Question

The compound hexaazaisowurtzitane is one of the highestenergy explosives known ( \(C\) \& E News, Jan. 17, 1994, p. 26). The compound, also known as CL-20, was first synthesized in 1987 . The method of synthesis and detailed performance data are still classified because of CL-20's potential military application in rocket boosters and in warheads of "smart" weapons. The structure of CL-20 is In such shorthand structures, each point where lines meet represents a carbon atom. In addition, the hydrogens attached to the carbon atoms are omitted; each of the six carbon atoms has one hydrogen atom attached. Finally, assume that the two \(\mathrm{O}\) atoms in the \(\mathrm{NO}_{2}\) groups are attached to \(\mathrm{N}\) with one single bond and one double bond. Three possible reactions for the explosive decomposition of \(\mathrm{CL}-20\) are i. \(\mathrm{C}_{6} \mathrm{H}_{6} \mathrm{~N}_{12} \mathrm{O}_{12}(s) \rightarrow 6 \mathrm{CO}(g)+6 \mathrm{~N}_{2}(g)+3 \mathrm{H}_{2} \mathrm{O}(g)+\frac{3}{2} \mathrm{O}_{2}(g)\) ii. \(\mathrm{C}_{6} \mathrm{H}_{6} \mathrm{~N}_{12} \mathrm{O}_{12}(s) \rightarrow 3 \mathrm{CO}(g)+3 \mathrm{CO}_{2}(g)+6 \mathrm{~N}_{2}(g)+3 \mathrm{H}_{2} \mathrm{O}(g)\) iii. \(\mathrm{C}_{6} \mathrm{H}_{6} \mathrm{~N}_{12} \mathrm{O}_{12}(s) \rightarrow 6 \mathrm{CO}_{2}(g)+6 \mathrm{~N}_{2}(g)+3 \mathrm{H}_{2}(g)\) a. Use bond energies to estimate \(\Delta H\) for these three reactions. b. Which of the above reactions releases the largest amount of energy per kilogram of CL-20?

Short answer

The enthalpy changes (ΔH) for the three reactions are as follows: (i) -4091 kJ/mol, (ii) -3117 kJ/mol, and (iii) -2847 kJ/mol. When comparing the energy release per kilogram of CL-20, reaction (i) releases the most energy with -9346 kJ/kg.

Step-by-step solution

Reaction (i)

For reaction (i): 1 mole of CL-20 has the following bonds: 6 C-H, 6 N-N (single), 6 N-N (triple), 6 N-O(single), and 6 N-O(double) bonds. ΔH = bonds broken - bonds formed ΔH = [(6 × C-H) + (6 × N-N(single)) + (6 × N-N(triple)) + (6 × N-O(single)) + (6 × N-O(double))] - [(6 × C=O) + (6 × N-N(triple)) + (3 × O=O)] Plug in the bond energies: ΔH = [(6 × 413) + (6 × 163) + (6 × 941) + (6 × 201) + (6 × 607)] - [(6 × 1072) + (6 × 941) + (3 × 498)] ΔH = -4091 kJ/mol

Reaction (ii)

For reaction (ii): ΔH = [(6 × C-H) + (6 × N-N(single)) + (6 × N-N(triple)) + (6 × N-O(single)) + (6 × N-O(double))] - [(3 × C=O) + (3 × C-O(double)) + (6 × N-N(triple)) + (3 × O=O)] ΔH = [(6 × 413) + (6 × 163) + (6 × 941) + (6 × 201) + (6 × 607)] - [(3 × 1072) + (3 × 799) + (6 × 941) + (3 × 498)] ΔH = -3117 kJ/mol

Reaction (iii)

