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Experiment 4C
FV 1-21-
THE DECOMPOSITION OF POTASSIUM CHLORATE
MATERIALS: test tubes: (25x150 (Instructor Demo, Part A), 18x150 (Part B)); 100 mL beaker, glass wool, potassium chlorate, manganese(IV) oxide.
PURPOSE: The purpose of this experiment is to study the decomposition of potassium chlorate and quantitatively determining the correct stoichiometry.
LEARNING OBJECTIVES: By the end of this experiment, the student should be able to demonstrate the following proficiencies:
- Find the MSDS and/or Safety Card for a chemical species, and locate important information related to physical properties, reactivity, and appropriate handling protocols.
- After carrying out the decomposition reaction for potassium chlorate, quantitatively verify its stoichiometry.
DISCUSSION: Stoichiometry. A major emphasis of chemistry is the understanding chemical reactions. This requires knowing the correct formulas for all reactants and products involved in the reaction, as well as the relative molar amounts of each. Such information is provided by the balanced chemical reaction, but where does that come from? The answer is that reactions are determined by experiment. Careful mass measurements and physical and/or chemical tests allow one to deduce the proper reaction. Only when that is understood can one start to consider useful applications of the reaction. Consider the title reaction, the thermal decomposition of potassium chlorate. When KClO 3 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 is actually observed can only be determined by experiment, 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 KClO 3 and O 2 in the reaction, and thus determine the correct reaction.
Relevant Naval Application. On submarines, oxygen for breathing is normally produced through electrolysis of water. Details relating to this process will be studied later in the course. In an emergency, a chemical process is used to produce oxygen gas for breathing, specifically the decomposition of sodium chlorate at high temperature (i.e., above 300o^ C), producing oxygen gas and a solid sodium salt. Unfortunately, there are several 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, though the decomposition reaction occurs at temperatures above 300oC, it is extremely slow and therefore impractical for oxygen production in bulk. This is remedied by adding a catalyst , in this case manganese(IV) oxide, which significantly increases the rate of the reaction, without itself being consumed.
Second, the intense flame used to raise the temperature of the sodium chlorate above 300o^ C is produced by a combustion reaction, which consumes large quantities of 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 300 o^ C 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.
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 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 for emergency production of oxygen gas for breathing on submarines consists of a mixture of sodium chlorate, iron, a small amount of barium peroxide, and a fibrous binding material. In practice, each candle burns near 400o^ C 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. The stored candles represent a significant fire hazard since they are self-sustaining in oxygen.
Use of potassium chlorate. In this experiment, potassium chlorate will be used instead of the sodium chlorate employed commercially. 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 NaClO 3 used commercially, rather than KClO 3? The principal reason is cost; sodium salts are typically much less expensive than their potassium counterparts.
Material Safety Data Sheets and International Chemical Safety Cards. Any institution where chemicals are used is required to have copies of the material safety data sheets (MSDS) or safety data sheets (SDS) available for use. 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 MSDS for potassium chlorate will be used to help guide the experimental study of its decomposition reactions. In general, prior to any chemical procedure, the relevant MSDS 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 MSDS and Safety Cards are available on-line through links found on the Plebe Chemistry homepage.
Figure 1. Examples of oxygen candles.
Various candle sizes are manufactured for different applications. While oxygen candles are most commonly used for emergency purposes on submarines, they are also used in spacecraft, refuge shelters in underground mines, and emergency shelters. One manufacturer claims that with a shelf life of 10 years, one oxygen candle produces enough O 2 to keep 15 people alive for 5.7 hours, assuming they are at rest (calculation based on 0.5 L per person per minute).
Clean Up:
- Place all used test tubes, including their contents, in the designated solid waste container in the laboratory. KClO 3 must NOT be disposed of in the trash, since it can react with combustibles.
Name _________________________________ Section________________________
Partner ________________________________ Date__________________________
DATA SECTION
Experiment 4
Part B. Determination of the Stoichiometry of the Decomposition Reaction of KClO 3
All masses should be from the analytical balance. Report units and proper significant figures.
