Textbook Question
a. Using bond-dissociation enthalpies from Table 4-2. (page 167), calculate the heat of reaction for each step in the free-radical bromination of methane
b. Calculate the overall heat of the reaction.
6. Thermodynamics and Kinetics
Enthalpy
4:44 minutesProblem 41c
Textbook Question
Use bond-dissociation enthalpies (Table 4-2, p. 167) to calculate values of ΔH° for the following reactions.
c. (CH3)3C—OH + HCl → (CH3)3C—Cl + H2O
Verified step by step guidance1
Identify the bonds broken and formed in the reaction. In this case, the bonds broken are the O-H bond in (CH3)3C—OH and the H-Cl bond in HCl. The bonds formed are the C-Cl bond in (CH3)3C—Cl and the O-H bond in H2O.
Look up the bond-dissociation enthalpies (BDEs) for each bond involved in the reaction. These values can be found in Table 4-2 on page 167. For example, the BDE for the O-H bond, H-Cl bond, C-Cl bond, and O-H bond in water should be noted.
Calculate the total energy required to break the bonds. This is done by summing the BDEs of the bonds broken: BDE(O-H in (CH3)3C—OH) + BDE(H-Cl).
Calculate the total energy released when the new bonds are formed. This is done by summing the BDEs of the bonds formed: BDE(C-Cl in (CH3)3C—Cl) + BDE(O-H in H2O).
Determine the enthalpy change (ΔH°) for the reaction by subtracting the total energy of the bonds formed from the total energy of the bonds broken: ΔH° = [Sum of BDEs of bonds broken] - [Sum of BDEs of bonds formed].
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Bond-Dissociation Enthalpy
Bond-dissociation enthalpy (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 thermodynamics and organic chemistry, 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.
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Enthalpy Change (ΔH°)
Enthalpy change (ΔH°) refers to the heat content change of a system at constant pressure during a chemical reaction. It can be calculated using the bond-dissociation enthalpies of the reactants and products. A negative ΔH° indicates that the reaction is exothermic (releases heat), while a positive ΔH° indicates an endothermic reaction (absorbs heat). Understanding ΔH° is essential for predicting reaction feasibility and energy requirements.
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Reaction Mechanism
A reaction mechanism describes the step-by-step sequence of elementary reactions by which overall chemical change occurs. It provides insight into how reactants transform into products, including the formation and breaking of bonds. In the context of the given reaction, understanding the mechanism helps in identifying the bonds involved and calculating the ΔH° accurately by considering the specific bonds broken and formed during the reaction.
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