CHEMISTRY 130 General Chemistry I OXIDATION-REDUCTION CHEMISTRY A solution of potassium permanganate is pink. [1] Color changes can often be used to monitor chemical reactions. DEPARTMENT OF CHEMISTRY UNIVERSITY OF KANSAS
OXIDATION-REDUCTION CHEMISTRY Introduction Oxidation-reduction occurs when electrons are transferred between reactants. Redox processes are ubiquitous in the environment and are crucial in all living systems. (See the Appendix for just a few examples.) There are four types of redox reactions: Redox Reaction Example combination 2 Fe + 3 Br 2 à 2 FeBr 3 decomposition 2 HgO à 2 Hg + O 2 displacement Zn + CuSO 4 à Cu + ZnSO 4 disproportionation 2 Hg + à Hg + Hg 2+ Probably the most familiar example of a redox reaction you use every day is the one occurring in the internal combustion engine of your car. Hydrocarbon fuels, such as the octane in gasoline, combine with oxygen to produce carbon dioxide (CO 2) and water (H 2O). 2 C 8H 18 + 25 O 2 à 16 CO 2 + 18 H 2O In all oxidation-reduction reactions, the transfer of electrons is reflected by changes in the oxidation numbers associated with the substances taking part in the reaction. To review or learn about oxidation numbers, see Section 4.9 of your textbook. In a previous lab period, you performed a vinegar titration to determine the concentration of acetic acid. Phenolphthalein was used as an acid-base indicator to visualize the endpoint. The color of your solution was dependent on the form of the indicator species present. In the case of phenolphthalein, the acid form is colorless and the basic form is pink. The titration was complete at the point when approximately half a drop of additional NaOH caused the solution to remain slightly pink. At this point, the moles of OH- ions from the NaOH were equal to (or in very slight excess of) the moles of protons from the acetic acid. This 1:1 stoichiometry is shown below. NaOH + HC 2H 3O 2 à NaC 2H 3O 2 + H 2O In this experiment, you will be investigating an oxidation-reduction reaction by performing a set of titrations and using your results to measure an unknown concentration of ferrous chloride. It would be convenient if there were some indicator to help you find this equivalence point. Fortunately, the color of a solution is frequently due to the oxidation state of a species in solution. Some commonly used redox reagents that are themselves redox indicators are potassium permanganate (KMnO4) and potassium dichromate (K2Cr2O7). These are internal indicators because of large differences in the colors of their oxidized (MnO 4- /purple, Cr2O7 2- /orange-yellow) and reduced forms (Mn 2+ /light pink, Cr 3+ /green). 2
Pre-lab Safety: Goggles must be worn at all times. POTASSIUM PERMANGANATE: KMnO 4 This solution contains an oxidizing agent which is corrosive. Direct contact with skin and eyes should definitely be avoided. Skin Contact: Dilute aqueous solutions may be mildly irritating. FIRST AID: Remove contaminated clothing and shoes immediately. Wash with soap or mild detergent and large amounts of water until no evidence of chemical remains (at least 15-20 minutes). Eye Contact: Dilute aqueous solutions may be only mildly irritating. FIRST AID: Wash eyes immediately with large amounts of water, occasionally lifting upper and lower lids, until no evidence of chemical remains (at least 15-20 minutes). Get medical attention immediately. IRON(II) CHLORIDE TETRAHYDRATE: FeCl 2 4 H 2O Eye Contact: may cause severe irritation, and possible eye burns. FIRST AID: Immediately flush eyes with plenty of water for at least 15 minutes, occasionally lifting the upper and lower lids. Get medical aid immediately. Skin Contact: Exposure may cause irritation and possible burns. FIRST AID: Immediately flush skin with plenty of soap and water for at least 15 minutes while removing contaminated clothing and shoes. Get medical aid if irritation develops or persists. PHOSPHORIC ACID: H3PO4 Although phosphoric acid is classified as a weak acid, the concentration being used is 6 M. It is corrosive and contact with skin and eyes should be avoided. FIRST AID: Immediately flush eyes with plenty of water for at least 15 minutes. If spilled on clothing, the clothing should be removed immediately and rinsed. If irritation persists, seek medical help. Test solutions should be neutralized before disposal in the sink. Ask you TA for instructions regarding the disposal of unused stock solutions. Pre-lab Assignment: Please write out the following in your lab notebook. This assignment must be completed before the beginning of lab. You will not be allowed to start the experiment until this assignment has been completed and accepted by your TA. 1) List all of the chemicals you will use for this week's experiment. For each chemical, list specific safety precaution(s) that must be followed. In order to find specific safety information, please obtain a Materials Safety Data Sheet (MSDS) on the chemical of interest. MSDSs can be found through an internet search (e.g., Google) or from the following website: www.hazard.com Read the MSDS and find specific safety concerns for each chemical. 