Use bond-dissociation enthalpies [TABLE 4-2], p. 167) to calculate values of ΔH° for the following reactions.a. CH3—CH3 + I2 —> CH3CH2I + HI

Verified step by step guidance

1. Identify the bonds broken and formed in the reaction: CH3—CH3 + I2 —> CH3CH2I + HI.
2. Calculate the total energy required to break the bonds in the reactants: C—C bond in ethane and I—I bond in iodine.
3. Calculate the total energy released when new bonds are formed in the products: C—I bond in ethyl iodide and H—I bond in hydrogen iodide.
4. Use the bond-dissociation enthalpies from the table to find the energy values for each bond broken and formed.
5. Apply the formula for the change in enthalpy: \( \Delta H^\circ = \text{(sum of bond energies of bonds broken)} - \text{(sum of bond energies of bonds formed)} \).

Key Concepts

Bond-Dissociation Enthalpy

Bond-dissociation enthalpy (BDE) is the energy required to break a specific bond in a molecule, resulting in the formation of two radicals. It is a crucial concept in thermochemistry, as it helps predict the stability of molecules and the energy changes during chemical reactions. BDE values are typically provided in kilojoules per mole (kJ/mol) and vary depending on the type of bond and the molecular environment.

Enthalpy Change (ΔH°)

The enthalpy change (ΔH°) of a reaction is the difference in enthalpy between the products and reactants under standard conditions. It indicates whether a reaction is exothermic (releases heat, ΔH° < 0) or endothermic (absorbs heat, ΔH° > 0). Calculating ΔH° using bond-dissociation enthalpies involves summing the BDEs of bonds broken in the reactants and subtracting the BDEs of bonds formed in the products.

Reaction Mechanism

A reaction mechanism describes the step-by-step sequence of elementary reactions by which a chemical change occurs. Understanding the mechanism is essential for predicting the products and energy changes in a reaction. In the given reaction, the mechanism involves the homolytic cleavage of the I—I bond and the formation of new C—I and H—I bonds, which can be analyzed using bond-dissociation enthalpies.