Lab: Decomposition of Potassium Chlorate, Lab Reports of Chemistry

Download Lab: Decomposition of Potassium Chlorate and more Lab Reports Chemistry in PDF only on Docsity! E4E-1 Experiment 4E FV 6/28/18 THE DECOMPOSITION OF POTASSIUM CHLORATE MATERIALS: Two test tubes: (18x150), clamp, ring stand, Bunsen burner, weighing boat, glass wool, pure KClO3, unknown mixture containing KClO3 and inert substance, MnO2 PURPOSE: The purpose of this experiment is to study the decomposition of potassium chlorate by quantitatively determining the correct stoichiometry, and to use that result to assess the purity of an unknown potassium chlorate mixture. LEARNING OBJECTIVES: By the end of this experiment, the student should be able to demonstrate the following proficiencies: 1. Explain the relationship between the mass of a substance and the number of moles of a substance. 2. Apply stoichiometric ratios between the moles of reactant(s) and product(s) in balanced chemical equations. 3. Calculate the percent by mass of a compound in a mixture. 4. Explain the purpose of a catalyst. DISCUSSION: Stoichiometry. Chemistry is making and breaking bonds. Because no atoms are created or destroyed, only the connecting bonds change, in a chemical reaction, a major emphasis requires knowing the correct formulas for all reactants and products involved in the reaction, as well as the relative molar amounts of each. Nature constrains us to having the same number and type of atoms before and after the reaction, a balanced chemical equation, but how do we know how the atoms are arranged—what compounds are the reactants and products? Where does that information come from? The answer is that reactions are determined by experiment. Care and thought in measurements are fundamental to science, and mass measurements and physical and/or chemical tests allow one to deduce the proper reaction. Only when it is realized that better measurements make a reaction understood, can one start to consider useful applications of the reaction. Consider the title reaction, the thermal decomposition of potassium chlorate. When KClO3 is heated strongly, it breaks down, releasing oxygen gas and leaving behind a thermally stable (i.e., heat-insensitive) solid residue of an ionic potassium compound. solid potassium chlorate  oxygen gas + solid residue There are at least three plausible reactions one can write for the process, but only one occurs to any significant extent. Which one of the three is actually observed can only be determined by experiments, such as those conducted here. By measuring the amount of oxygen lost when a sample of potassium chlorate is heated, we will be able to determine the stoichiometric coefficients of KClO3 and O2 in the reaction, and thus determine the correct reaction. Relevant Naval Application. On submarines, oxygen for breathing is normally produced by the electrical decomposition of an aqueous solution. . Details relating to this electrolysis process will be studied later in the course. In case of problems with the electrol,ytic oxygen generators, a secondary supply source is also available to produce oxygen gas for breathing. This is a chemical process, directly analogous to your experiment: the decomposition of sodium chlorate (NaClO3) at high temperature (above 300oC) in a canister called a “chlorate candle. Unfortunately, there are a few complications associated with this reaction which must be remedied if the production of oxygen gas for breathing is to be performed safely and efficiently in this practical application. First, the intense flame used to raise the temperature of the sodium chlorate above 300oC is produced by a combustion reaction, which consumes oxygen gas, whereas the purpose of the overall process is to produce oxygen gas. While this issue cannot be completely remedied, small amounts of iron metal are mixed in, reacting with some of the oxygen to produce iron oxide and releasing large quantities of energy which helps maintain the mixture above the 300oC decomposition temperature. After the candle is ignited, the oxygen-consuming flame used to initiate the decomposition reaction is replaced by this iron combustion process, making it more self-sustaining. E4E-2 Second, though the decomposition reaction occurs at temperatures above 300oC, the direct reaction is still extremely slow and therefore impractical for oxygen production in bulk. This is remedied by adding a catalyst, in this case a small amount of manganese(IV) oxide, MnO2, which significantly increases the rate of the reaction, without itself being consumed. Third, while the desired decomposition reaction predominates, there is another decomposition reaction which produces toxic chlorine gas, oxygen gas and sodium oxide. This is remedied by including small amounts of barium peroxide (BaO2) in the mixture, which reacts with the toxic chlorine gas to produce barium chloride and oxygen gas. In summary, the “chlorate” or “oxygen” candle used in production of oxygen gas for breathing on submarines consists of a mixture of NaClO3, MnO2, iron, a small amount of BaO2, and a fibrous binding material. In practice, each candle burns near 400oC for 45-60 minutes, and produces approximately 115 SCF (standard cubic feet) of oxygen gas at 0.5 psig (pounds per square inch, gauge pressure), which is enough oxygen for about 100 people. As you might suspect, since they are self- sustaining in oxygen, the stored candles represent a significant fire hazard. Use of potassium chlorate. In this experiment, potassium chlorate will be used instead of the sodium chlorate employed commercially (see Figure 1). As you should suspect, analogous reactions occur, with all of the same complications. The only remedy that will be applied here will be the inclusion of the manganese(IV) oxide catalyst. Since all of the procedures will be carried out in the fume hood, any toxic chlorine gas produced will be safely carried away in the ventilation system. Why is NaClO3 used