Question

In this laboratory exploration various reactions of copper will illustrate these properties. First, begin with copper metal. When concentrated nitric acid is added, dramatic changes occur and the metal dissolves forming a brightly colored copper ion-containing solution.

Cu_(s) + 4HNO_3(aq) à Cu(NO₃)_2(aq) + 2NO_2(g) + 2H₂O_(l)                                       _____________ equation

Cu_(s) + 4H⁺ + 4NO₃^-_(aq) à Cu⁺²_(aq) +2NO₃^-_(aq)+ 2NO_2(g) + 2H₂O_(l)                  _____________ equation

Reaction type_______________

What advantage(s) is/are there to the second version of the reaction equation?

However, if diluted nitric acid was used instead, the following reaction would have occurred:

3Cu_(s) + 8HNO_3(aq) à 3Cu(NO₃)_2(aq) + 2NO_(g) + 4H₂O_(l)                                                        _____________ equation

3Cu_(s) + 8H⁺ + 8NO₃^-_(aq) à3Cu⁺²_(aq) + 6NO₃^-_(aq) + 2NO_(g) + 4H₂O_(l)                 _____________ equation

Reaction type_______________

Why is the more caustic concentrated nitric acid used if both the concentrated and dilute acid solutions both produce the copper (II) ion, our desired result?

What are disadvantages of this method?    Safety concerns?

At this point, water is added to transform all the copper (II) ion into the water complex, forming a complex ion. This color change signals a Lewis acid/base reaction in which the water is the Lewis base and the metal ion is the Lewis acid. It also serves to dilute the acid, useful in the precipitation step to keep reaction temperatures down, and serves to disperse the gas formed.

Cu(NO₃)_2(aq) + 6H₂O_(l) à Cu(H₂O)₆⁺²_(aq)   + 2NO₃^-_(aq)                                             _____________ equation

Reaction type_______________

The next step in the copper atoms journey is the formation of copper (II) hydroxide when sodium hydroxide solution is added to the reaction mixture. This process must be done carefully, slowly and with a cool water bath, to prevent skipping this step and creating the copper (II) oxide product directly, the next step in the process, as well as redissolving the copper (II) hydroxide as a complex.

Cu(H₂O)₆⁺²_(aq) + 2NaOH_(aq)    à Cu(OH)_{2 (s)}   + 6H₂O_(l) + 2Na⁺_(aq)                       _____________ equation

Reaction type_______________

Adding excess hydroxide should be avoided as it results in redissolving the precipitate and forming a copper complex. Therefore the sodium hydroxide is added 1 drop at a time, mixed with the vortexer, briefly cooled in a cool water bath, and the reaction progress noted after each drop. Once the precipitate remains after mixing , 2 additional drops are added to ensure complete precipitation.

Why is complete precipitation necessary?

Once the copper (II) hydroxide precipitation reaction is completed , the reaction mixture is carefully centrifuged and the supernatant analyzed for complete precipitation.

How can it be determined that the reaction is complete?

Why was it necessary to add the sodium hydroxide dropwise and utilize a cool water bath?

What safety precautions are necessary for centrifuge use?

If complete precipitation has occurred, the supernatant is decanted and the remaining solid carefully heated (D) in a moderate Bunsen burner flame to transform it to the copper (II) oxide.

Cu(OH)_{2 (s)} + D à CuO_(s)   + H₂O_(l)  

What safety precautions are necessary for Bunsen burners?

When heating mixtures?

At this point, the CuO solid will be dissolved in preparation for the final recovery of copper metal. The CuO will be dissolved using the careful dropwise addition of acid (in this case 6M sulfuric acid) followed by vortexing (similarly to the copper (II) hydroxide precipitation earlier). All following processes may be done in either the hood or at the benches. In this step, avoiding addition of excess sulfate ion (a problem in the final step) is our goal. Although temperature control is not an issue during the dissolving process, a cool water bath will be necessary during the final reaction.             

CuO_(s) + H₂SO_4(aq) à CuSO_4(aq) + H₂O_(l)                                                   _____________ equation          

CuO_(s) + 2H⁺ + SO₄^-2_(aq) à Cu⁺²_(aq) + SO₄^-2_(aq) + H₂O_(l)                         _____________ equation

CuO_(s) + 2H⁺_(aq)   à Cu⁺²_(aq) + H₂O_(l)                                                             _____________ equation

Reaction type_______________

What advantage(s) is/are there to the various versions of the reaction equation?

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In the final step, the copper metal is regenerated by reacting the copper (II) ion and magnesium metal.

Cu SO_4(aq) + Mg_(s) à Cu_(s)+ MgSO_4(aq)                                                                                                         _____________ equation

Cu⁺²_(aq) + SO₄^-2_(aq) + Mg_(s) à Cu_(s)+ SO₄^-2_(aq) + Mg_(aq)⁺²                    _____________ equation

Cu⁺²_(aq) + Mg_(s) à Cu_(s)+ Mg⁺²_{(aq) )}                                                             _____________ equation

Reaction type_______________

Using the species in the first equation, can any other products be predicted to form during the isolation of copper solid from solution?

What advantage(s) is/are there to the various versions of the reaction equation?

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How can it be determined that the reaction is complete?

Once the reaction is complete, the centrifuge will be used to separate the copper metal formed from the reaction mixture. Once separated, the solid will be rinsed 3 times with deionized water , using the centrifuge to separate the solid from the rinse water after each rinse.

What is the purpose of the rinses?

There are multiple cautionary notes for this reaction. If excess sulfate is present in solution due to excess sulfuric acid being used to dissolve the copper (II) oxide, an additional solid will form, complicating the rinsing process and affecting yield.

What is the equation for this side reaction?

How will the yield be affected if the rinses are incomplete?

If the reaction mixture is allowed to get too warm another side reaction will occur, what is it?

If the cool water bath was not used, what would be the effect upon the copper metal yield? Why?

If magnesium is added once the copper ion reaction is complete, a further reaction of the metal will occur.

What is it?

After the final rinse, the copper metal is in the test tube.

How (experimentally) will the copper metal recovered % yield be determined?