pts
Lab is written according to basic lab instructions
Rubric is included
Questions answered with question clearly indicated with the answer
Lab physically completed within one week of original date assigned
Prelab and post lab questions have answers circled and work shown. The factor label method
is utilized and proper sig figs and units are reported for all mathematical questions.
Prelab questions 1-4f are completed
Data table completely filled in with answers circled and work shown. Proper sig figs and units
are reported.
Results table is completely filled in with answers circled and work shown. The factor label
method is utilized and proper sig figs and units are reported.
Post lab question 1 Clearly state which starting material is the limiting reactant and which material is
present in excess. Show calculations and all work for your determination of this answer.
Post lab question 2 - Clearly explains what physical evidence you have that shows that cupric chloride
is the limiting reactant?
Post lab question 3 – Clearly explain why is it unnecessary to carefully measure the amount of water
used to dissolve the CuCl2?
Post lab question 4 – Discuss reasonable potential sources of error in this experiment.
Post lab question 5 - Discuss potential reasons why the percent yield of recovered copper may be
greater than 100%.
Post lab question 6- Clearly explain why you think scientists add excess of one or more chemicals
when performing a reaction rather than combining the exact stoichiometric ratio?
Post lab question 7- Clearly explain what factors you think may lead scientists (in industry, for
example) to choose one certain starting material as the limiting reactant and another as the excess
chemical?
Data Table
Initial mass of CuCl2
2 H2O (g)
Initial mass of aluminum wire (g)
Mass of left over aluminum wire (g)
Actual mass of aluminum reacted (g)
Mass of watch glass and copper (g)
Mass of watch glass (g)
Actual Mass of recovered dry copper (g)
Results Table:
Moles of CuCl2
2H2O
Moles of aluminum that should react (theoretical)
Mass of aluminum that should react (theoretical)
Percent error for reacted aluminum
Predicted mass of recovered copper (theoretical)
Percent error for recovered copper
Percent yield of recovered copper