commercially, rather than KClO3? The principal reason is cost; sodium salts are typically much less expensive than their potassium counterparts. Also, because of the lower molar mass, there is a slightly higher mass percentage of oxygen in the sodium compound than there is in the potassium compound. Energetics. As should be clear from the discussion of the Navy’s chlorate candles, practical applications almost always have to consider energy changes associated with reactions. Most chemical and physical processes are accompanied by changes in energy – some release energy as they proceed, and some require an input of energy in order to sustain the process. We will examine some elementary concepts of energy changes associated with the reactions observed over the course of the semester. Safety Data Sheets and International Chemical Safety Cards. Any institution where chemicals are used is required to have copies of the Safety Data Sheets, SDS, (formerly material safety data sheets, MSDS) available for inspection by anyone using those materials. These sheets provide key information relating to health hazards, appropriate storage, handling and disposal arrangements, fire and explosive hazards, required control measures, physical/chemical properties, and reactivity data. In this experiment, the SDS for potassium chlorate will be used to help guide the experimental study of its decomposition reactions. (See https://beta-static.fishersci.com/content/dam/fishersci/en_US/documents/programs/education/regulatory- documents/sds/chemicals/chemicals-p/S25482.pdf .) In general, prior to any chemical procedure, the relevant SDS should be consulted to assure safe and proper procedures are followed. Another system which provides similar information is the International Chemical Safety Card system. Both material SDS and Safety Cards are available on-line through links found on the Chemical Safety Information page of the Plebe Chemistry website, https://www.usna.edu/ChemDept/plebeChem/safety.php. Figure 1. Examples of chlorate (oxygen) candles. Various candle sizes are manufactured for different applications. While oxygen candles are most commonly used for back-up purposes on submarines, they are also used in spacecraft, refuge shelters in underground mines, and emergency shelters. One manufacturer claims that one oxygen candle produces enough O2 to keep 15 people alive for 5.7 hours, assuming they are at rest (calculation based on 0.5 L per person per minute). It has a shelf life of 10 years, so long- term storage for emergency use is practical. E4E-5 Name _____________________________________ Section _____________ Date _______________ IN-LAB QUESTIONS  Experiment 4E Complete these questions during lab. 1. a. Record your observations as the mixture is heated. b. How do you know when the decomposition of potassium chlorate is near completion? 2. a. What is the purpose of MnO2 in this experiment? b. Why doesn’t the MnO2 affect the stoichiometry of the reaction? 3. Why do you think it would be advisable to heat the sample a second time? E4E-6 DATA TREATMENT Experiment 4E Part A. Determination of the Stoichiometry of the Decomposition Reaction of Potassium Chlorate In this section the balanced chemical equation for the decomposition of potassium chlorate will be determined. From the mass information recorded in the Data Section, you will calculate the number of moles of potassium chlorate present in the test tube before heating (question A.1) and the moles of oxygen gas generated (question A.2). After these two calculations, determine the stoichiometric ratio between potassium chlorate and oxygen gas (A.3) and generate the correct balanced chemical equation (A.4). This equation should also include the chemical formula for the solid material left in the test tube after heating. Show all work. (A.1) From the mass of potassium chlorate used, calculate the number of moles of potassium chlorate that decomposed. (A.2) From your mass loss data, calculate the number of moles of oxygen gas (O2) evolved. (A.3) Based on your answers above, calculate the numerical value of the following mole ratio (your instructor may ask you to record your ratio on the board): 𝑚𝑜𝑙𝑒𝑠 𝑂2 𝑝𝑟𝑜𝑑𝑢𝑐𝑒𝑑 𝑚𝑜𝑙𝑒𝑠 𝐾𝐶𝑙𝑂3 𝑑𝑒𝑐𝑜𝑚𝑝𝑜𝑠𝑒𝑑 = (A.4) Balance the three reactions below, as done in the Pre-Lab, and show the ratio moles O2/moles KClO3 for each reaction. 𝑚𝑜𝑙𝑒𝑠 𝑂2 𝑝𝑟𝑜𝑑𝑢𝑐𝑒𝑑 𝑚𝑜𝑙𝑒𝑠 𝐾𝐶𝑙𝑂3 𝑑𝑒𝑐𝑜𝑚𝑝𝑜𝑠𝑒𝑑 = a. ___ KClO3(s)  ___ KClO2(s) + _____ O2(g) _______ b. ___ KClO3(s)  ___ KClO(s) + _____ O2(g) _______ c. ___ KClO3(s)  ___ KCl (s) + _____ O2(g) _______ (A.5) Which of the three reactions best fits your experimental data? Explain your answer below. E4E-7 Part B. Determination of the Percent by Mass of Potassium Chlorate in an Unknown Mixture Having determined the balanced chemical equation for the decomposition of potassium chlorate (from page E4E-6), it is now possible to determine the percent by mass of potassium chlorate in an unknown mixture containing KClO3 and an inert ingredient. As before, heating the sample will cause the decomposition to occur and oxygen gas will be produced. By calculating the number of moles of oxygen produced (B.1), the moles and mass of potassium chlorate originally present in the unknown sample (B.2) can be determined. From that the mass percent potassium chlorate in the unknown can be obtained (B.3). Show your work. (B.1) From your mass loss data, calculate the number of moles of O2 evolved when the unknown was heated. (B.2) Using the correct stoichiometric equation, calculate the mass of potassium chlorate that must have been decomposed to provide the O2 loss observed for the unknown. (Use the correct stoichiometry of the reaction identified in question (A.4), not you experimental mole ratio.) (B.3) From your data, calculate the percent by mass of potassium chlorate in the unknown sample. Pay attention to significant figures. 𝑚𝑎𝑠𝑠 % = 𝑚𝑎𝑠𝑠 𝐾𝐶𝑙𝑂3 𝑚𝑎𝑠𝑠 𝑢𝑛𝑘𝑛𝑜𝑤𝑛 𝑥 100 Unknown number: _____________ Percent by mass of KClO3 in unknown: ________________