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STOICHIOMETRY and THERMOCHEMISTRY of EXPLOSIVES: a Group Challenge Problem

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STOICHIOMETRY and THERMOCHEMISTRY of EXPLOSIVES: a Group Challenge Problem STOICHIOMETRY AND THERMOCHEMISTRY OF EXPLOSIVES: A Group Challenge Problem Ref.: Consult the Learning Objectives for the topic of “Explosives” on the Plebe Chemistry Webpage: http://www.usna.edu/ChemDept/plebeChem/ By this time in the General Chemistry course, you have developed skills in handling stoichiometry, in calculating energy changes during reactions, and in using the gas laws to describe physical parameters where appropriate. These skills permit examination of some of the phenomena associated with the performance of explosives. Clearly, this is an application of chemistry which is of significant interest in military operations, and which is important in the civilian sector as well. Introduction: An explosive is a material, either a compound or a mixture, that contains oxygen and fuel in a proportion that supports rapid combustion in an enclosed, airless space. An explosion generates gases that expand several thousand times over that of the original sample; this occurs in a fraction of a second, and usually at a high temperature. Because of this expansion, an explosion exerts a mechanical force sufficient to burst a sealed container or force a projectile out of a gun. There are two classifications of explosives, categorized according to the speed with which the reaction moves through the system. A low explosive deflagrates, meaning that it burns and causes heaving, but does not produce a shock wave. This is the explosive of choice for purposes such as the quarrying of large blocks of stone for building, and for use as a propellant in guns. Such materials are often mixtures of separate oxidizer and fuel compounds. A high explosive detonates, meaning the chemical reaction moves through the explosive at a speed greater than the speed of sound. To shatter rock into gravel, or in other applications (such as artillery blasts) where maximum destructive effect is desired, a high explosive would be chosen. Typically, high explosives are very reactive compounds that, in effect, contain their own oxygen and fuel within an individual molecule of the explosive. The different effects depend on how fast heat and gas are produced by the reaction; this, in turn, depends on how fast the reaction moves through the material. Three thousand feet per second is the speed that has been chosen to mark the division between a low explosive and a high explosive. Black powder, a mixture of potassium nitrate, charcoal, and sulfur, is a low explosive, used in guns and in quarrying rock for building stone. TNT (trinitrotoluene), and RDX (cyclotrimethylene trinitramine) are examples of high explosives. RDX is a military explosive with a detonation velocity as high as 27,000 fps. Interestingly, stoichiometry and energy considerations do not really distinguish the two types of explosives; both release large volumes of gas, and significant amounts of heat during the combustion. Basic principles of chemistry can be used to calculate such energy and volume changes that accompany the reaction of an explosive. Stoichiometry: Nearly all explosives in this exercise contain some combination of carbon, oxygen, hydrogen, and nitrogen. Since the explosion is a combustion, we should expect that typical combustion products will be formed. The actual products of any chemical reaction must be determined experimentally, but some generalizations can be used to predict the products of related reactions, such as those of explosives. Follow these guidelines in order: . All the nitrogen in the explosive is converted to nitrogen gas (N2). All the hydrogen in the explosive is converted to water vapor (steam). The carbon is usually converted to carbon dioxide. If not enough oxygen is present to convert all of the carbon to carbon dioxide, then some carbon monoxide forms. If there is not enough oxygen to accomplish that, then only carbon monoxide and soot (solid carbon), or soot alone, forms. 1 Oxygen Balance Zero Oxygen Balance: Explosive has just enough oxygen to convert all of its hydrogen to water and all of its carbon to carbon dioxide. Positive Oxygen Balance: Extra oxygen atoms remain after all of the hydrogen has been converted to water and all of the carbon to carbon dioxide. O2 is a product. Negative Oxygen Balance: There is insufficient oxygen in the explosive compound to convert all of the hydrogen and carbon to H2O and CO2, respectively. Thermochemistry: Enthalpy is the amount of heat absorbed or released at constant pressure. The heat is evidence that a change in the energy of the system has occurred. Even though an explosion does not occur at constant pressure, the enthalpy of formation for an explosive can be used (along