Many compounds that do not occur
naturally or are difficult
to obtain may be synthesized in the laboratory. A particular
synthesis may proceed through several individual steps that involve the
formation of many
intermediate chemical compounds on the way to the final product.
Some of these steps do not produce 100% yields and so you obtain less
product than you would have expected. Chemists often devote a
great deal of time and effort investigating the reaction conditions to
improve the final yield. However, there are even more subtle
problems: one's laboratory skills. Even if the reaction can
to provide a 100% yield, if you drop part of it on the way to the
balance, or do not fully dry it, or leave some of it on a piece of
filter paper, you may significantly reduce your final yield.
Today's synthesis begins with an oxidation-reduction reaction where Cu (s) dissolves with nitric acid to form Cu2+(aq):
3 Cu(s) + 8 HNO3(aq) 3 Cu(NO3)2(aq) + 2 NO(g) + 4 H2O(l)
3 Cu(s) + 8 H+(aq) + 8 NO3-(aq) 3 Cu 2+(aq) + 6 NO3-(aq) + 2 NO(g) + 4 H2O(l)
3 Cu(s) + 8 H+(aq) + 2 NO3-(aq) 3 Cu 2+(aq) + 2 NO(g) + 4 H2O(l)
The three equations above, represent the
balanced chemical equation or formula
the total ionic equation and net ionic equation for the oxidation of Cu(s) to Cu2+(aq)
by the NO3-(aq) ion.
solution goes through several color changes that represent different
states of copper. Finally, a blue aqueous solution of Cu2+
ions is produced. At the same time the reduction of NO3-(aq)
ion produced the colorless gas nitrogen monoxide, NO. However, NO
and is converted to the orange-brown gas nitrogen dioxide, NO2
as soon as it comes in
with the O2 in the air. Nitrogen dioxide is the same
gas you see over highly polluted urban areas.
The next step in the synthesis involves an acid-base reaction of Cu 2+(aq) to form Cu(OH)2 (s):
NaOH(aq) + HNO3(aq) NaNO3(aq) + H2O(l)
Na+(aq) + OH -(aq) + H+(aq) + NO-3(aq) Na +(aq) + NO3-(aq) + H2O(l)
OH-(aq) + H +(aq) H2O(l)
In this series of reactions, a solution of sodium hydroxide neutralizes the acidic solution. After neutralization of H+(aq) ions by the base, excess sodium hydroxide increases the OH-(aq) concentration. The litmus paper tests for sufficient base. The excess OH-(aq) ions then react with the Cu2+(aq) ions to form the insoluble chalky, light blue hydroxide, copper (II) hydroxide, Cu(OH)2(s):
Cu(NO3)2(aq) + 2 NaOH(aq) Cu(OH)2(aq) + 2 NaNO3(aq)
Cu2+(aq) + 2 NO3-(aq) + 2 Na +(aq) + 2 OH -(aq) Cu(OH)2(aq) + 2 Na+(aq) + 2 NO3-(aq)
Cu2+(aq) + 2 OH-(aq) Cu(OH)2(s)
The formation of solid Cu(OH)2(s) is a precipitation (metathesis) reaction.
The next step in the synthesis involves a decomposition reaction in which heat is used to decompose the the light blue copper (II) hydroxide into the black copper (II) oxide, CuO(s) :
Cu(OH)2(s) CuO(s) + H2O(g)
The CuO(s) solid is then dissolved using sulfuric acid to form the hydrated copper (II) ion, Cu(H2O)42+(aq):
CuO(s) + H2SO4(aq) + 3 H2O(l) Cu(H2O)42+(aq) + SO42-(aq)
In this final step of the synthesis, the
hydrated copper (II) ion,
is reacted with zinc. Since zinc is higher on the
activity series than copper, the zinc will dissolve and displace
the copper as a precipitate:
Cu(H2O)42+(aq) + Zn(s) Cu(s) + Zn2+(aq) + 4 H2O(l)
Dissolve the solid Copper:
Accurately weigh a clean, dry 100 mL beaker to 0.001g. Add a
spiral of copper wire to the beaker and determine it mass
difference. Dissolve the copper
by adding 5 mL of concentrated nitric acid, HNO3. This
be carried out UNDER YOUR SNORKEL HOOD.
The reaction, which should take about 10-15 minutes, converts copper
to the water-soluble ionic compound copper (II) nitrate, Cu(NO3)2.
