Download Stoichiometry Lab: Leftover Aluminum Wire

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Stoichiometry Lab: Leftover Aluminum Wire
introduction
The single-displacement reaction between copper(ll) 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 balanced equation and stoichiometry calculations to predict the
amount of aluminum that should react withthe copper{ll) chloride and compare this to the actual amount
of leftover" aluminum wire.
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 infomiation about the reaction It represents and about the substances involved in the reaction. For
example, the coefficients in a balanced 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 detemiined.
Consider the following unbalanced chemical equation representing the reaction that will be
performed in this lab:
CUCI2 (aq) + Al(s)
AICI3 (aq) + Cu (s)
In the experiment, aluminum wire will be added to an aqueous solution of copper(ll) chloride
causing a single-displacement reaction to take place. Aluminum metal will fonn aluminum(lll) ions, which
is apparent by the dissolving of the aluminum wire to forni aluminum(lll) chloride (AICI3). The
simultaneous formation of copper metal from copper(ll) ions will occur and solid copper metal will
precipitate from the solution. As this reaction progresses, 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 first determine which material is the limiting reactant. The
limiting reactant (I.R) is the reagent that is used up in the reaction and on which the overall yield of the
product depends. The quantities of copper(ll) chloride and aluminum used In this lab are such that the
aluminum is in excess and the copper(ll) chloride is the limiting reactant in the reaction (as evident from
the title of the lab). 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, limitthe amount of product formed. Consider, for
example, the reaction between hydrogen gas and oxygen gas to produce water. The balanced equation
is as follows:
2H2(g) + 02(g) -> 2H20(I)
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If10.0 grams H2 are mixed with 10.0 grams of O2, which one will "run out"first and act as the
limiting reactant? Wemust first determine the number ofmoles ofeach reactant thatwe are starting with:
10.0gH^x
=4.95molH,
10.0g0;x
' 32.0g0j
=0.313mo/Q,
'
Then we must determine which of these reactants limits the amount of productformed. Consider
H2 first. The4.95 moles of H2 could theoretically produce 4.95 moles of H2O. This is determined by
looking at the balanced chemical equation to determine the stoichiometricratio, which in this case is two
to two. That is, foreverytwomoles of hydrogen that react, two molesof watercan be generated:
4.95 mo/
X
^ ^
2 molH2
Now consider O2. The 0.313 mole of O2 could theoretically produce 0.626 moles of H2O. This is
determined from the stoichiometric ratio, which inthis case is one to two. That Is, for every one mole of
oxygen that reacts, two moles of water can be generated:
0.313mo/0, X
ImolO^
= 0.626molH,0
^
Therefore, if all the H2 reacted, 4.95 molesof H2O could theoretically form while only0.626 moles
of H2O could form from the available O2. The O2 is therefore the limiting reactant inthis examplesince O2
limits the amount of H2O produced. The O2 will "run out" first while some of the H2 will remain in excess.
then, is the maximum number of grams of product expected from the
stoichiometric reaction when the limiting reactant (LR) is completelyconsumed, with side reactions,
reversibility, losses, and the like ignored. The theoretical yield is calculated from the expression:
Theoretical Yield =moles of
ofProduct ^ #g product
moles ofLR
moles of product
The theoretical yield is the maximum amountof product that can be produced from the quantities
of reactants used. However, the amountof product predicted by the theoretical yield is seldom actually
obtaineddue to side reactions, losses, or other complications. The actual yieldof productis oftengiven
as a percentage of the theoretical \fleld. This is called the percent yield which describes the efficiency of
the reaction and is calculated from the expression:
Q. . .,
% yield =
actual yield
theoretical yield
X100%
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Procedure
1. Set up a hotwater bath in a 150 mL beaker using approximately 100 mL oftap water.
Bring the water to a boil using a hot plate while performing steps 2-5.
2. Obtain approximately 0.4 grams of copper(ll) chloride dihydrate crystals. Use a balance
to determinethe exact mass to the nearest hundredth of a gram. Record the mass in
the data table on page 4. (Safety precaution: Copper(ll) chloride solution, also known as
cupric chloride solution, is toxic by ingestion and is a body tissue imtant; avoid contact
with body tissues.)
3. Place the copper(ll) chloride crystals in a small test tube. Fill the test tube about 4/5 full
with distilled water and stir to dissolve the crystals.
4. Cut a piece of aluminum wire about 12 cm long with wire cutters.
5. Measure and record the exact mass of the aluminum wire to the nearest hundredth of a
gram in the data table on page 4.
6. Lower the aluminum wire into the test tube containing the copper(ll) chloride solution.
The wire will extend beyond the height of the tube. Place the test tube into the boiling
water bath for approximately 5-10 minutes. The reaction is complete when the solution
becomes completely colorless.
7. Measure the mass of a clean, dry weigh-boat and record the mass in the data table on
page 4.
8.
Remove the wire from the test tube. USE CAUTION SO AS NOT TO DISTURB THE
COPPER ON THE WIRE.
9. Use a spatula to scrape as much solid copper as possible from the aluminum wire into
the weigh-boat.
10. Rinse the aluminum wire with tap water to remove any impurities and dry the wire with a
paper towel. Once dry, measure and record the "leftover" mass of the aluminum wire in
the data table on page 4.
11. Allow the solid copper in the weigh-boat to dry overnight. (Note: the copper will slightly
oxidize and change color to green or dark brown) Once dry, measure the mass of the
copper and the weigh-boat. Recortf the mass in the data table on page 4.
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Prelab Questions
Complete prior to starting the lab procedure.
1) Write the balanced chemical equation for the reaction that occurred between the aluminum
wire and the copper(ll) chloride solution.
2) What is the molar mass of CuCl2-2H20 ?
3) What is the molar mass of Al ?
4).Define oxidation and reduction
a. Using this lab as a reference, provide an example of each.
5) How will you know when the reaction is done?
6. What are the products in this reaction?
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Data Table
Mass of CuCl2-2H20 used (g)
Initial mass of Al wire used (g)
Mass of "leftover" Al wire (g)
Mass of clean, dry weigh-boat (g)
Mass of weigh-boat and Cu (g)
Calculations
SHOW YOUR WORK &put your answers on the line
1) Calculate the mass of Al that reacted (initial mass - leftover mass).
2) Using the mass of the limiting reactant, calculate the theoretical yield of Cu in grams.
3) Calculate the actual yield of Cu in grams.
4) Calculate the percent yield of Cu.
PER
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Analysis Questions
The answers these questions should not include phrases such as "human error" or "calculation
error^ or "measured incorrectly." You need to specifically talk about parts of the procedures or
assumptions that the lab makes... DO NOT talk about mistakes that you may have made.
1. Discuss reasonable and potential sources of error in this experiment. In other words,
explain what could have caused mistakes to occur.
Be Specific!!
2. Discuss potential reasons why the % yield of recovered copper may be less than 100%.
3. Discuss potential reasons why the % yield of recovered copper may be greater than
100%.
4. Why do you think that scientists add excess of one or more chemicals when performing
a reaction rather than combine the exact stoichiometric ratio?