with H values for other reactants and products) in a calculation of the energy change for the explosive reaction, as a first approximation. Bond dissociation energies can also be used. However, these are average values, based on bond strengths in many stable compounds. Because explosives are such reactive molecules, however, the use of standard bond dissociation energies may not give very accurate results. Still, they do provide an ‘order of magnitude’ estimate of the energy released during the reaction. In an explosion, much of the potential energy stored in the chemical bonds of the explosive material is converted into kinetic energy of the gaseous products formed. (Other ambient gases, such as N2 in air, would also absorb that heat.) To estimate the temperature that the gases could reach, we need a value of the heat capacity for these materials. For this exercise, a molar heat capacity of (7/2)R can be used for reactants or products that are gases, as a first approximation. This value is a general one; the actual heat capacity will vary with temperature and the nature of the gas. Gas Laws: In determination of pressures and volumes of gases, we will use the ideal gas law as a first approximation, even though the system is far from the ideal condition of one atmosphere pressure and room temperature. More exact equations can (and must) be applied if greater accuracy is required. DATA Explosive Hf° (kJ/mol) Density (g/mL) nitrogylcerine (C3H5O9N3) 380.1 1.591 HMX (C4H8O8N8) -75.0 1.90 PETN (C5H8O12N4) 506.7 1.77 TNT (C7H5O6N3) 74.6 1.65 Methane (CH4) -75 Ethylene oxide (C2H4O) -53 ANFO C10H22 -177 1.75 (mixture) NH4NO3 -365.6 stoichiometric 2 CH O 3 O C N C C N O C C O TNT H C H N O O H O O H C O N O H C O N Trinitroglycerine H C O N O O H O O O N O H H H N C N O H C N HMX N C H O N C N H H O H N O O O O N O H C H O H H O PETN N O C C C O N O H H O H C H O N O O 3 Explosives Project Report Submission Purpose: Perform the calculations needed to estimate (a) the pressure that would be generated upon detonation of 1.00 cm3 of an explosive if it were confined to its original volume and (b) the volume to which the product gases of the explosive will expand in returning to room temperature and pressure (1 atm, 25C. Report Submission: Memorandum (one-page should be sufficient, but it may be longer if needed) Completed Worksheet (see attached) One submission per group with all members the team providing a signature on the memo to indicate that they have equally contributed to the project. Memo Guidance: Paragraph 1: Introduction: State the purpose of the project and report the key findings (i.e., results, actual values). Paragraph 2: Approach: A narrative description of the strategy used to obtain the results. Provide a description of the process that leads from the initial assumptions to the final results explaining what intermediate results needed to be calculated along the way. Don’t provide values!! Describe the process. Paragraph 3: Conclusions: This is a critical evaluation of the results. Several simplifying assumptions have been employed in these calculations. State the approximations. Comment on the validity of each of these when applied to the problem of explosives (i.e., consider our discussions in class about these approximations and the conditions under which they are valid and the conditions under which they are likely to yield poor results). Peer Evaluation: Within a Team: Email to [email protected] with a brief statement of what each member of the team did and a ranking of the effort from 1 – 4. 1 = most significant effort on the team. 4 = least significant effort on the team. All four numbers must be used (no ties!). Between Teams. Each team is charged with assigning a grade to one other team’s memo. Team Bonus: A quiz on the topic of explosives will be given. Team bonuses on that quiz may be awarded if all members of a given team demonstrate high performance on the quiz. 4 SC111 – Explosives Module Worksheet Names _____________________________________ 1. a. Write the balanced chemical reaction for the decomposition of your explosive, using the assumptions presented in the Stoichiometry section (page 1) and the information above. Include the physical states (s, l, g). b. What type of oxygen balance does your explosive have? Zero, Positive, or Negative? 2. a. Using information from this document and your textbook, calculate Hrxn per mole of your explosive. Report your final answer with units of kJ/mol. Hrxn = b. Calculate the heat evolved, q, when 1.00 cm3 of the explosive reacted. Report with units of kJ. q = 5 3. Calculate the change in temperature (in Kelvin) of the gases formed by the explosion (using 1.00 cm3 of explosive), assuming that all the energy produced by the reaction appears as an increase in the temperature of the gases before they expand from their original volume.
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