The Reaction of Copper Metal with Elemental Sulfur
Beginning Question
Does the empirical formula of the reaction product depend on the mass of copper metal
consumed during the reaction?
Safety Considerations
Stay safe by tying hair back and using crucible tongs to avoid touching hot porcelain or
ironware. Perform the experiment under the hood to avoid breathing fumes that may
form when elemental sulfur combines with oxygen in the air. Sulfur dioxide is an irritant
and may form an acid when it combines with the moisture in the nose and lungs (see
MSDS sheets for sulfur dioxide).
Procedure
1. Heat a crucible and its lid to constant mass. Find the mass of the lid separately in
case it falls on the floor and breaks.
2. Measure a piece of copper wire to a length of 15-25 cm.
3. Remove the surface coating on the copper wire; clean the wire by rubbing it with
steel wool.
4. Coil the copper wire around a pencil and compress the coils but do not allow the coils
to touch each other. Place the coil in the empty crucible and find the total mass.
5. Completely cover the copper coil with powdered sulfur.
6. With the lid completely on the crucible, heat until visible signs of reaction stop.
7. Repeat the process of adding more sulfur to cover the coil and heat again to constant
mass.
8. Find the mass of the product.
Data, Observations, Calculations, and Graphs
Observations
1. The copper metal sample is shiny and flexible, and copper in color.
2. The sulfur powder is yellow and has the consistency of flour or corn starch.
3. When the crucible gets hot, purple flames and a white vapor escape. The vapor is
irritating to the nose and throat.
4. Some sulfur melts and drips out.
5. The product is a thick, gray-black coil (thicker than the copper coil). It is brittle.
When broken apart, it is clear that there is no more pure copper metal – it has all been
converted to the black product.
Data (K & N)
Mass of crucible and lid
Mass of lid alone
Mass of crucible, lid, and copper, Cu
Mass of crucible, lid, and product after reaction
Mass of copper metal before reaction
Mass of black product after reaction
Mass of sulfur converted to black product
46.83 g
9.28 g
47.76 g
47.99 g
0.93 g
1.20 g
0.27 g
Calculations
Mass of copper metal before heating = 46.83 g - 27.76 g = 0.93 g Cu
Mass of black product after reaction = 47.99 g - 46.83 g = 1.16 g product
Mass of sulfur converted to black product = 1.16 g - 0.93 g = 0.23 g
Empirical formula of the black powder:
0.93 g Cu/63.55 g/mol Cu = 0.01463 mol Cu
0.23 g S/32.07 g/mol S = 0.007172 mol S
mole ratio of Cu:S = 0.01463/0.007172 = 2.04 ~ 2 mol Cu:1 mol S
Thus, the empirical formula for the black powder (a sulfide of copper) is is Cu2S.
Class Data
Team
Mass Cu (g)
Moles Cu
Mass S (g)
Moles S
Mole Ratio:
Cu:S
M&C
0.44
0.006924
0.13
0.004054
1.7
M&J
0.47
0.007396
0.13
0.004054
1.8
K&N
0.93
0.01463
0.23
0.007172
2.0
T&G
0.90
0.01416
0.25
0.007795
1.8
*M & R
0.65
0.01023
0.25
0.007795
1.3
M&K
0.51
0.008025
0.16
0.004989
1.6
*J & J
0.82
0.01290
0.32
0.009978
1.3
D&M
0.60
0.009441
0.19
0.005925
1.6
*anomalous data
The class average mole ratio (Cu:S) = 1.8
Claim
The empirical formula of the product, Cu2S, does not depend on the mass of copper
consumed during the reaction.
Evidence
Experimentally, we found that 0.93 g Cu (0.01463 mol Cu) combined with 0.23 g S
(0.007172 mol S) in a ratio of 2.04 moles Cu to 1.00 mol S, a 2:1 ratio. This is supported
by the overall class average of 1.8 moles Cu to 1.00 mole S, also a 2:1 ratio. Based on
the varying amounts of copper consumed, as reported by various teams, the empirical
formula for the product does not depend on the mass of copper consumed. Note that,
despite the variation in the mass of Cu consumed, from 0.44 g (M & C) to 0.93 g (K &
N), the empirical formula came out the same, about 2:1.