2) For each of the four types of redox reactions below, provide an example chemical reaction that is not already mentioned in this lab document. a. combination b. decomposition c. displacement d. disproportionation 3
3) To review how titration calculations work, here s a problem related to acid-base titrations. A 0.0100 L sample of H2SO4 was titrated with 0.150 M NaOH. It took 23.3 ml of NaOH from the buret--that's 0.0233 L--to reach the titration endpoint. a. Balance the chemical equation for this titration by placing coefficients in the spaces below. H 2 SO 4(aq) + NaOH (aq) à H 2 O (l) + Na 2 SO 4(aq) b. Based on the coefficients of the equation that you just balanced, how many moles of NaOH would be consumed by one mole of H 2 SO 4? c. How many moles of NaOH are contained in the 0.0233 L volume of 0.150 M NaOH that is delivered from the buret? d. How many moles of H 2 SO 4 must be present to react completely with the NaOH delivered from the buret? e. What is the molarity (M) of the H 2 SO 4 solution titrated? Hint: Molarity refers to the number of moles of H 2SO 4 per liter, i.e., mol divided by L. Use the number of moles (question #3d. above) along with the actual volume of H2SO4 titrated, 0.0100 L.) In addition to these pre-lab requirements, a short quiz may be given at the beginning of lab based on the material in this lab write-up. Procedure Part 1 Balancing the Redox Equation Reagents: 0.020 M KMnO 4, 0.20 M FeCl 2, and 6 M H 3PO 4 Each group will examine one of the solution mixtures below. In the laboratory, you will find prepared solutions of KMnO 4, FeCl 2, and H 3 PO 4. (In this reaction, phosphoric acid is necessary to prevent interference from chloride ions in the solution and to make the solution acidic.) Using the chart below, each group will place appropriate amounts of each reagent in a small beaker. Use volumetric pipets for the iron solution, and auto-pipetters for the acid and the permanganate solution. Report your observations on the board. Line up the labeled beakers on the bench top so that they may be compared by all the groups. Determine how many moles of each reagent are present in your team s solution and report those amounts. Be sure to record the class data in your laboratory notebook. Solution Conc. (M) Group 1 Group 2 Group 3 Group 4 Group 5 KMnO 4 0.020 M 5 ml 10 ml 15 ml 20 ml 25 ml FeCl 2 0.20 M 10 ml 10 ml 10 ml 10 ml 10 ml H 3 PO 4 6 M 10 ml 10 ml 10 ml 10 ml 10 ml As a class, discuss the results of this preliminary investigation. Using the collective data, balance the oxidation-reduction reaction that occurs between MnO4 and Fe 2+. Fe 2+ (aq) + MnO 4 - (aq) + H + (aq) à Fe 3+ (aq) + Mn 2+ (aq) + H 2 O (l) 4
Part 2 Determining the Unknown FeCl2 Concentration Reagents: 0.020 M KMnO 4, 6 M H 3 PO 4, and FeCl 2 solution (unknown concentration) 1. Using the information you obtained in Part 1, plan an investigation that will allow you to determine the concentration of a FeCl 2 solution of unknown concentration that your TA provides. (Hint: If you need to review an example titration, you may wish to consult the write-up, procedure, and/or data from your Determination of Acetic Acid in Vinegar experiment.) 2. Complete your investigation and determine the concentration of the FeCl 2 solution. Show balanced equations and calculations in your notebook, and be certain that all your numerical data is expressed with the proper units and significant figures. Lab Clean-up: Acidic solutions should be neutralized before disposal in the sink. Stock solutions of acids should be put in a separate waste receptacle. If you have questions, ask your TA. Glassware, including test tubes, should be thoroughly cleaned using deionized water and then returned to your lab drawer. Post-Lab Questions After completing the lab, answer the following questions in your laboratory notebook. 