To further understand how the various components of Δ G dictate whether a process occurs spontaneously, we now look at a simple and familiar physical change: the conversion of liquid water to water vapor. If Δ G 0, the process is not spontaneous as written but occurs spontaneously in the reverse direction.For a system at constant temperature and pressure, we can summarize the following results: (This substance destroyed Texas City, Texas, in 1947 see Chapter 3 "Chemical Reactions", Section 3.3.1 "Interpreting Chemical Equations".) Ammonium nitrate was also used to destroy the Murrah Federal Building in Oklahoma City, Oklahoma, in 1995. An example of such a process is the decomposition of ammonium nitrate fertilizer. Multiplying both sides of the inequality by − T reverses the sign of the inequality rearranging, Δ H sys − T Δ S sys > 0) would therefore occur spontaneously. We have stated that for a spontaneous reaction, Δ S univ > 0, so substituting we obtain Δ S univ = Δ S sys + Δ S surr > 0 = Δ S sys − Δ H sys T > 0 Thus the entropy change of the surroundings is related to the enthalpy change of the system. Although very few chemical reactions actually occur under conditions of constant temperature and pressure, most systems can be brought back to the initial temperature and pressure without significantly affecting the value of thermodynamic state functions such as G. The criterion for predicting spontaneity is based on Δ G, the change in G, at constant temperature and pressure. Gibbs, whose work was translated into French by Le Châtelier, lived with his sister and brother-in-law until his death in 1903, shortly before the inauguration of the Nobel Prizes. His series of papers entitled “On the Equilibrium of Heterogeneous Substances” was the foundation of the field of physical chemistry and is considered one of the great achievements of the 19th century. He was appointed professor of mathematical physics at Yale in 1871, the first such professorship in the United States. In 1863, Gibbs was awarded the first engineering doctorate granted in the United States. A criterion of spontaneity that is based solely on the state functions of a system would be much more convenient and is provided by a new state function: the Gibbs free energy.īorn in Connecticut, Josiah Willard Gibbs attended Yale, as did his father, a professor of sacred literature at Yale, who was involved in the Amistad trial. This is not particularly useful for two reasons: we are normally much more interested in the system than in the surroundings, and it is difficult to make quantitative measurements of the surroundings (i.e., the rest of the universe). Unfortunately, using Δ S univ requires that we calculate Δ S for both a system and its surroundings. The sign of Δ S univ is a universally applicable and infallible indicator of the spontaneity of a reaction. Conversely, if Δ S univ < 0, a process cannot occur spontaneously if Δ S univ = 0, the system is at equilibrium. We have developed one such criterion, the change in entropy of the universe: if Δ S univ > 0 for a process or a reaction, then the process will occur spontaneously as written. One of the major goals of chemical thermodynamics is to establish criteria for predicting whether a particular reaction or process will occur spontaneously.
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