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CONSERVATION OF MATTER AND CHEMICAL
PROPERTIES
I.
OBJECTIVES AND BACKGROUND
The object of this experiment is to demonstrate the conservation of matter- or more
particularly, the conservation of "atoms" in chemical reactions and how this conservation is
represented using balanced chemical equations. Beginning with copper metal, you will
perform a series of chemical reactions; the last of which will result in the formation of
copper metal. The Law of Conservation of Matter suggests that the mass of copper you start
with will be the same as the copper you recover at the end--how close can you get to this?
II. CHEMICALS AND EQUIPMENT
Balance
Disposable plastic pipet
Glass stirring rod
Cu (copper) foil
Graduated cylinder
Mg (magnesium) ribbon
2 - 50 mL beakers
6M HNO3 solution, nitric acid
150 mL beaker
6 M NaOH solution, sodium hydroxide
Büchner funnel
6 M HCl solution, hydrochloric acid
Vacuum filter flask
Celite
Filter paper
acetone
Red litmus paper
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III. PROCEDURE
Preparation of Cu(NO3)2 from Cu metal:
copper (solid) + nitric acid (aqueous solution) → copper(II) nitrate (aqueous solution)
+ nitrogen dioxide (gas) + water (liquid)
1.
While one lab partner is obtaining the copper and performing the first reaction step,
the other partner can set up the Büchner funnel apparatus in Step 4. Place a 50 mL beaker
on a lab balance and press the tare button to zero the balance. Add copper foil to the beaker
until the mass reading is between 0.28 and 0.32g. (Caution: do not try to tear the copper
foil with your hands; you may end up cutting yourself. Use the scissors provided. Record the
mass to as many decimal places as the balance gives you. Place the beaker under a hood
and use a plastic pipet to slowly add 5mL of 6M HNO3 one mL at a time (CAUTION!!
NITRIC ACID IS HIGHLY CORROSIVE. AVOID ALLOWING IT TO COME IN CONTACT
WITH YOUR SKIN, EYES OR CLOTHING. IF THIS SHOULD HAPPEN, WASH THE
AFFECTED AREA WITH COLD WATER AND ASK YOUR TA FOR HELP). The brownish
red gas which is evolved is nitrogen dioxide and the bluish color in the solution is due to the
copper(II) nitrate produced. Keep the beaker in the hood until no more of the gas is visible.
2.
Take the beaker back to your bench area. Once all of the copper metal has dissolved,
very slowly and carefully dilute the solution by adding about 5 mL of distilled water.(The
addition of water to concentrated acid results in the generation of heat; if the water is
added too quickly, the solution will begin to boil and spatter acid.) The resulting light blue
solution contains the copper(II) nitrate product. We could recover this product as a blue
crystalline solid by evaporating off the water, if we wished, but we will leave it dissolved in
water for the next reaction.
Preparation of Cu(OH)2 from Cu(NO3)2:
copper(II) nitrate (aqueous solution) + sodium hydroxide (aqueous solution) →
copper(II) hydroxide (solid)+ sodium nitrate (aqueous solution)
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3. Obtain ~4mL (it doesn't have to be exact!) of 6M NaOH solution (CAUTION: THIS
SOLUTION IS CAUSTIC). Using a plastic pipet, add the NaOH solution to to the Cu(NO3)2
solution from Step 2 one mL at a time, stirring after each addition, until 3 mL have been
added. The light blue solid (called a precipitate) which forms is the copper(II) hydroxide.
Continue to add NaOH drop by drop until new formation of the blue solid can no longer be
observed, then begin testing the solution after each drop with red litmus paper to determine
whether there is any unreacted (excess) NaOH present. This can be done by touching a drop
of the solution to the litmus paper using the glass stirring rod. You can use each strip of
litmus paper several times. Stop adding NaOH as soon as you obtain a positive litmus test
(i.e., a blue spot). Do not add a large excess of NaOH.
4. When all of the copper(II) hydroxide is precipitated (as indicated by a positive litmus
test), we can separate it from the soluble sodium nitrate byproduct by filtration. Add one
scoopula of Celite to the beaker. Celite is an inert material which makes it easier to filter off
the copper hydroxide product without clogging the filter paper. Stir the contents of the
beaker thoroughly and before the slurry has a chance to resettle, filter it using a Büchner
funnel and vacuum filter flask (Your instructor will demonstrate the use of this equipment).
Rinse the beaker two or three times with ~2 mL aliquots of distilled water and pour each of
the washings into the funnel. This is done to remove the last traces of solid from the beaker.
Allow the vacuum to run until the precipitate has formed a solid matte in the funnel.
