Chemistry 151 Last Updated Dec Lab 11: Oxidation-Reduction Reactions

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1 Chemistry 151 Last Updated Dec Lab 11: Oxidation-Reduction Reactions Introduction Oxidation-reduction ( redox ) reactions make up a large and diverse part of chemical systems. A few examples include the following. Electron transfers between elements and monatomic ions Perhaps the simplest type of redox reaction is one where electrons are transferred between an element and a monatomic ion, such as the reaction between nickel (II) ion and magnesium. Ni 2+ (aq) +Mg(s) Ni(s) + Mg 2+ (aq) Here, nickel (II) is undergoing reduction because it gains electrons and magnesium is undergoing oxidation because it loses electrons. Nickel (II) is also referred to as the oxidizing agent, as it is the chemical removing electron from magnesium. Likewise, magnesium can be referred to as the reducing agent, since it s the chemical donating the electrons to nickel (II). However, combining any ion with an element doesn t necessarily guarantee a reaction will occur. For example, the reverse of the above reaction, between magnesium ion and nickel, will not readily occur. Ni(s) + Mg 2+ no reaction From this, we can conclude that nickel (II) is a stronger oxidizing agent than magnesium ion, and that magnesium metal is a stronger reducing agent than nickel. Such ideas will be discussed in greater detail in General Chemistry II. Reactions between metals and acids If a metal is a strong enough reducing agent, it will react with an acid to produce hydrogen gas, such as the reaction between iron and sulfuric acid Fe(s) + H 2 SO 4 (aq) FeSO 4 (aq) + H 2 (g) The stronger the reducing agent, the more quickly this reaction will occur, which in this case can be observed by the increased production of gas. Reactions with water In this course water is routinely used as the solvent for aqueous reactions, but water can undergo reactions of its own, many of which are redox reactions. For example, sodium will react vigorously with water to produce hydrogen gas and sodium hydroxide. 2Na(s) + 2H 2 O(l) 2NaOH(aq) + H 2 (g) Notice that this reaction is similar to the reaction between a metal and an acid (think of water as HOH instead of H 2 O). Water s ability to behave as an acid is another concept that will be discussed further in General II. Combustion Reactions Combustion reactions involve the conversion of elemental oxygen, which has an oxidation number of zero, to either a metal or nonmetal oxide, where oxygen will have an oxidation number of 2. Therefore, all combustion reactions are also redox reactions. The combustion of methane (CH 4 ) is shown below. CH 4 (g) + 2O 2 (g) CO 2 (g) + 2H 2 O(g)

2 Peroxides and bleaches Two common household oxidizing agents are bleach (sodium hypochlorite, NaClO) and hydrogen peroxide (H 2 O 2 ). When stains are removed with bleach, colored compounds are oxidized into colorless ones as hypochlorite is reduced to chloride ion. H 2 O 2 will also act as an oxidizing agent, as seen in the following reaction with iron (II). 2Fe 2+ (aq, colorless) + H 2 O 2 (aq) + 2H + (aq) 2Fe 3+ (aq, yellow) + 2H 2 O(l) Indicator solutions Most reagents that are used as chemical tests or indicators are oxidizing or reducing agents. Benedict s reagent, for example, is a common reagent used for testing diabetes. The reagent is a basic solution of copper (II) that is reduced to copper (I) in the presence of glucose. The copper (I) forms as Cu 2 O, a red solid. 2Cu 2+ (aq, blue) + C 6 H 12 O 6 (aq) + 5OH - (aq) Cu 2 O(s, red) + C 6 H 11 O 7 - (aq) + 3H 2 O(l) Early versions of breathalyzer tests used potassium dichromate. Dichromate compounds, which are usually yellow to orange in color, are reduced to a green chromium (III) compound in the presence of alcohols, such as ethanol (CH 3 CH 2 OH). 3CH 3 CH 2 OH(aq) + 2Cr 2 O 7 2- (aq, orange) + 16H + (aq) 3CH 3 COOH(aq) + 2Cr 3+ (aq, green) + 11H 2 O(l) Here, ethanol is converted to acetic acid (CH 3 COOH), the main ingredient in household vinegar. In this lab you will observe a series of redox reactions, including some of the ones mentioned above. You will also compare the strengths of different oxidizing and reducing agents. Procedure Part I: Reactions Between Elements and Monatomic Ions A. Metals and Metal Ions Use a set of small test tubes to prepare the mixtures outlined in the table below. In each case, use approximately 2 ml of solution and g of metal. If needed, clean the metal with sandpaper. Allow the solution to set for 5 minutes then record your observations. Sample Solution (0.2 M) Metal 1 CuSO 4 Fe (steel wool) 2 ZnSO 4 Fe (steel wool) 3 CuSO 4 Zn 4 FeSO 4 Zn 5 FeSO 4 Cu 6 ZnSO 4 Cu B Nonmetals and Nonmetal Ions 1. Transfer 2 ml of 0.1 M potassium iodide to a test tube 2. Add 3-4 drops of bromine solution (the solvent is cyclohexane, which is immiscible with water, so it will form two layers). 3. Cover the opening of the test tube with a small piece of Parafilm, place your thumb over the opening and gently shake to mix. 4. Add a few drops of starch (which turns blue in the presence of I 2 ) to the test tube and mix. 5. Repeat steps 1-3 using 0.1 M potassium bromide and iodine solution.