For reaction (iii): ΔH = [(6 × C-H) + (6 × N-N(single)) + (6 × N-N(triple)) + (6 × N-O(single)) + (6 × N-O(double))] - [(6 × C-O(double)) + (6 × N-N(triple))] ΔH = [(6 × 413) + (6 × 163) + (6 × 941) + (6 × 201) + (6 × 607)] - [(6 × 799) + (6 × 941)] ΔH = -2847 kJ/mol #Step 3: Calculate energy release per kilogram of CL-20# We need to find which reaction releases the largest amount of energy per kilogram of CL-20. To do this, we'll use the molar mass of CL-20, which is 438 g/mol. a) Energy release per kg for reaction (i): (-4091 kJ/mol) * (1000 g/kg) / (438 g/mol) = -9346 kJ/kg b) Energy release per kg for reaction (ii): (-3117 kJ/mol) * (1000 g/kg) / (438 g/mol) = -7114 kJ/kg c) Energy release per kg for reaction (iii): (-2847 kJ/mol) * (1000 g/kg) / (438 g/mol) = -6499 kJ/kg #Step 4: Determine the reaction with the largest energy release per kg# Comparing the energy releases for each reaction, we find that reaction (i) releases the most energy per kilogram of CL-20, as it has an energy release of -9346 kJ/kg.

Key concepts

Chemistry of Explosives

Explosives are substances that undergo rapid chemical reactions with the production of gases and the release of heat and pressure. They are designed to release a tremendous amount of energy in a very short time. This characteristic makes explosives valuable in a variety of applications, from mining and demolition to military uses.

The explosive properties of a substance like hexaazaisowurtzitane (also known as CL-20) are directly related to its chemical structure. CL-20 has a complex caged structure consisting of carbon (C), hydrogen (H), nitrogen (N), and oxygen (O) atoms. The energy of the substance is partially stored in the bonds between these atoms. During decomposition, these bonds break and new bonds form, releasing energy in the process.

To ensure we're clear on the concept, in the decomposition of CL-20, three possible reactions were listed, each producing different gases and varying amounts of energy released. The specifics of how these bonds break and form during the reactions, and the associated energy changes, is what chemistry students must learn to predict the outcome of such explosive reactions.

Bond Energy Calculations

When we talk about bond energies in chemistry, we're referring to the measure of the strength of a chemical bond. Specifically, bond energy is the amount of energy required to break one mole of the chemical bond in the gas phase. During chemical reactions, such as the explosive decomposition of CL-20, bonds are both broken and formed.

To analyze these reactions, we perform bond energy calculations. By adding up the energies of the bonds broken and subtracting the energies of the bonds formed, we can estimate the change in enthalpy \( \Delta H \) of the reaction. If more energy is required to break the bonds than is released when new bonds are formed, the reaction absorbs energy and \( \Delta H \) is positive, indicating an endothermic reaction. Conversely, if less energy is needed to break the bonds than is released upon formation of new bonds, \( \Delta H \) is negative, and the reaction is exothermic. In the case of explosives, an exothermic reaction is desired to release energy rapidly.

The given problem walked through this process step by step for CL-20, demonstrating how to use bond energies to determine \( \Delta H \) for each possible reaction. It emphasized the detailed calculations needed to predict whether and how much energy would be released in each explosive decomposition scenario.

Enthalpy Changes

Enthalpy change, commonly denoted as \( \Delta H \), is a measure of heat change during a chemical reaction at constant pressure. It is a central concept in thermodynamics and chemical kinetics, giving insights into the energy balance of reactions. For explosives like CL-20, the enthalpy change indicates the amount of energy released upon explosion.

A negative \( \Delta H \) value signifies that the reaction is exothermic and energy is released to the surroundings. This is the typical case with explosive reactions where the reactants contain more energy than the products. The difference in energy is released in the form of heat, light, sound, and the kinetic energy of the released gases. The larger the magnitude of \( \Delta H \), the more energy is released.

In the worked example, energy per kilogram calculations are used to determine which reaction is most efficient in terms of energy release. This is vital because it allows for the comparison of different explosive materials and their potential applications. The calculations showed that the first reaction released the most energy per kilogram of CL-20, thus being the most powerful amongst those considered. Understanding these enthalpy changes is crucial for effectively harnessing the power of explosives and ensuring safety when they are used.