Small molecules with C'C double bonds, called monomers, can join with one another to form long chain molecules called polymers. Thus, acrylonitrile, H2C'CHCN, polymerizes under appropriate conditions to give polyacrylonitrile, a common starting material for producing the carbon fibers used in composites. (b) Use the bond dissociation energies in Table 7.1 to calculate ΔH per H2C'CHCN unit for the conversion of acrylonitrile to polyacrylonitrile. Is the reaction endothermic or exothermic?

Verified step by step guidance

1. Step 1: Identify the bonds that are broken and formed during the polymerization of acrylonitrile to polyacrylonitrile. In this case, the C=C double bond in acrylonitrile is broken and a C-C single bond is formed in the polymer.
2. Step 2: Look up the bond dissociation energies for the bonds involved in the reaction in Table 7.1. You will need the energy for breaking a C=C double bond and forming a C-C single bond.
3. Step 3: Calculate the energy change for the breaking of the bonds in the reactants. This is done by multiplying the number of each type of bond broken by the bond dissociation energy for that bond. The total energy required to break the bonds is the sum of these values.
4. Step 4: Calculate the energy change for the formation of the bonds in the products. This is done by multiplying the number of each type of bond formed by the bond dissociation energy for that bond. The total energy released in forming the bonds is the sum of these values.
5. Step 5: Calculate ΔH for the reaction by subtracting the total energy required to break the bonds in the reactants from the total energy released in forming the bonds in the products. If ΔH is negative, the reaction is exothermic (releases heat). If ΔH is positive, the reaction is endothermic (absorbs heat).

Key Concepts

Bond Dissociation Energy

Bond dissociation energy (BDE) is the energy required to break a specific bond in a molecule, resulting in the formation of free radicals. It is a crucial concept in thermochemistry, as it helps determine the stability of molecules and the energy changes during chemical reactions. In the context of polymerization, knowing the BDE of the bonds involved allows for the calculation of the overall energy change (ΔH) when monomers like acrylonitrile are converted into polymers.

Polymerization

Polymerization is the chemical process through which monomers, small molecules with reactive functional groups, chemically bond to form a polymer, a large and complex molecule. This process can occur through various mechanisms, including addition and condensation polymerization. Understanding the type of polymerization and the conditions required is essential for predicting the properties of the resulting polymer, such as polyacrylonitrile, and its applications in materials science.

Enthalpy Change (ΔH)

Enthalpy change (ΔH) is a measure of the heat content of a system at constant pressure, indicating whether a reaction absorbs or releases energy. A negative ΔH signifies an exothermic reaction, where energy is released, while a positive ΔH indicates an endothermic reaction, where energy is absorbed. Calculating ΔH for the polymerization of acrylonitrile involves summing the bond dissociation energies of the bonds broken and formed, providing insight into the energy dynamics of the reaction.