Textbook Question
Without concerning yourself with the mechanism of the reaction, calculate the equilibrium constant for the following equilibrium processes. (Assume T = 298 K.)
(a)
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6. Thermodynamics and Kinetics
Enthalpy
4:19 minutesProblem 47a
Textbook Question
Calculate the ∆H° value for the following reaction:
Verified step by step guidance1
Step 1: Identify the bonds broken and formed in the reaction. In the given reaction, CH₄ reacts with Cl₂ under UV light to form CH₃Cl and HCl. Bonds broken: one C-H bond in CH₄ and one Cl-Cl bond in Cl₂. Bonds formed: one C-Cl bond in CH₃Cl and one H-Cl bond in HCl.
Step 2: Use bond dissociation energy (BDE) values to calculate the energy required to break the bonds. Look up the BDE values for the C-H bond in CH₄ and the Cl-Cl bond in Cl₂. For example, the BDE for a C-H bond in CH₄ is approximately 412 kJ/mol, and for a Cl-Cl bond, it is approximately 243 kJ/mol.
Step 3: Use bond dissociation energy values to calculate the energy released when bonds are formed. Look up the BDE values for the C-Cl bond in CH₃Cl and the H-Cl bond in HCl. For example, the BDE for a C-Cl bond is approximately 338 kJ/mol, and for an H-Cl bond, it is approximately 431 kJ/mol.
Step 4: Calculate the total energy change (∆H°) for the reaction using the formula: ∆H° = (Sum of BDEs of bonds broken) - (Sum of BDEs of bonds formed). Substitute the values from Steps 2 and 3 into this formula.
Step 5: Interpret the sign of ∆H°. If the value of ∆H° is negative, the reaction is exothermic, meaning it releases energy. If the value is positive, the reaction is endothermic, meaning it absorbs energy.
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4mKey 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 using standard enthalpies of formation for the reactants and products, applying the formula ∆H° = Σ(∆H°f products) - Σ(∆H°f reactants).
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Bond Dissociation Energy
Bond dissociation energy is the energy required to break a specific bond in a molecule, resulting in the formation of free radicals or atoms. In the context of the given reaction, understanding the bond energies of C-H and Cl-Cl bonds is crucial for calculating the overall energy change during the reaction. The difference in energy between the bonds broken and formed determines the reaction's enthalpy change.
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Photochemical Reactions
Photochemical reactions are chemical reactions that are initiated by the absorption of light. In the provided reaction, the presence of light (hν) facilitates the breaking of the Cl-Cl bond, leading to the formation of chlorine radicals that react with methane (CH4). Understanding the role of light in these reactions is essential for analyzing the mechanism and energy changes involved.
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