Reading, Reflections, and Post-lab Questions
Post-lab Questions
Two samples containing mercury and oxygen appear to be the same compound. When
each is heated, the samples decompose, with oxygen gas escaping into the atmosphere. A
residue of pure liquid mercury metal is left behind in each case. When 1.2996 grams of
sample A is heated, 1.2036 g Hg remains. When a 0.8244-g sample of sample B is
heated, 0.0617 g of oxygen is released. (a) What are the empirical formulas of samples A
and B? (b) What is the percent composition of samples A and B? (c) Are they the same
compound?
Sample A:
1.2036 g Hg/200.6 g/mol Hg = 0.006000 mol Hg
0.096 g O/16.0 g/mol O = 0.0060 mol O
empirical formula:
0.0060000 mol Hg/0.0060 mol O = 1.0;
The empirical formula is HgO.
percent composition: 1.2036 g Hg/1.2996 g total = 92.61 % Hg by mass
0.0966 g O/1.2996 g total = 7.43 % O by mass
Sample B:
0.7627 g Hg/200.6 g/mol Hg = 0.003802 mol Hg
0.0617 g O/16.0 g/mol O = 0.00386 mol O
empirical formula:
0.003802 mol Hg/0.00386 mol O = 0.985;
The empirical formula is HgO.
percent composition: 0.7727 g Hg/0.8344 g total = 92.61 % Hg by mass
0.0617 g O/0.8344 g total = 7.40 % O by mass
Thus, samples A and B are the same compound!
Reading and Reflection
The empirical formula, Cu2S, does not depend on the mass of copper metal used.
Although each group used a different length of copper wire (to “cover” the range from
15-25 cm of wire), it was determined that the mole ratio of Cu to S remained relatively
constant: 1.8 mol Cu:1.00 mol S. There were two anomalous data (*); the appearance of
the final product for those two samples was “rusty.” It could be possible that some type
of side reaction occurred creating a product that was not copper (I) sulfide. We learned
from reading about copper-sulfur compounds that copper (II) sulfide melts at a
temperature of 103 C (p. 4-50 of the CRC handbook of Chemistry and Physics, 74th
Edition), so our product could not have been copper (II) sulfide, CuS. No matter how
much sulfur was used, copper was the limiting reactant (text, p. 110, course lecture notes,
9-27-06). The copper combined with sulfur following the law of constant composition
(text, p. 44, course lecture notes, 9-13-06): in a ratio of two moles of copper to one mole
of sulfur, according to the equation
2 Cu + S = Cu2S
The empirical formula, Cu2S, has the smallest whole number ratio of moles Cu to moles
S (text, p. 64, course lecture notes, 10-06-06). Copper is in the “+1” oxidation state and
sulfur is in the “-2” oxidation state (text, p. 148, course lecture notes, 9-20-06) in this
compound. This reaction is both a combination reaction (two elements combine to form
a new compound) (text, p.146, course lecture notes, 9-22-06) and also an oxidationreduction reaction (copper metal is oxidized to the copper (I) ion (2 Cu = 2 Cu+1 + 2 e-)
and elemental sulfur is reduced to the sulfide ion (S + 2 e- = S-2) (text, p.148, course
lecture notes, 9-22-06).
References
CRC handbook of Chemistry and Physics, 74th Edition, Lide, D.R. and Frederikse, P.P.R.,
Eds., Boca Raton, FL: CRC Press, Inc., 1993
MSDS sulfur dioxide, URL: http://avogadro.chem.iastate.edu/MSDS/MSDS_S.html,
accessed 10-06.
Silberberg, M. S. 2003. Chemistry: The Molecular Nature of Matter and Change, 3rd ed.
Dubuque, IA: McGraw Hill.
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