17 Copper-Silver Nitrate Reaction

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            Copper-Silver Nitrate Reaction
               In this experiment you will weigh a sample of solid silver nitrate and prepare
           a water solution of it. You will also weigh a piece of coiled copper wire, place it
            in the silver nitrate solution, and observe its behavior. By weighing the copper
           wire at the close of the experiment you will be able to determine the amount of
           copper reacted. Using these and other measurements you will be able to deter-
           mine a quantitative relation between reactants and products.
               In keeping with chemical practice, we will refer to the chemical substances by
           using appropriate symbols. Copper is an element; it contains only one kind of
           atom. The symbol for copper is Cu. Silver nitrate is a compound and has the
           formula AgNO 3 . The symbol Ag is for the silver part and the symbols NO 3 for
           the nitrate part. The group of atoms NO 3 , consisting of one nitrogen atom and
           three oxygen atoms, is often found in chemical compounds and has the name
               Before coming to the laboratory, prepare a table in your notebook so that you
           can record the data you will observe. Use the entries suggested at the end of
           the procedure section.

a.    Obtain a 30 cm length of bare copper wire,             It will interact with them.) Fasten the tube in
     a clean dry 100 ml beaker, an 18 x 150 mm               a clamp on a ring stand as shown in
     test tube, and a vial containing about 3 grams          Figure 7-1. Stir the solution with a stirring
     of silver nitrate crystals.                             rod to dissolve the crystals and mix the
b.   Weigh the clean dry beaker, the wire, and               solution. Drain the stirring rod by touching
     the vial containing silver nitrate. Determine           the end of the rod to the inside of the test
     all masses to the nearest 0.01 gram.                    tube near the mouth. Rinse the stirring rod
                                                             with a small amount of distilled water into
c.   Measure about 25 ml of distilled water into
                                                             the test tube.
     the test tube. Add the solid silver nitrate
     from the vial to the water in the test tube.       d.   Coil the copper wire by wrapping it around
     Do not rinse or wash the vial. Save it to               a glass stirring rod.     See Figure 7-2A.
     be weighed as directed in step f. (Caution !            Stretch the coiled wire as shown in Figure
     Be careful not to get any of the silver nitrate,        7-2B until it is about 2 cm longer than the
     solution or solid, on your skin or clothing.            test tube. Leave the wire tightly coiled near
18                                                                                                 EXPERIMENT 7

Figure 7-1                                                       Figure 7-3

Test tube containing silver nitrate solution clamped in place.   Coiled copper wire placed in silver nitrate solution.

      one end and stretched out and straightened                 e.   Place the coiled wire into the silver nitrate
      toward the other. Weigh the wire to ±0.01                       solution with the closely coiled end at the
      gram.                                                           bottom of the test tube. Place a one hole
Figure 7-2A                   Figure 7-2B                             rubber stopper in the test tube with the end
                                                                      of the wire extending through the hole. See
Coiled copper wire.           Coiled copper wire, stretched.
                                                                      Figure 7-3. Be careful not to shake or in any
                                                                      way disturb the contents of the test tube.
                                                                      Record observations in your laboratory note-
                                                                      book. Allow the reaction to continue for
                                                                      30 minutes.
                                                                 f.   While the reaction is proceeding, weigh the
                                                                      empty vial with cap in order to determine the
                                                                      mass of silver nitrate used in the experiment.
                                                                 g.   Shake the crystals from the copper wire.
                                                                      Allow the wire to remain in the solution for
                                                                      a few seconds to see if the reaction is com-
                                                                      plete. Using a wash bottle, rinse the wire
     EXPERIMENT 7                                                                                            19

          into the weighed 100 ml beaker.             See
          Figure 7-4. Empty the contents of the test
          tube into the 100 ml beaker. Add a few ml
          of water to transfer all the crystals from the
          test tube. Dip the copper wire in acetone,
          and then set it aside to dry. Weigh the copper
          wire when it is dry.
    h.    Let the crystals of silver settle in the beaker
          and carefully decant the solution. Decant
          means to pour off the liquid, leaving solid be-
          hind. Pour the liquid along a glass stirring
          rod as shown in Figure 7-5. Wash the residue
          three or four times using about 10 ml of dis-
          tilled water each time. Carefully decant after
          each washing. A few very tiny particles of
          silver may float off as you decant. They have
          a very small mass and may be neglected.
          After the final washing, the sample must be
          dried. Your teacher will suggest a suitable
          method. Be sure to identify your beaker           Figure 7-5
          uniquely.                                         Decanting the liquid from a solid.
    j.    After the residue is dry, allow the beaker to
          cool to room temperature. Weigh the beaker
          containing the product. Make all weighings
                                                                 on the same balance. To make sure the
                                                                 sample is dry, you should subject it to further
                                                                 drying and weigh it again.
                                                            k.   Cover the beaker with a watch glass and
                                                                 save the dry silver crystals. They will be used
                                                                 later in Experiment 8.

                                                                      Your Data Table Should Include the
                                                                 mass of clean dry 100 ml beaker
                                                                 mass of vial, cap, and silver nitrate
                                                                 mass of empty vial with cap
                                                                 mass of the coiled copper wire before
                                                                 mass of the coiled copper wire after reaction
                                                                 mass of the beaker plus silver crystals

                                                            PROCESSING THE DATA
    Figure 7-4                                              1)   Determine the change in mass of the
    Rinsing the copper wire using a wash bottle.                 copper wire during the experiment.
20                                                                              EXPERIMENT 7.

2)   Determine the number of moles of copper          8)    Using the results of your experiment, write
     which reacted. Recall that the mass of a              whole number coefficients in the following
     substance divided by the mass per mole                statement :
     equals the number of moles.                           One mole of Cu plus              mole(s) of
3)   Determine the mass of silver obtained.                AgNO3 in solution produces          mole(s)
                                                           Ag plus one mole of Cu(NO 3 ) 2 in solution.
4)   Determine the number of moles of silver.
                                                      9)   How many atoms of copper were removed
5)   Divide the moles of silver by moles of
     copper to determine the ratio Ag/Cu. Be               from the wire used in your experiment?
     sure to express your answer as a decimal        10)   How many atoms of silver metal were
     and to the correct number of significant              formed in your experiment?
                                                           A QUESTION TO WONDER ABOUT
6)   Determine the moles of AgNO 3 used in the
                                                     What caused the color in the solution to appear
                                                     as the reaction proceeded?
7)   Divide the moles of silver by moles of silver
     nitrate to determine the ratio Ag/AgNO 3 .
     Express your answer as a decimal using the
     correct number of significant figures.