5. SEQUENCE OF REACTIONS

The procedure for the synthesis of a substance may occasionally be relatively complex, involving a sequence of several chemical reactions. In this exercise copper metal, the starting material, is carried through a number of sequential reactions, eventually leading to a final product. Throughout the process, the student will make observations about the nature and appearance of the products formed. Since matter is neither created nor destroyed in a chemical reaction, the experimenter will be able to make quantitative judgments from the mass of final product. The reaction sequence utilizes a variety of types of reactions. The following introductory material is organized in the order that each reaction is performed, with that type of reaction highlighted.

Oxidation. Metallic copper may be oxidized by nitric acid. In this reaction, the insoluble metal dissolves as the copper +2 cation. The nitrate anion, NO3-, is reduced to nitrite anion, NO2-. The copper(II)nitrate is a strong electrolyte and exists as independent ions in the water solution. The hydrated copper ion in acidic solution has a light blue color.

Cu(s) + 4 HNO3(aq) Cu(NO3)2(aq) + 2 NO2(g) + 2 H2O(l)

Cu(s) + 4 H+(aq) + 2 NO3- (aq) Cu+2 (aq) + 2 NO2(g) + 2H2O(l)

Precipitation. The addition of a strong base such as sodium hydroxide results in the precipitation of insoluble copper hydroxide, Cu(OH)2. Copper(II)hydroxide is so extremely insoluble that a very fine gelatinous mass is formed which is extremely difficult to filter.

Cu(NO3)2(aq) + 2 NaOH(aq) Cu(OH)2 (s) + 2 NaNO3(aq)

Cu+2 (aq) + 2 OH-(aq) Cu(OH)2 (s)

Dehydration. The light blue gelatinous copper(II)hydroxide may be converted to black copper(II)oxide simply by heating the solution.

Cu(OH)2(s) CuO(s) + H2O(l)

The solid copper(II)oxide may then be separated from the solution by filtration.

Acid Base Reaction. The insoluble, and basic, copper(II)oxide will react with dilute sulfuric acid to form the very soluble copper(II)sulfate. Copper(II) sulfate is a strong electrolyte and exists as independent ions in solution.

CuO(s) + H2SO4(aq) CuSO4(aq) + H2O(l)

CuO(s) + 2 H+(aq) Cu+2 (aq) + H2O(l)

Reduction. In the final stage you will be directed by your laboratory instructor to use one of two possible options. In the first option, the copper(II) is reduced by iodide ion to copper(I) and the iodide ion is oxidized to iodine, I2. The copper(I) cation then reacts with excess iodide ions to form the insoluble white copper(I) iodide which may then be collected and weighed. The iodine, I2, that was formed is also insoluble and must be removed to avoid an incorrect mass of copper (I) iodide. This may be accomplished by adding a second reagent, sodium bisulfite, NaHSO3, that will solubilize the iodine so that it does not interfere.

2 CuSO4 + 4 NaI 2 CuI + I2 + 2 Na2SO4

2 Cu+2(aq) + 4 I-(aq) 2 CuI(s) + I2(s)

H2O(l) + I2(s) + NaHSO3(aq) H2SO4(aq) + HI(aq)

The second option involves reacting copper(II) ions in solution with solid zinc metal to produce metallic copper, which may then be separated from the solution. The unreacted zinc metal must be removed by reaction with hydrochloric acid.

CuSO4(aq) + Zn(s) Cu(s) + ZnSO4(aq)

Cu+2(aq) + Zn(s) Cu(s) + Zn+2(aq)

You will be directed to make a quantitative evaluation of this series of reactions from the original mass of copper metal and the final mass of either copper iodide or copper metal.

PROCEDURE

  1. Obtain a length of copper wire that weighs between 0.8 and 1.0 g and weigh it to the nearest milligram.
  2. Coil the wire into a loose spiral and place it in a 150-mL beaker. Add 10 mL of concentrated nitric acid, cover the beaker with a watch glass, and place it in the hood.
  3. The copper wire should dissolve completely in five to ten minutes. If it does not, and if the reaction appears to have stopped, add an additional 1 to 2 mL of HNO3, cover with a watch glass, place the beaker on a hot plate and warm it.
  4. After dissolution is complete add 25 mL of water and then slowly add, while stirring, 22 mL of 20% sodium hydroxide. The mixture should be basic at this point. Check the solution with pH paper to assure this fact.
  5. Heat the mixture, stirring to prevent bumping and spattering, until the boiling point is reached. Continue to heat for a few minutes to better coagulate the copper (II) oxide and to insure that all the blue copper (II) hydroxide has decomposed.
  6. Assemble the Buchner funnel and vacuum flask assembly as was done in the second experiment. See Figure 6. Filter with suction and wash the precipitate with three 10-mL portions of water. Use the portions of water to rinse the solid copper(II)oxide out of the beaker and onto the filter paper. Discard the filtrate.
  7. Transfer the copper(II)oxide to a 150-mL beaker using a small spatula or stirring rod. There will be a small amount of solid which remains on the filter paper.
  8. Remove the remainder by pouring back and forth, over the precipitate and through the filter paper, a dilute sulfuric acid solution. Add this solution to the beaker containing the copper(II)oxide. Finally rinse the filter paper, funnel and beaker with 10 mL of water and add this to the sample solution.
  9. If the copper(II)oxide is not completely dissolved at this point, heat the solution on a hot plate for a few minutes. In some cases a small amount of additional dilute sulfuric acid may be necessary.
  10. OPTION ONE: Add to the blue copper(II)sulfate solution, with stirring, 50 mL of the sodium iodide solution. Then slowly add, with stirring, the sodium bisulfite solution until all the I2 has been reduced and the precipitate is white. Heat the mixture to boiling for one minute. Suction filter the precipitate through very fine filter paper. It may be necessary to cycle the solution through the paper to insure all of the solid has been retained. Dry the filter paper and weigh. Calculate the mass of copper in the copper iodide recovered and compare this with the mass of original copper metal.
  11. OPTION TWO: Add, in small portions, with stirring, up to two grams of zinc metal until all of the blue color disappears. Add 10 mL of the hydrochloric acid solution to dissolve the excess zinc metal. The solution may be warmed slightly to speed up the reaction. When the zinc is completely dissolved, allow the finely divided copper to settle, and decant the solution. Wash the solid with two separate 25- mL portions of water, decanting carefully after each washing. Accurately weigh a clean, dry evaporating dish. Transfer all of the copper to the dish, using as little water as possible. Warm the dish until nearly all of the water has evaporated. Remove the dish from the heat when a few milliliters of water remain, and allow the heat in the dish to evaporate the last of the water. Cool and weigh the dish. Calculate the mass of copper recovered and compare this with the mass of original copper metal.