Chemical Reactions
Lab 2: The Stoichiometry of the Reaction Between Aluminum and Copper(II) Sulfate
Purpose
The purpose of this experiment is to use the mole-to-mole relationship of a balanced chemical equation to
identify the limiting reactant and to find the percent yield of the reaction.
Introduction
The limiting reactant in a chemical reaction is the one that determines how much product can be formed.
When two reactants in a chemical reaction are not present in the exact amounts needed for reaction, one
reactant will be used up and some of the other reactant will be left over. The one that is used up is called
the limiting reactant because it limits the amount of product that can be formed. Naturally, the reaction
must stop when that reactant is used up. The other reactant is said to be in excess. In this experiment,
you will carry out a reaction and use the mole amounts of each one to find out which is limiting and which
is in excess. In addition, you will calculate your theoretical yield and compare the amount of product you
get to this theoretical yield to find a percent yield.
Brief Description
You will use a measured of amount of aluminum to react with a measured amount of a solution of copper
(II) sulfate. You will determine the limiting reactant, the amount of product you should get and the percent
yield.
Safety
You must wear safety glasses throughout this experiment.
Wash your hands if you spill any of the copper(II) sulfate solution on them.
Materials
2 50-mL portions of 0.250 M copper(II) sulfate
2 7 cm x 7 cm squares of aluminum foil
3 150-mL beakers
2 stirring rods
2 pieces of filter paper
Small amount of >1M strong monoprotic acid
100-mL graduated cylinder
funnel support
ring stand
rulers
scissors
2 funnels
Procedure
1.
Use one of the 150-mL beakers to get ~50 mL of 0.250 M copper(II) sulfate solution and take it to
your desk.
2.
Cut two squares of aluminum foil, 7 cm x 7 cm. Mass each piece of foil. Record the mass of each
piece on the data sheet. Make sure you know which piece of aluminum is piece #1 and which
one is piece #2.
3.
Measure 50.0 mL of the copper solution and transfer it to a clean 150 mL beaker. Measure a
second sample of the copper(II) sulfate solution into another clean 150 mL beaker. Signal to the
teacher that you are ready for the addition of acid to your solution.
4.
Tear the aluminum foil into strips and carefully add the strips of the aluminum foil to the beaker
with the copper solution. Stir the solution from time to time. You should do one piece of foil while
your partner does the second piece.
5.
When all the foil has been added, let the reaction continue until all foil has been dissolved, at
least 5 minutes.
6.
7.
8.
9.
Your teacher will show you how to fold a piece of filter paper. Mass each piece of filter paper.
Record the mass of each piece on the data sheet. Be sure you know which one you will call filter
paper #1 and which one will be filter paper #2. Fold each piece of filter paper and place it into a
funnel. Using your stirring rod as a directing device, filter off the majority of the liquid. When you
get to the last amount of liquid, swirl the contents of the beaker and filter the liquid with the copper
that has formed.
When there is room in the filter paper, use a squirt bottle with distilled water to rinse all the copper
into the funnel. Use your stirring rod to help you transfer the copper to the funnel. You must get
all the copper into the funnel.
When the liquid has all drained from the funnel, carefully pick up the filter papers and lay them on
a piece of paper towel to dry.
When the contents of the funnel have dried, mass the filter paper and contents and record the
masses.
Data Sheet
Lab 2: The Stoichiometry of the Reaction Between Aluminum and Copper(II) Sulfate
Trial 1
Trial 2
Mass of aluminum square
_______________
_______________
Mass of filter paper + solid
_______________
_______________
Mass of filter paper
_______________
_______________
Mass of solid
_______________
_______________
Questions
Lab 2: The Stoichiometry of the Reaction Between Aluminum and Copper(II) Sulfate
1.
Calculate the number of moles of aluminum used for each trial. Calculate the number of moles of
copper(II) sulfate used for each trial. Be sure to show sample calculations.
2.
Based on what you have learned in class, write and balance a molecular equation for this
reaction.
3.
Use the balanced equation and your mole amounts of aluminum and copper(II) sulfate, determine
your limiting reactant. Be sure to show sample calculations.
4.
Calculate the theoretical yield for your experiment. Calculate your percent yield. Be sure to show
sample calculations.
5.
Propose another experiment to investigate what factors affect the yield of the aluminum-copper(II)
reaction. Discuss in detail your reasoning for investigating the factor(s) and propose a hypothesis
based on your understanding of the system.
Teacher’s Notes
-
It is critical that the aluminum be the limiting reactant. Because aluminum foil thickness and mass
can vary, adjust the size of the foil as necessary for your class to have a mass of 0.25g or less for
the foil sample, or raise the concentration of the copper (II) sulfate solution to compensate
-
The reaction occurs at the surface of the foil. If you test your set-up and find the reaction runs too
slowly, have students tear their foil into strips, but ensure all foil makes it into the beaker.
-
The acid is used mainly to break down any coating or oxide that forms on the foil surface.
Crumbling then re-opening the foil several times before adding it to the beaker assists in this
process and, if done properly, can negate the need for the addition of acid. Without acid, though,
the reaction does run slightly more slowly.
-
When run carefully, students will typically have yields in excess of 95%. Some students may find
yields in excess of 100%. This is typically due to the formation of copper (II) oxide on the product
from reaction with air. This provides an excellent opportunity to discuss oxidation, and form a
relationship with what students already know. (Statue of Liberty, rusting metal, etc.) To avoid the
formation of oxides, use an incubator, oven, or desiccator to dry the filter papers in.