Textbook Question
Isooctane, C8H18, is the component of gasoline from which the term octane rating derives (b) The standard molar heat of combustion of isooctane (l) is -5461 kJ/mol. Calculate ΔH°f for isooctane(l)
Ch.9 - Thermochemistry: Chemical Energy
Chapter 9, Problem 123
What are the two terms that make up the free-energy change for a reaction, ΔG, and which of the two is usually more important?
Verified step by step guidance1
Step 1: Understand that the free-energy change for a reaction, denoted as ΔG, is a thermodynamic quantity that predicts the direction of a chemical reaction and its spontaneity.
Step 2: Recognize that ΔG is composed of two main terms: the enthalpy change (ΔH) and the entropy change (ΔS), which are related by the equation ΔG = ΔH - TΔS, where T is the temperature in Kelvin.
Step 3: The enthalpy change (ΔH) represents the heat absorbed or released during a reaction at constant pressure. It indicates whether a reaction is exothermic (releases heat) or endothermic (absorbs heat).
Step 4: The entropy change (ΔS) measures the change in disorder or randomness of the system. A positive ΔS indicates an increase in disorder, while a negative ΔS indicates a decrease.
Step 5: Consider that the relative importance of ΔH and TΔS depends on the specific reaction and conditions, but generally, at lower temperatures, ΔH tends to be more significant, while at higher temperatures, TΔS can become more influential.
Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Gibbs Free Energy (ΔG)
Gibbs Free Energy (ΔG) is a thermodynamic potential that measures the maximum reversible work obtainable from a thermodynamic system at constant temperature and pressure. It is a crucial indicator of the spontaneity of a reaction; a negative ΔG suggests that a reaction can occur spontaneously, while a positive ΔG indicates non-spontaneity.
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Gibbs Free Energy of Reactions
Enthalpy (ΔH)
Enthalpy (ΔH) is a measure of the total heat content of a system. It reflects the energy required to break and form bonds during a chemical reaction. In the context of Gibbs Free Energy, ΔH contributes to the overall energy change, influencing whether a reaction is exothermic or endothermic, which can affect the spontaneity of the reaction.
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Entropy (ΔS)
Entropy (ΔS) is a measure of the disorder or randomness in a system. In thermodynamics, it plays a critical role in determining the spontaneity of a reaction. Generally, an increase in entropy (positive ΔS) favors spontaneity, and in many cases, the contribution of entropy to ΔG is considered more significant than that of enthalpy, especially at higher temperatures.
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Related Practice
Textbook Question
Isooctane, C8H18, is the component of gasoline from which the term octane rating derives. (a) Write a balanced equation for the combustion of isooctane(l) with O2 to yield CO2(g) and H2O(l)
Textbook Question
What does entropy measure?
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Textbook Question
How is it possible for a reaction to be spontaneous yet endothermic?
Textbook Question
Tell whether the free-energy changes, ΔG, for the processes listed in Problem 9.127 are likely to be positive, negative, or zero.