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We can assess the spontaneity of the process by calculating the entropy change of the universe. If Δ S univ is positive, then the process is spontaneous. At both temperatures, Δ S sys = 22.1 J/K and q surr = −6.00 kJ.
The spontaneous process whereby the gas contained initially in one flask expands to fill both flasks equally therefore yields an increase in entropy for the system. Figure \(\PageIndex{3}\): This shows a microstate model describing the flow of heat from a hot object to a cold object.
Key points. The second law of thermodynamics says that the entropy of the universe always increases for a spontaneous process: Δ S universe = Δ S system + Δ S surroundings > 0. . At constant temperature and pressure, the change in Gibbs free energy is defined as Δ G = Δ H − T Δ S. .
However, entropy is a state function; using the figure of speech that we introduce in Section 7.21, we can find the entropy change for the spontaneous process by evaluating \(\Delta S\) along a second and reversible path that connects the same initial and final states.
Describe the dispersal of matter and energy that accompanies certain spontaneous processes. In this section, consider the differences between two types of changes in a system: Those that occur spontaneously and those that occur only with the continuous input of energy.
The second law of thermodynamics states that the total entropy of a system either increases or remains constant in any spontaneous process; it never decreases. An important implication of this law is that heat transfers energy spontaneously from higher- to lower-temperature objects, but never spontaneously in the reverse direction.
We find that these questions of uphill and downhill nature of a reaction can be answered using concepts such as spontaneity, entropy, and free energy. Chemical reactions which proceed downhill, are said to be spontaneous.
