Chemical Reactions of Copper - Ms. Hoogenboom\'s …



Chemical Reactions of Copper

Introduction

Chemistry is the study of matter and the changes it undergoes. In this experiment, you will carry out a series of reactions involving the element copper. You will observe the four classes of chemical reactions--synthesis, decomposition, single and double replacement. You will be expected to write balanced equations for these various reactions. You will see evidence of chemical change and the conservation of matter. You will learn common lab practices and lab techniques. Finally, the concept of recycling is introduced.

Procedure

Day 1 Conversion 1 -- Changing Cu to Cu(NO3)2

Label a 125 mL flask with your name and your lab partner's name. Weigh and record its mass. Take approximately 1.5 grams of copper wire; lightly sand with steel wool, weigh it and record its mass. Loosely fold the wire so that it is flat and place the wire in the flask and carefully add 20 mL of 6.0 M nitric acid (HNO3). (Caution: The nitric acid is concentrated and dangerous to skin and clothing.) Stopper the flask and briefly observe the reaction that takes place. It may help to place the flask on a sheet of white paper to observe the color of new substances being produced. Unstopper the flask and set the flask in the fume hood.

The brown gas produced by the reaction in nitrogen dioxide, a common component of atmospheric pollution. The blue color of the solution is characteristic of many copper compounds dissolved in water. Expressed in words, the reaction is

copper + nitric acid --> copper (II) nitrate + nitrogen dioxide + water

Day 2 Conversion 2 -- Changing Cu(NO3)2 to Cu(OH)2

Put a laboratory apron on. Test your solution from Conversion 1 with pH paper, by dipping the end of a stirring rod into the solution and then touching a piece of pH paper. It should indicate that the solution is acidic due to a little excess HNO3. (pH paper should indicate a pH less

than 7.) Obtain 35 mL of 6.0 M sodium hydroxide. Test it in the same manner. Record both test paper results.

Read this entire paragraph before starting!

Add about 150 mL of ice water to a 400 mL beaker. (This is called a cold water bath.) Place the flask into the beaker so that the water comes up almost to the neck of the flask. (Why do you think this is necessary?) Add, slowly and cautiously, 5 mL portions of the 35 mL of sodium hydroxide to your copper (II) nitrate in the flask. Continuously mix the solution with a stirring rod. Proceed until all 35 mL of NaOH have been added. The sodium hydroxide, a base, neutralizes the excess nitric acid and then reacts with the copper ions in the solution. The neutralization reaction produces considerable heat, so stir well and add the sodium hydroxide solution slowly.

Test the resulting solution with the acid/base test paper and record the result. If the solution is still acidic, add more NaOH (as before) and test again. The solution must be basic. (pH paper should indicate a pH more than 7.)

Observe the solution. It should be royal blue (not green) and thick, the result of the production of copper (II) hydroxide, a solid. (A solid which results from a chemical reaction is called a precipitate.)

copper (II) nitrate + sodium hydroxide --> copper (II) hydroxide + sodium nitrate

Day 2 Conversion 3 -- Changing Cu(OH)2 to CuO

Add approximately 40 - 50 mL of distilled water to the flask containing the

copper (II) hydroxide precipitate. Place the flask higher than normal over your Bunsen burner and as an added precaution, clamp the flask to the ring stand so it cannot be knocked off the

ring clamp. Heat the precipitate gently and stir constantly, until all the material

is converted to a black substance, copper (II) oxide. Then turn off the Bunsen

burner. Continue stirring for 30 seconds after the heat is turned off.