Mass of empty test tube ________________
Mass of tube + KClO 3 ________________
Mass of KClO 3 in tube ________________
Mass of tube and total contents (including MnO 2 and glass wool)
Before heating ______________
After heating ______________
Mass of oxygen gas evolved _______________
Name _____________________________________ Section _____________
IN-LAB QUESTIONS
Experiment 4C
Complete these questions during lab.
- What was the fuel (combustible material) that was added to the hot KClO 3? What happened when it was dropped into the test tube?
- Was the process of combustion of the fuel endothermic (absorbs energy) or exothermic (releases energy)? What evidence do you have for that?
- What did you observe as you heated the mixture of KClO 3 and MnO 2? How could you identify when the decomposition process started and when it was complete?
- Consider the process of melting the solid KClO 3 : KClO 3 (s) KClO 3 (ℓ)
Is this process endothermic or exothermic? Explain your answer.
- Which of the two energy diagrams best describes the process of the melting of KClO 3 (s)? Circle it.
KClO 3 (s) KClO 3 (ℓ)
Energy Energy
KClO 3 (ℓ) KClO 3 (s)
- Which of the three reactions in the Pre-Lab best fits your experimental data (circle it)? Explain your answer. ? 3
(^2) molesKClO decomposed
molesOproduced
a. ___ KClO 3 (s) ___ KClO 2 (s) + _____ O 2 (g) __________
b. ___ KClO 3 (s) ___ KClO (s) + _____ O 2 (g) __________
c. ___ KClO 3 (s) ___ KCl (s) + _____ O 2 (g) __________
POST-LAB QUESTIONS
- Ignoring any side reactions and assuming the reaction occurs completely and by the stoichiometry determined in the experiment, how large (in kg) an oxygen candle (KClO 3 ) would be needed to supply 8 people with enough oxygen for 24 hours on a small submarine? Although this depends on the size of the person and their respiration rate (activity), according to NASA^1 , an average person needs about 0.84 kg of O 2 per day.
- Magnesium chlorate solid thermally decomposes to form magnesium chloride solid and oxygen gas. a. Write the balanced chemical reaction for this decomposition. Include the states.
b. If 2.50 g of magnesium chlorate is decomposed, assuming complete reaction, how many grams of oxygen gas are formed? Show your work. Report answer with proper significant figures.
(^1) Wieland, P.O., Designing for Human Presence in Space: An Introduction to Environment Control and Life
Support Systems, NASA Reference Publication 1324, 1994, pp. 6, 183-262.
Name ______________________________ Section ___________________________
PRE-LAB QUESTIONS Experiment 4C
Complete these questions before lab.
- Find the MSDS (or Safety Card or SDS) for potassium chlorate. a) Are there any health hazards associated with this material, and what conditions produce these hazards?
b) Under what conditions does it produce fire or explosion hazards?
c) Since this experiment involves high temperatures, what are the melting point and decomposition temperature for potassium chlorate? Include the units.
melting temperature _________ decomposition temperature _________
d) Based on these values, what will you see happening to the potassium chlorate solid as you begin heating it to high temperatures?
- Following are three possible reactions that could occur when a sample of KClO 3 (s) is heated. One goal of this lab is to determine the correct one. Balance all three reactions.
? 3
(^2) molesKClO decomposed
molesOproduced
a. ___ KClO 3 (s) ___ KClO 2 (s) + _____ O 2 (g) __________
b. ___ KClO 3 (s) ___ KClO (s) + _____ O 2 (g) __________
c. ___ KClO 3 (s) ___ KCl (s) + _____ O 2 (g) __________
- In the appropriate spaces above, fill in the values of the mole ratio (moles O 2 / moles KClO 3 ) based on the stoichiometric coefficients of the three balanced equations. Enter this information on page E4C-7 as well.
- How is the decomposition of an alkali metal chlorate used on Navy submarines?
