Reactivity: The Copper Cycle

1 lab period; work in pairs. Complete the Preparation page before laboratory.
Goals

To explore the chemistry of copper
To enhance your skills of observation
To learn techniques of synthesis and separation

Background

The passion of the chemist is to make new molecules and explore their physical properties and chemical reactivity. Choosing and carrying out appropriate chemical reactions to accomplish this is a combination of skill and artistry, which must be learned and practiced. This experiment provides an opportunity for you to start with a pure chemical substance and subject it to a series of chemical reactions. Based on what you observe, you will fill in as much information about the nature and products of these reactions as you can.

Several important types of reactions are involved in this experiment. These are called oxidation-reduction (redox), acid-base, and precipitation reactions. Redox reactions involve the transfer of electrons from one chemical substance to another. As a result of the process, one element loses electrons and another gains them. The substance that loses electrons becomes more positive in charge, and is said to be oxidized. The substance gaining electrons becomes more negative in charge and is said to be reduced. Several redox reactions are analyzed in Table 1.

Table 1: Redox Reactions
Reaction	Substance oxidized	Substance reduced
2Na(s) + Cl₂(g) --> 2NaCl(s)	Na goes to Na⁺	Cl in Cl₂ goes to Cl^-
2Fe³⁺(aq) + Fe(s) --> 3Fe²⁺	Fe(s) goes to Fe²⁺	Fe³⁺ goes to Fe²⁺

Acid base reactions involve the transfer of hydrogen ions, H⁺, from one substance to another. The substance donating the hydrogen ion is called an acid, and the substance accepting it is called a base. Some examples of acid-base reactions are given in Table 2.

Table 2: Acid-Base Reactions
Reaction	Acid	Base
HCl(aq) + OH-(aq) --> H₂O + Cl-(aq)	HCl	OH-
H₂SO₄(aq) + Na₂CO₃(aq) --> H₂CO₃(aq) + Na₂SO₄(aq)	H₂SO₄	CO₃^2-

Precipitation reactions result in the formation of a solid when two solutions are mixed. The solid is called a precipitate. Some examples of precipitation reactions are given in Table 3.

Table 3: Precipitation Reactions
Reaction	Precipitate
HCl(aq) + AgNO₃(aq) --> HNO₃(aq) + AgCl(s)	AgCl
H₂SO₄(aq) + CaBr₂(aq) --> 2HBr(aq) + CaSO₄(s)	CaSO₄

Copper is a transition metal that is beautifully colored in the metallic state and in many of its compounds. The metal is used in jewelry and coins, and in roofing and plumbing materials. Malachite is a green mineral with formula Cu₂(CO₃)(OH)₂, used for making ornamental objects and jewelry. Similarly, azurite, a carbonate of copper with formula Cu₃(OH)₂(CO₃)₂, is a blue semiprecious stone. In this experiment, you will begin with a sample of copper metal and will carry it through a series of reactions of the types discussed above. You should have fun finding out where you end!

Focus Questions

As you proceed through the experiment, keep the following questions in mind. When you have finished the experiment, provide answers to them.

What are some of your observations of the oxidation-reduction reactivity of copper?
What are some of your observations of the acid-base reactivity of copper compounds?
Propose balanced chemical equations for reactions (1)-(4).
What type of reaction is occurring in each step, BL acid-base, precipitation, or redox?
What is the overall percent yield of the cycle?
In which steps was product lost? How could the procedure be improved to increase the yield?

Equipment and Materials

Assorted beakers and Erlenmeyer flasks
Stirring rod
Pasteur pipets/bulb
aspirator trap
filter paper
Source of Cu (for example, a pre-1981 penny)
concentrated HNO₃ (15.9 M)
concentrated H₂SO₄ (12 M)
Zn metal
saturated NaHCO₃

Safety

Safety goggles must be worn at all times in the laboratory. Concentrated sulfuric acid is corrosive and generates much heat on exposure to water. Concentrated nitric acid is corrosive and strongly oxidizing. Wear gloves when handling them. In the event of skin contact, flush the affected area with copious quantities of cold water. THE REACTION OF Cu WITH NITRIC ACID PRODUCES NO₂ GAS, WHICH IS EXTREMELY POISONOUS. CARRY OUT THIS REACTION IN A FUME HOOD.