Remove the stirring rod. Use your wash bottle to rinse off any black particles of CuO that are on the stirring rod back into the flask. Let the solution cool for three minutes. Then decant the extra liquid, using your stirring rod to direct the flow of liquid into the sink, as shown by your teacher. Don't lose any solid! You don't need to pour off every drop of liquid--just most of it. Now repeat this washing process with another 40 - 50 mL of deionized water. Stir, let the solid CuO settle and decant off the extra liquid. Repeat a third time. This process is called washing the precipitate.

copper (II) hydroxide --> copper (II) oxide + water

Day 3 Conversion 4 -- Changing CuO to CuCl2

Add 30 mL of 3.0 M hydrochloric acid to the flask containing the black copper (II) oxide. Swirl the flask gently, while leaving it flat on the counter. The oxide will soon dissolve, leaving a clear aqua-blue solution of copper (II) chloride. If the oxide has not all dissolved after 3 minutes, add more HCl, a few mL at a time, until it does.

copper (II) oxide + hydrochloric acid --> copper (II) chloride + water

Day 3 Conversion 5 -- Changing CuCl2 to Cu

Cut about 0.7 grams of aluminum wire. Bend it half, and in half again. Then bend it to form an "L" shape, so that the bottom of the "L" will just fit in the bottom the flask. Put the wire in the flask and watch what happens; the aluminum wire seems to "grow copper hair." Occasionally shake the wire to dislodge the copper. Since the reaction produces hydrogen gas, the absence of bubbles will indicate when the reaction has stopped. The solution will gradually turn grayish as gas is produced; later you will find that it has cleared when the reaction is complete. Let the flask stand overnight.

There are two reactions going on simultaneously. One is the reaction that results in the recovery of the metallic copper. The other is the reaction of the aluminum with a left-over hydrochloric acid; this produces the hydrogen gas.

(1) copper (II) chloride + aluminum --> aluminum chloride + copper

(2) hydrochloric acid + aluminum --> aluminum chloride + hydrogen gas

Day 4 -- Washing and drying the recovered copper

The next day you will find that the copper has settled. There should be no blue color at all left in the solution. Remove any remaining aluminum wire from the flask. Decant and discard the clear liquid. Wash the copper three times with 50 - 80 mL portions of warm distilled water, decanting the liquid each time. Don't lose any copper! After the final decanting, the flask should contain only pure, but wet, copper. Place it in an oven for drying overnight.

Day 5 -- Weighing the recovered copper

Weigh the flask containing the dry copper on the same balance you used on Day 1. Also make observations about your copper. Does it appear pure? Is there any discoloration? (Remember that compounds containing copper are usually blue or green in color.)

Scrape as much copper into a garbage can as possible. Wash the flask with soap and water. Return all equipment.

Analysis: Each question should be answered in complete sentences and typed.

1. Write balanced equations for each reaction that occurred. Include states of matter for each substance involved (Refer to your observations and lab procedure to help determine the states of matter.

a. Categorize each reaction as single replacement, double replacement, decomposition, synthesis or none of the above.

b. For each reaction, state one piece of specific evidence that a chemical change was taking place.

4. Answer the following questions for the reaction performed in conversion 1:

a. Where did the copper atoms from the wire go?

b. What elements were in the gas molecules released by the reaction?

c. Where did these atoms come from?

d. Where did the atoms in the water produced come from?

5. What is a precipitate? What is the purpose of washing a precipitate? What is the purpose of decanting?

6. Calculate the percent of copper recovered. Show all work.

7. Was your percent recovery over or under 100%? Why do you think this is? Explain two specific places in the lab where a mistake could have been made causing your percent to be above or below 100%. (This answer should pertain to YOUR percent, so it should be answered based on your answer to question #6.)

7. What was the purpose of the cold water bath in Conversion 2 on Day 2?

8. How is the concept of recycling related to this experiment?

9. Draw a flow chart that illustrates the transformations of copper throughout this lab.

10. Use the balanced chemical equation from Day 1 to answer the following question: How many grams of copper(II) nitrate did you produce from the amount of copper(grams) you started with on Day 1? Show ALL YOUR WORK!!!!

11. Use the first balanced chemical equation from Day 3 to answer the following question: How many grams of copper were produced from the amount of aluminum(grams) you started with on Day 3? Show ALL YOUR WORK!!!!

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