5. Turn off the vacuum and using a spatula, remove the filter paper full of precipitate. Place
the filter and precipitate in a clean 150 mL beaker, taking care not to lose any of the
precipitate. Scrape out as much of the solid remaining in the filter funnel as possible and
add it to the beaker. Now examine the solution in the filter flask. If it is nearly colorless, it
can be discarded. If it still has a distinct blue color, not all of the copper(II) nitrate has
reacted. If this is the case, pour the solution into a 250 mL beaker and add more NaOH
solution. If more precipitate forms, treat it according to steps 3 and 4 and add the additional
filter and precipitate to the first filter in the 150 mL beaker.
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Preparation of CuCl2 from Cu(OH)2:
copper(II) hydroxide (solid) + hydrochloric acid (aqueous solution) →
copper(II) chloride (aqueous solution) + water
6. Add about 2 mL of 6M HCl solution (CAUTION: THIS SOLUTION IS CORROSIVE) to
the beaker in step 5 containing the precipitate and filter paper and-stir the mixture until
the blue copper(II) hydroxide has dissolved and a pale green milky mixture remains. The
filter paper and inert Celite remain unaffected. Clean the Büchner funnel and filter flask by
rinsing them two or three times with distilled water. Place a clean piece of filter paper in
the funnel and filter the mixture in the beaker. Rinse the residue in the beaker two or three
times with 2 mL portions of distilled water, and add each washing to the Büchner funnel.
At this point, the filter paper and the Celite in the Büchner funnel should be colorless. The
Celite residue remaining in the Büchner funnel and the filter paper may be discarded. The
filtrate (the bluish green solution in the filter flask) contains the copper(II) chloride, which,
if we so wished, could be further isolated as a green solid by evaporating off the water. If
this liquid appears cloudy due to residual Celite, transfer it to a 50 mL beaker, clean out
the funnel and filter flask, and, using a fresh piece of filter paper, repeat the filtration.
Preparation of Cu metal from CuCl2:
copper(II) chloride (aqueous solution) + magnesium (solid) →
copper (solid)+magnesium dichloride (aqueous solution)
7. Pour the liquid in the filter flask into a clean 50 mL beaker and place the beaker under a
hood. Obtain ~0.2 g of magnesium. Add ~10 pieces of Mg metal to the solution, taking care
to prevent excess foaming. The foaming is due to a side reaction between the Mg metal and
the excess HCl which leads to the evolution of dihydrogen gas:
Mg(s) + 2 HCl(aq) → MgCl2(ag) + H2(g)
Use the stirring rod, from time to time, to break up any pieces of Mg metal that have
become coated with Cu. As soon as all of the Mg disappears, add more, and repeat this
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process until the blue-green color of the copper dichloride disappears. (Note: it may not be
necessary to add all of the magnesium metal to completely react with the copper(II)
chloride. If you add too much magnesium, it will be time-consuming to get rid of the excess
magnesium.) Then add 1 mL of 6M HCl (CAUTION: THIS SOLUTION IS HIGHLY
CORROSIVE) to destroy any excess Mg present according to the above equation. The
precipitated Cu metal is unaffected by the hydrochloric acid. Wait until the bubbling of
dihydrogen gas is completed and no more of the magnesium metal can be observed in the
flask.
8. Clean the Büchner funnel and filter flask, place a piece of filter paper in the funnel, and
filter off the precipitated Cu metal prepared in step 7. Turn the vacuum off and pour 2-3
mL of acetone onto the Cu (CAUTION!! ACETONE IS FLAMMABLE. THERE SHOULD
BE NO OPEN FLAMES PRESENT IN THE LABORATORY). Turn the vacuum back on
and, when all of the acetone has passed through the filter, add a second 2-3 mL of acetone,
this time keeping the vacuum running. When the filtration is complete, continue drawing
air through the filter paper for at least five minutes to dry the Cu precipitate. While your
copper is drying, measure the mass of a clean watch glass.
9. Carefully scrape the precipitate off of the filter paper and onto to the watch glass, and
measure the mass of the watch glass containing the Cu product. Obtain the mass of the Cu
produced as the difference between this mass and the mass of the watch glass alone.
IV. DATA AND CALCULATIONS
Fill out the Report Sheet on the next page.
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Report Sheet: Conservation of Matter
Name
Partner's Name
A. Original mass of Cu used in step 1
B. Mass of Cu and watch glass in step 9
C. Mass of watch glass in step 8
D. Mass of Cu recovered
E. Percent of Cu recovered
Date
Instructor's Initials
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V.
QUESTIONS
1. Based on the word description given at the beginning of each section, write a balanced
chemical equation for each of the transformations of Cu in this experiment. Be sure to
include the states of each of the reactants and products as well. Hint: for the first reaction,
use 2 for the coefficient for NO2(g)
2. Classify each of the reactions in question 1.
3. For each of the reactions in question 1, indicate what observations were used to
determine when the reactions were completed (e.g., appearance or disappearance of a color,
an external chemical test, etc.).
4. Comment on your percentage of recovered Cu. How would you account for the fact that
you did not get 100% recovery?