Ch. 5 - Chemical Reaction Analysis: Thermodynamics and Kinetics

Mullins - Organic Chemistry: A Learner Centered Approach 1st Edition

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Mullins 1st Edition

Ch. 5 - Chemical Reaction Analysis: Thermodynamics and Kinetics

Problem 15d

Chapter 4, Problem 15d

Calculate ∆H° for the following equilibrium processes.
(d)

Verified step by step guidance

Step 1: Identify the two conformations of the molecule in the equilibrium process. In this case, the molecule is methylcyclohexane, and the equilibrium involves the axial and equatorial positions of the methyl group on the cyclohexane ring.

Step 2: Recall that the ∆H° for this equilibrium process is related to the energy difference between the axial and equatorial conformations. The equatorial position is generally more stable due to reduced steric hindrance.

Step 3: Use the standard value for the energy difference between axial and equatorial positions of a methyl group on cyclohexane. This value is typically around -7.6 kJ/mol, favoring the equatorial conformation.

Step 4: Write the expression for ∆H° for the equilibrium process. Since the equatorial conformation is more stable, the ∆H° will be negative, indicating that the equilibrium favors the equatorial conformation.

Step 5: Confirm the direction of equilibrium and the sign of ∆H° based on the stability of the conformations. The equatorial conformation is lower in energy, so the equilibrium constant will favor this conformation, and ∆H° will reflect this preference.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Enthalpy Change (∆H°)

Enthalpy change (∆H°) is a measure of the heat content of a system at constant pressure. It indicates whether a reaction is exothermic (releases heat, ∆H° < 0) or endothermic (absorbs heat, ∆H° > 0). Understanding how to calculate ∆H° is crucial for analyzing chemical reactions and their energy profiles.

Equilibrium Processes

Equilibrium processes refer to the state in which the rates of the forward and reverse reactions are equal, resulting in constant concentrations of reactants and products. The equilibrium constant (K) can be used to relate the concentrations of species at equilibrium, and changes in enthalpy can affect the position of equilibrium according to Le Chatelier's principle.

Hess's Law

Hess's Law states that the total enthalpy change for a reaction is the sum of the enthalpy changes for individual steps, regardless of the pathway taken. This principle allows for the calculation of ∆H° for complex reactions by breaking them down into simpler steps, making it essential for solving problems involving enthalpy changes in equilibrium processes.

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Related Practice

Textbook Question

Calculate ∆H° for the following equilibrium processes.

(a)

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Textbook Question

Which conformation in each of the following pairs has the least strain energy?

(a)

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Textbook Question

Calculate ∆H° for the following equilibrium processes.

(b)

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Textbook Question

Which conformation in each of the following pairs has the least strain energy?

(c)

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Textbook Question

Considering the process described in Assessment 5.13, will it be favored or disfavored at a temperature higher than the one you calculated? How about at a temperature below what you calculated?

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Textbook Question

Which conformation in each of the following pairs has the least strain energy?

(b)

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Calculate ∆H° for the following equilibrium processes.(d)

Verified video answer for a similar problem:

Key Concepts

Enthalpy Change (∆H°)

Equilibrium Processes

Hess's Law

Calculate ∆H° for the following equilibrium processes.
(d)