Experimental

Record all data and observations in your notebook.

Part 1: Reaction of copper with nitric acid. Use a penny as your source of copper. Weigh the penny and place it in the 50-mL beaker. IN THE HOOD, add 1 mL of water and 0.5 mL of concentrated nitric acid, HNO₃. BE SURE TO ADD THE WATER FIRST, SO THAT CONCENTRATED ACID IS ADDED TO WATER. Use a Pasteur pipet to dispense the nitric acid. (To use a Pasteur pipet will require that you know how many drops per mL the pipet dispenses.) Let the reaction run for about 2 minutes, swirling the beaker intermittently, until about 0.10 g of Cu dissolves. What do you observe? STILL IN THE HOOD, pour the solution into a second beaker, leaving the penny behind in the original beaker. Add a SMALL volume of distilled water to the penny beaker, swirl to rinse the penny, and pour the water into the second beaker. The penny and solution may now be taken out of the hood. Place the penny on a piece of paper towel, thoroughly dry it, and reweigh it.

Do your observations during this part of the experiment allow you to speculate on the copper containing product of the reaction below?

(1): Cu(s) + HNO₃ ---> Copper product 1 + NO₂(g) + H₂O Part 2: Reaction of Copper Product 1 with NaHCO₃. With vigorous stirring or swirling, dropwise add the solution from Part 1 to 7.5 mL of saturated NaHCO₃. What do you observe when the solution from Part 1 contacts the bicarbonate solution? Filter the mixture by suction using your Hirsch funnel and sidearm test tube, and wash with 1 mL water. Suction the product dry.

Do your observations allow you to speculate on details of reaction (2)?

(2): Copper Product 1 + NaHCO₃(aq) ---> Copper Product 2 + NaNO₃(aq) + CO₂(g) + H₂O

Part 3: Reaction of Copper Product 2 with Sulfuric Acid. Using your stirring rod or spatula, quantitatively transfer the dry solid from Part 2 to a small beaker. (A quantitative transfer is one in which as much of the solid as possible is transferred.) Add 7 mL of water to the beaker, then slowly add 0.5 mL 12 M H₂SO₄. Cool in an ice bath. What do you observe? Do your observations allow you to speculate on details of reaction (3)?

(3): Copper Product 2 + H₂SO₄ ---> Copper Product 3 + H₂O + CO₂(g)

Part 4: Reaction of Copper Product 3 with Zinc. To the solution from Part 3, add approximately 0.12 g of granular Zn and stir continuously. What do you observe? Do your observations allow you to speculate on the products of reaction 4?

(4): Copper Product 3 + Zn(s) ---> Copper Product 4 + Zinc product

Your goal is to use up all of Copper Product 3, but to be left with no unreacted zinc. You may have to add either more zinc or more sulfuric acid to achieve this goal. Isolate the solid by filtration, and wash with about 5 mL distilled water. Suction until no water remains in the funnel. Open the atmosphere vent of the aspirator trap to release suction, and empty the filtrate from the receiver tube. Reapply suction, and wash the solid with about 5 mL of acetone. Again release suction, and pour the acetone from the receiver tube into a solvent waste bottle in the hood.

Reapply suction to dry the product.

Weigh the final product.

Calculate the overall percent yield for the series of reactions.

Clean-up. When you have finished all of your work:

Clean glassware by recommended procedures, shake off excess water, and place in the drying oven.
Clean Pasteur pipets, graduated pipets, and vials by recommended procedures, shake off excess water, and place in the drying oven.
Return all borrowed equipment to the instructor before leaving lab.
Clean up your work area before leaving lab.

Disposal Methods

Place all solutions in the appropriately marked waste bottles. After weighing the product, place it in the appropriate waste bottle.

Preparation
Reactivity: The Copper Cycle

Preparation Questions