1) The redox reaction in this experiment is Fe 2+ (aq) + MnO 4 - (aq) + H + (aq) à Fe 3+ (aq) + Mn 2+ (aq) + H 2 O (l) For each element (Fe, Mn, O, H) in the chemical equation, determine if it is oxidized, reduced or unchanged during the reaction using oxidation numbers. (See Section 4.9 of your textbook.) 2) The goal of this experiment was to determine the concentration of FeCl 2, ferrous chloride, a compound that is important in wastewater treatment. Ferrous chloride helps control levels of hydrogen sulfide in the water. a) What is the chemical formula of hydrogen sulfide? b) Why would you want to remove hydrogen sulfide from the water supply? c) Treated wastewater usually becomes drinking water downstream. Do you think you could use this titration method to determine the concentration of excess ferrous chloride in drinking water? Why or why not? Base your answer on concentration, and cite any external information you use. 5
Report Introduction Experimental Procedures Results Discussion Conclusion A short (2 page) report is due at the beginning of next week s lab period. It is a partial, rather than full, report and should be prepared and submitted individually, rather than in groups. It will consist only of the Experimental Procedures and Conclusions sections of a full report. You have practiced writing these sections of a lab report already. This is your opportunity to improve based on the feedback you received from your TA. To review: The Experimental Procedures ideally should be clear enough that someone who hasn t done your experiment could reproduce your results. As an alternative to written text, your Experimental Procedures may also be presented in the form of a detailed flow chart. The Experimental Procedures section should also include any equations you used and example calculations. A person reading this section should also be able to use their raw data and calculate results that are the same as (or similar to) your results. The Conclusions section should (1) summarize the objective of the experiment, (2) state your overall results, (3) and briefly outline the interpretation of your results. (4) Describe what you learned or concluded from doing the experiment. Often, scientists make mention of (5) what they might try to do differently next time. Reference(s) [1] http://www.sciencebrothers.org/the-chemical-chameleon/, accessed Feb. 26, 2016. 6
Glossary Indicator a substance that provides a visual cue that a threshold level of some chemical change has occurred; indicators often change color upon reaching some concentration (for example, phenolphthalein turns pink when a solution has an excess of hydroxide present and becomes basic) Oxidation the loss of electrons; an increase in oxidation number Reducing agent a substance that facilitates the reduction of another substance; a substance that becomes oxidized. Titration (volumetric analysis) a laboratory procedure in which a solution of unknown concentration is reacted with a solution of known concentration, in order to determine the concentration of the unknown. Oxidation number (oxidation state) the hypothetical charge an atom would have if the atom or compound containing it were composed only of ions Oxidizing agent a substance that facilitates the oxidation of another substance; a substance that becomes reduced Oxidation-reduction reaction (redox reaction) a type of chemical reaction that involves the transfer of one or more electrons between reactants; a chemical reaction involving a change in oxidation number for atoms participating in the reaction Reduction the gain of electrons; a decrease in oxidation number 7
Appendix 1: Oxidation-reduction Reactions Environmental Redox Cycle Site Oxidation-reduction reactions are extremely important to the environment. While they are always at work around you, they may go unnoticed. Important examples include the carbon, nitrogen, and sulfur cycles. The transformation of carbon dioxide into organic compounds, which in turn produces oxygen needed by plants as well as animals to metabolize carbon compounds, is particularly integral to all life on the planet. Learn more about the chemical reactions involved in these processes at the following web site. http://www.greenfacts.org/glossary/def/environmental-cycles.htm Life-Cycle Redox Processes Without oxidation-reduction reactions, living organisms would be dead! To learn more about these very important processes, visit the web site below. http://makahiki.kcc.hawaii.edu/chem/everyday_metabolism.html Active Metal Series How can scientists predict if an oxidation-reduction reaction will occur? Check the web for the answer. http://www.chem1.com/acad/webtut/aquatic/fallelect.html