Textbook Question
a. Using the BDEs in Table 4-2 (page 167), compute the value of ΔH° for each step in the iodination of methane.
b. Compute the overall value of ΔH° for iodination.
6. Thermodynamics and Kinetics
Enthalpy
4:03 minutesProblem 19
Textbook Question
If the following reaction is favorable, what can we say about the sign of ∆H°? Explain your answer.
Verified step by step guidance1
Step 1: Understand the concept of enthalpy change (∆H°). ∆H° represents the change in enthalpy (heat content) during a chemical reaction under standard conditions. It indicates whether a reaction absorbs or releases heat.
Step 2: Recall the relationship between reaction favorability and ∆H°. A reaction is considered favorable if it proceeds spontaneously under given conditions. While spontaneity is primarily determined by Gibbs free energy (∆G°), ∆H° plays a role in determining ∆G° along with entropy (∆S°).
Step 3: Consider the sign of ∆H° for favorable reactions. If a reaction releases heat (exothermic), ∆H° is negative, which often contributes to making the reaction favorable. Conversely, if a reaction absorbs heat (endothermic), ∆H° is positive, but the reaction can still be favorable if entropy changes significantly.
Step 4: Analyze the context of the problem. Since the question asks about the sign of ∆H° for a favorable reaction, we can infer that a negative ∆H° (exothermic reaction) is more likely to contribute to favorability, as it lowers the energy of the system.
Step 5: Conclude that for a favorable reaction, the sign of ∆H° is typically negative, but it is important to consider other factors like entropy and temperature that influence the overall spontaneity (∆G°).
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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. A negative ∆H° indicates that a reaction releases heat (exothermic), while a positive ∆H° signifies that the reaction absorbs heat (endothermic). Understanding the sign of ∆H° is crucial for predicting the favorability of a reaction, as exothermic reactions are generally more favorable.
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Gibbs Free Energy (∆G)
Gibbs Free Energy (∆G) combines enthalpy and entropy to determine the spontaneity of a reaction. A reaction is considered favorable (spontaneous) when ∆G is negative. The relationship between ∆G, ∆H, and entropy (T∆S) is given by the equation ∆G = ∆H - T∆S, where T is the temperature in Kelvin. This concept is essential for understanding how enthalpy influences reaction favorability.
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Thermodynamic Favorability
Thermodynamic favorability refers to the likelihood of a reaction occurring under standard conditions. A reaction is thermodynamically favorable if it leads to a decrease in free energy (negative ∆G). This often correlates with a negative ∆H° for exothermic reactions, suggesting that heat is released, which can drive the reaction forward. Understanding this concept helps in predicting the behavior of chemical reactions.
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