Pre-Lab Exercises Lab 5: Oxidation and Reduction Name Date Section 1. What is oxidation? 2. What is reduction? 3. Look at the reaction 2 H 2O 2 H 2 + O 2. Is this an oxidation-reduction reaction? If so, what is being oxidized and what is being reduced? 4. Give two common, everyday examples of oxidation-reduction reactions. Page 1 of 5
Chemistry 100 Lab 5: Oxidation and Reduction DISCUSSION Oxidation and reduction reactions both involve the transfer of electrons. In oxidation, a substance loses electrons. Reduction occurs when a substance gains those electrons. Although we may discuss and diagram these reactions separately, they always occur together. Electrons lost by one substance must be gained by the other, and vice versa. In this lab, we will look at some common examples of oxidation-reduction (or redox) reactions. Two different metals, when placed in an electrolyte solution (a solution of ions), develop a voltage between them. That is, they become a battery. This occurs as a result of a series of oxidation-reduction reactions at the surface of the metals. You will use a pickle as the electrolyte solution, and copper, zinc and magnesium as the metals. When the voltmeter registers a positive voltage, it means that electrons are flowing between the 2 electrodes. The metal attached to the black connector is losing electrons and is being oxidized. It is called the anode. These electrons released from the oxidized metal can flow through the electrolyte solution. Once the electrons reach the other metal attached to the red connector (called the cathode), they cause the metal to be reduced. An electric current, or a flow of electrons, can cause a chemical reaction. For example, a 9-volt battery, placed in a circuit, pushes electrons out of its negative (black) pole and pulls electrons into its positive (red) pole. When this occurs through a conductive solution, a chemical reaction occurs in a process called electrolysis. For the chemicals in the solution, reduction will occur at the anode (the black connector, negative pole) because the metal of the anode itself is being oxidized and pushing electrons into the solution. Oxidation of chemicals in the solution will occur at the cathode (the red connector, positive pole) because the metal of the cathode itself is being reduced, pulling electrons from the surrounding solution at that end. Iron, our most important structural metal, is attacked readily by oxygen in the presence of water. The iron corrodes, and rust (iron oxide) is formed. Corrosion is accelerated in the presence of salt because there are then more free ions to allow electron movement in the solution, and ultimately oxidation. Rust can be prevented, or at least slowed, by coating the iron with other metals or with paint, oil, etc. to keep the oxygen and water from contacting the iron. The coating is a sort of sacrificial substance that can be oxidized by the oxygen, usually much more slowly, rather than the iron. Page 2 of 5
Homemade Battery PROCEDURES 1. There are three different pairs of metals that you can make from the three strips. For one of the pairs, insert one metal strip into one of the pre-cut slits in the pickle; place another strip in the other slit. 2. Attach one voltmeter connector to one metal and the other connector to the other metal. If the voltage is negative, switch the connectors. Record the voltage you observe. 3. Repeat for the other two pairs of metals. 4. Locate a small buzzer (see picture to the right). Place each pair of metals into the pickle, as above, but this time touch the electrodes on the buzzer to the metals in the pickle. You may have to bend the metals a bit to bring them into contact with the buzzer s electrodes. What combination (or combinations) of metals make the buzzer work? Electrolysis 1. Under the hoods, you will find a squeeze bottle containing dilute (3 M) sulfuric acid (diluted battery acid). Place about 15mL of this solution into a 50-ml beaker. a. Insert metal electrodes attached to a 9-volt battery. Observe what happens at the ends of the electrodes. You will see bubbles form at one or at both of the electrodes (depending on the type of electrode used). Answer the questions in the report. b. Remove the electrodes from the solution and rinse them. 2. Find another squeeze bottle containing potassium iodide (the ingredient that provides iodine in iodized salt) solution. Place about 15mL of this solution into a 50-ml beaker. Add one drop of phenolphthalein solution. This ph indicator, as you might remember, is colorless in acid but turns pink in base. a. Dip the electrodes into the solution. Look carefully for changes in the solution around the electrodes. At one electrode, the I - ions in the solution are changed to neutral and colored I 2. At the other, water is changed to OH - ions and H 2 gas. Answer the questions in the report. b. Remove the electrodes as soon as you make your decision. If the solution is too discolored to reuse, discard it and add fresh solution. 3. Find another squeeze bottle containing copper nitrate solution (0.1 M). Place about 15mL of this solution into a 50-mL beaker. a. Insert the electrodes. One electrode will turn dark, and bubbles may form at the other. Metallic copper is being formed from Cu 2+ ions at the electrode that turns dark. Answer the questions in the report. b. Rinse the electrodes and wipe off the dark copper deposit with a paper towel. You may see a copper coating underneath, evidence of the electroplating of copper onto the electrode. Corrosion 1. Obtain two plain nails and one galvanized (zinc-coated) nail. 2. Slide each nail into a test tube, and add enough water to immerse half the nail. 3. Add a little salt to the test tube containing one of the plain nails. Stir to dissolve the salt, and label the tubes so you will know which one has the salt. 4. Put the test tubes in a small beaker, label it with your name, and leave it until the next lab period. 5. Observe the nails to see if any rust has formed. Page 3 of 5
Lab Station Check Out # Homemade Battery REPORT Name Lab Partner Date Section Combination Metal in contact with red (+) connector (cathode) Metal in contact with black (-) connector (anode) Voltage Did the buzzer work? 1 2 3 Which metal combination(s) made the buzzer work best? Electrolysis Sulfuric Acid 1. If you see bubbles at only one electrode, is it the anode or the cathode? 2. Is the gas being produced as a result of oxidation or as a result of reduction? 3. Is the gas hydrogen, or is it oxygen? If bubbles are produced at both electrodes, one gas is hydrogen and the other is oxygen. Can you tell which is which? (Hint: oxidation of water produces oxygen, while reduction of water produces hydrogen.) Potassium Iodide 1. What color will the OH - ions turn the phenolphthalein? 2. At which electrode is the I 2 is produced, and at which electrode are the OH - and H 2 produced? Copper Nitrate 1. What is occurring, oxidation or reduction, at the electrode that turns dark? 2. Is this the anode or cathode? 3. Is this consistent with what you observed in the previous 2 sections? 4. If bubbles form at the other electrode, is it oxygen or hydrogen? Page 4 of 5
Corrosion 1. Describe your experiment with the three nails. a. How long did you let the experiment run? b. Which nail has the most rust? Why? c. Do any nails have no rust at all? Why? Post-Lab Analysis 1. The Statue of Liberty is covered with a layer of copper, which is the color of pennies. Why is she green? 2. It is estimated that the Statue of Liberty was completely green within 20 years of completion. Very few 20-yearold pennies are green. Can you formulate a hypothesis to explain this? How could you test your hypothesis? Page 5 of 5