Spontaneity
Overview
In thermodynamics, spontaneity refers to the ability of a process to occur on its own, without external assistance. This section provides a comprehensive analysis of the factors that influence spontaneous processes, primarily focusing on the concepts of entropy, enthalpy, and Gibbs free energy.
Key Points
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Spontaneous Processes: These are reactions that can occur without external energy. For instance, the expansion of a gas into a vacuum is spontaneous due to its natural tendency to occupy all available space.
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Heat Flow Direction: Spontaneous processes, such as heat transfer, occur in one direction—from higher to lower temperature. Non-spontaneous processes, such as heat flowing from a cooler to a warmer body, do not happen without external work.
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Entropic Consideration: The notion of entropy (
S
), a measure of disorder, is crucial in determining spontaneity. Generally, the total entropy of an isolated system tends to increase, suggesting a natural direction towards disorder.
-
Gibbs Free Energy: Defined as
G = H - TS
, where
H
is enthalpy and
T
is temperature. The change in Gibbs free energy (
ΔG
) is a major determinant of spontaneity: if
ΔG < 0
, the process is spontaneous.
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Relation Between ΔH and ΔS: Although a decrease in enthalpy often favors spontaneity, there are endothermic reactions (with positive ΔH) that are spontaneous due to sufficiently large increases in entropy.
Conclusion
Understanding spontaneity is essential for chemical thermodynamics, as it helps predict the feasibility of reactions based on their enthalpy and entropy changes. The use of Gibbs free energy unifies these concepts, offering a clear criterion for spontaneous processes.