![]() ![]() The state variable "entropy" was introduced by Rudolf Clausius in 1865, see the inset for his text, when he gave a mathematical formulation of the second law of thermodynamics. I have deliberately constructed the word entropy to resemble as much as possible the word energy, since both quantities to be named by these words are so closely related in their physical meaning that a certain similarity in their names seems appropriate to me. As I deem it better to derive the names of such quantities - that are so important for science - from the antique languages, so that they can be used without modification in all modern languages, I propose to call the quantity S the entropy of the body, after the Greek word for transformation, ἡ τροπή. The same statements can be made about the free energy change for vaporization below and above the boiling point of water.Translation: Searching for a descriptive name for S, one could - like it is said of the quantity U that it is the heat and work content of the body - say of the quantity S that it is the transformation content of the body. Above the melting point, because of the raised temperature, the changes in enthalpy and entropy combine to produce a negative change in the free energy for melting, so melting is spontaneous (favorable). Below the melting point, the changes in enthalpy and entropy combine to produce a positive change in free energy for melting, so melting is nonspontaneous (unfavorable). When water changes from a solid to a liquid (melting), or from a liquid to a gas (vaporization), the change in entropy is also positive. When heat is added to a substance the change in enthalpy is positive. The is positive because the water becomes more disorder as it change from a solid to a liquid, and from a liquid to a gas (See Figure 1)įigure 1: The the heating of water. The is positive because heat is being absorbed. A good example of the last situation where both and are positive, is the heating of solid water (ice) to convert it first to a liquid and then to a gas. The "| |" brackets mean "the absolute value of". The conditions for and, which make a process spontaneous or not The conditions for, which make a process spontaneous or not The following tables lay out the conditions for when a process is spontaneous and when it is not. Where is the change in the free energy, is the change in the enthalpy, is the change in the entropy, and T is the absolute temperature in Kelvin. This is summarized in the following equation: The free energy change combines the enthalpy change and the entropy change together, along with the temperature, to produce a quantity that can be used to determine if a process is spontaneous or not. For that you need to determine the change in the free energy. But like enthalpy, changes in entropy alone cannot be used to predict whether an overall change is spontaneous. When a change entropy is positive, it makes the change more spontaneous (favorable). Entropy is a measure of disorder when a system become more disordered, the change in entropy is positive. There is another factor that must be considered and that is the entropy. While a decrease in the enthalpy makes a process more spontaneous (favorable), the change in enthalpy alone cannot be used to predict whether an overall change is spontaneous. When heat is released, the change in the enthalpy for the system that is releasing the heat decreases, whereas when heat is absorbed, the change in the enthalpy increases. Spontaneous changes are ones in which the free energy of a system decreases. ![]()
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