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Ch. 5 - Chemical Reaction Analysis: Thermodynamics and Kinetics
Mullins - Organic Chemistry: A Learner Centered Approach 1st Edition
Mullins1st EditionOrganic Chemistry: A Learner Centered ApproachISBN: 9780137566471Not the one you use?Change textbook
All textbooksMullins 1st EditionCh. 5 - Chemical Reaction Analysis: Thermodynamics and KineticsProblem 37a
Chapter 4, Problem 37a

Calculate ∆H° for the following reactions.
(a) Chemical reaction equation showing reactants and products with methane and hydrogen.

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Step 1: Identify the bonds broken and formed in the reaction. In the given reaction, bonds in the reactants are broken, and new bonds are formed in the products. Specifically, a C-H bond in CH4 is broken, and a C-H bond in the alkane is formed.
Step 2: Use bond dissociation energy (BDE) values to calculate the energy change for breaking and forming bonds. Bond dissociation energy is the energy required to break a bond. Look up the BDE values for the C-H bond in CH4 and the C-H bond in the alkane.
Step 3: Calculate the total energy required to break the bonds in the reactants. Sum the BDE values for all bonds broken in the reaction.
Step 4: Calculate the total energy released when new bonds are formed in the products. Sum the BDE values for all bonds formed in the reaction.
Step 5: Determine ∆H° for the reaction by subtracting the total energy released (from Step 4) from the total energy required (from Step 3). Use the formula: ∆H° = Σ(Bonds Broken) - Σ(Bonds Formed).

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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). Calculating ∆H° for a reaction involves determining the difference in enthalpy between products and reactants, often using standard enthalpy values.
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Reaction Mechanism

A reaction mechanism describes the step-by-step sequence of elementary reactions by which overall chemical change occurs. Understanding the mechanism helps in predicting the products and the energy changes involved. In the given reaction, knowing how the alkene interacts with methane to produce another alkene and hydrogen is crucial for calculating ∆H°.
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Bond Energies

Bond energies refer to the amount of energy required to break a bond between two atoms. In thermodynamic calculations, the total energy required to break bonds in the reactants minus the energy released when new bonds form in the products gives the overall enthalpy change. This concept is essential for calculating ∆H° in the provided reaction, as it involves breaking and forming specific bonds.
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Related Practice
Textbook Question

Calculate ∆H° for the following reactions.

(c) CH3CH3 + HOOH → CH3CH2OH + H2O

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Textbook Question

Calculate ∆H° for the following reactions.

(b) CH3Br + HCl → CH3Cl + HBr

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Textbook Question

Write the rate law for the following reaction and identify which molecules are present in the rate-determining step. Draw a possible transition state and propose a mechanism.


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Textbook Question

Using bond-dissociation energies, identify the most stable radical. Justify the difference in stability based on the structure.

(a)

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Textbook Question

Third-order reactions are rare. Why do you think that is?

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Textbook Question

All things being equal, would you expect a first-order reaction to be faster or slower than a second-order reaction?

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