3 Part II: Reactions Between Metals and Acids 1. Label three test tubes 1-3. Place a small piece (~0.2 g) of zinc in test tube #1, ~1-2 cm of magnesium ribbon in #2, and a smaller piece (~0.2 g) of copper in #3. Clean each metal with sandpaper, as needed. 2. Add 2 ml of 10% hydrochloric acid to each test tube and record your observations. Part III: Reactions Between Metals and Water Repeat Part II, substituting deionized water for hydrochloric acid. Part IV: Combustion Reactions A. Combustion of Cellulose 1. Attach a ring clamp to a ring stand and set a clay triangle on the ring clamp. 2. Obtain piece of filter paper, approximately 5.5 cm in diameter and crumble it so that it will fit inside a crucible. Place the crucible on the clay triangle. 3. Heat the crucible with a Bunsen burner and record your observations. Make sure the paper doesn t come in direct contact with the flame. B. Combustion of Magnesium 1. Obtain a small piece of magnesium (~ 1 cm in length) and clean it with sandpaper to give it a shiny surface. 2. Holding the magnesium with a pair of tongs, use the flame of your Bunsen burner to light the metal. Record your observations. [Safety Note: Once the magnesium begins to burn, don t look directly at the resulting light.] Part V: The Reaction of Fe 2+ with Hydrogen Peroxide 1. Transfer a few crystals of FeSO 4 to a medium test tube and dissolve with 2 ml of water. 2. Add 2-3 drops of 3% hydrogen peroxide and record your observations. Part VI: The Reaction of Cu 2+ (Benedict s Solution) with Glucose 1. Assemble a hot-water bath as follows: a. Place a Bunsen burner on a ring stand. b. Attach a metal ring to the ring stand then place a wire gauze on the ring. c. Half-fill a ml beaker with water and set it on the wire gauze. Place a second ring clamp around the beaker to reduce the chance of an accidental spill. d. Begin heating the water with the beaker. 2. Place a small amount of glucose in a medium test tube (enough to cover the bottom) and dissolve with 2-4 ml of water. 3. Add 2 ml of Benedict s solution to the test tube and heat the solution in the hot-water bath. Record your observations. Part VII: The Reaction of Dichromate with Ethanol 1. Add a few crystals of potassium dichromate to a medium test tube and dissolve with 2-3 ml of 3.0 M sulfuric acid. 2. Add 2-4 ml of ethanol and gently swirl to mix. Record your observations. Waste Disposal All waste should be disposed of in the waste hood.

4 Name: Section: Data Part I: Reactions Between Elements and Monatomic Ions A. Metals and Metal Ions Sample Solution (0.2M) Metal Results 1 CuSO 4 Fe (steel wool) 2 ZnSO 4 Fe (steel wool) 3 CuSO 4 Zn 4 FeSO 4 Zn 5 FeSO 4 Cu 6 ZnSO 4 Cu Based on your results, rank the three cations in order of increasing oxidizing strength (weakest oxidizing agent) < < (strongest oxidizing agent) Rank the three metals in order of increasing reducing strength (weakest reducing agent) < < (strongest reducing agent). Explain your rankings. B Nonmetals and Nonmetal Ions From your observations, which is the stronger oxidizing agent, bromine or iodine? Which is the stronger reducing agent, bromide or iodide?

5 Part II: Reactions Between Metals and Acids Sample Metal Results 1 Zn 2 Mg 3 Cu Part III: Reactions Between Metals and Water Sample Metal Results 1 Zn 2 Mg 3 Cu Rank the three metals in order of increasing reducing strength (weakest reducing agent) < < (strongest reducing agent). Part IV: Combustion Reactions A. Combustion of Cellulose B. Combustion of Magnesium Part V: The Reaction of Fe 2+ with Hydrogen Peroxide

6 Part VI: The Reaction of Cu 2+ (Benedict s Solution) with Glucose Part VII: The Reaction of Dichromate with Ethanol

7 Name: Section: Post-Lab Questions 1. Are the results of Parts I III consistent with each other? Explain. 2. A chemist accidentally leaves a bottle of iron (II) chloride open overnight. The next morning the once colorless solution is now light yellow. What happened? 3. You will occasionally hear stories of people suddenly undergoing spontaneous combustion (bursting into flaming without an ignition source). Based you observations in this lab, comment on the likelihood of such events. 4. Ovens with a self-cleaning feature offer a convenient way to remove grease and other cooking byproducts. Based on your observations today, how do you think they work? 5. A popular item during Independence Day (and other fireworks-based celebrations) are sparklers that emit a bright light when lit. Given today s observations, what are these sparkler most likely made of? 6. Cooks will usually avoid using aluminum foil or aluminum cookware when prepare acidic foods. Give two reasons why? 7. In the Introduction section, the equation for the reaction between iron (II) and hydrogen peroxide also includes hydrogen ion as a reactant, but only solutions of FeSO 4 and H 2 O 2 was mixed in a test tube. If an acid is required for the reaction to work, what was the source of H + for this reaction. (No, we didn t add it to the peroxide bottle beforehand; it s the same stuff you d buy at a store)

8 Name: Section: Pre-Lab Questions 1. Based on what was said in the Introduction, how many of the mixtures in Part I should lead to a reaction? 2. Identify the oxidizing and reducing agent in the following equations. Explain your answers. a) 2Na(s) + 2H 2 O(l) 2NaOH(aq) + H 2 (g) Oxidizing agent: Reducing agent: b) Al 3+ (aq) + 3Li(s) 3Li + (aq) + Al(s) Oxidizing agent: Reducing agent: b) CH 4 (g) + 2O 2 (g) CO 2 (g) + 2H 2 O(g) Oxidizing agent: Reducing agent: 3. What is the formula of cellulose? What are the predicted products when it undergoes combustion? 4. Write balanced equations for each of the following reactions (assuming one occurs). Phases aren t required. a) Iron (II) ion and copper b) Magnesium and hydrochloric acid c) Magnesium and water d) The combustion of magnesium