It is very important to make sure all of the copper has
dissolved. You may not see any copper, but if there is still any
orange-brown gas coming off, the reaction is not complete.
Precipitate of Cu(OH)2:
Add 15 mL of 6N sodium hydroxide, NaOH, to the copper (II) nitrate solution. Stir with a glass stirring rod and check with litmus paper to be sure that the solution is basic. Use your glass stirring rod to place a drop of the solution onto a piece of red litmus paper. If it is not basic, add 5 mL more of sodium hydroxide. This particular compound, copper (II) hydroxide, Cu(OH)2, which falls to the bottom (precipitates) is not very stable and may be readily converted to a more stable compound, copper (II) oxide, CuO, by heating.
Conversion of Cu(OH)2 to CuO:
Convert Cu(OH)2 to CuO by heating on a hot plate with plenty of STIRRING. The thick mass of CuO will spatter VERY READILY if overheated without adequate stirring. WATCH OUT. DON'T STOP STIRRING UNTIL AFTER HEAT HAS BEEN REMOVED. To obtain your best final yield, it is critical that any blue or green color be convert to the black CuO! When all of the blue Cu(OH)2 has been converted to black CuO, quantitatively transfer the CuO to a 50 mL centrifuge tube. You will have to use a minimum amount of water because the centrifuge tube only holds 50 mL. Tightly screw on the top to the centrifuge tube and place it in the centrifuge, but make sure there is another tube opposite yours that has the same amount of liquid (distilled water will do). Close and lock the centrifuge and run it for 1-2 minutes. Wait for it to completely stop before opening and retrieving your sample. You will notice that all of the black CuO is now at the bottom of the centrifuge tube. Open the tube and carefully pour out the clear liquid, making sure that none of the black CuO escapes. You do not need to get rid of every single drop, but remove as much as you can. Make sure you review how to use a centrifuge BEFORE coming to lab!
Conversion of CuO to Cu(H2O)4 2+:
Now add 20 mL of 3 M H2SO4
to the centrifuge tube and put the cap on tightly. Shake the tube
and you will see a series of color changes as the black CuO is
dissolves. Make sure there is no more black CuO before proceeding
to the next step.
Conversion of Cu(H2O)4 2+ back to Cu:
At this point you have a solution of Cu2+
ions in your centrifuge tube. Quantitatively transfer this solution
to a 150 mL beaker using distilled water; you want to have a final volume of about 75-100 mL. Add a piece
of solid zinc to this solution. You should
notice an immediate reaction as the zinc displaces the copper from
solution. Just as you saw in the Empirical Formula lab, the
copper will coat the zinc. So you will have to physically remove
by shaking the zinc plate periodically. After the copper has been
completely removed from solution (solution is colorless), there still
may be a small amount of copper clinging to the zinc. Use your
scoopula to gentle scrape any remaining copper from the zinc.
After you are done with the zinc, clean it with water, dry it with
paper towels and return to your instructor.
You will isolate the copper solid by suction filtration. It is important to weigh your filter paper before you begin this filtration process. Set your filter paper with water and quantitatively transfer the copper to the filter. Wash the copper with several 5 mL portions of distilled water to remove any water soluble contaminants. Continue the suction for an additional 5 minutes after all of the liquid has been removed. While the copper is air drying, weigh your watch glass (be sure to place some sort of identifying mark on it first, your grease pencil works well for this). Transfer all of the copper and the filter paper to the watch glass. Try to get the filter paper to transfer in a single piece. Now place your watch glass in the oven (~100°C) for about 10 minutes. Allow the watch glass to cool to room temperature and weigh. Continue this heating, cooling, weighing cycle until you obtain two consecutive weights that agree to within 0.01 g. This process is known as weighing to constant dryness, and assures us that all of the water has been removed from the copper. You can now determine the amount of copper obtained by difference.
The theoretical yield of product is the amount expected on the basis of starting amounts with no consideration for loss along the way. Assuming the same number of moles of product as of starting material, calculate the mass of copper expected (the "theoretical yield"). In this case, you converted elemental copper through a series of reactions back into elemental copper. So, your theoretical yield should be the amount of copper that you started with. Convert the actual yield to a percent of the theoretical value as follows:
(Updated 11/15/13 by C.R. Snelling)