Chapter Summary – Chapter 19 – Thermodynamics ....from your text Central Science
Introduction and Section 19.1 In this chapter we examined some of the aspects of chemical thermodynamics, the area of chemistry that explores energy relationships. Most reactions and chemical processes have an inherent directionality: They are spontaneous in one direction and nonspontaneous in the reverse direction. The spontaneity of a process is related to the thermodynamic path the system takes from the initial state to the final state. In a reversible process, both the system and its surroundings can be restored to their original state by exactly reversing the change. In an irreversible process the system can't return to its original state without there being a change in the surroundings. Any spontaneous process is irreversible. A process that occurs at a constant temperature is said to be isothermal.
Section 19.2 The spontaneous nature of processes is related to a thermodynamic state function called entropy, denoted S. For a process that occurs at constant temperature, the entropy change of the system is given by the heat absorbed by the system along a reversible path, divided by the temperature: The way entropy controls the spontaneity of processes is given by the second law of thermodynamics, which governs the change in the entropy of the universe, The second law states that in a reversible process in an irreversible (spontaneous) process Entropy values are usually expressed in units of joules per kelvin,
Section 19.3 Molecules can undergo three kinds of motion: In translational motion the entire molecule moves in space. Molecules can also undergo vibrational motion, in which the atoms of the molecule move toward and away from one another in periodic fashion, and rotational motion, in which the entire molecule spins like a top. A particular combination of motions and locations of the atoms and molecules of a system at a particular instant is called a microstate. Entropy is a measure of how many microstates are associated with a particular macroscopic state. If the total number of accessible microstates is W, the entropy is given by The number of available microstates, and therefore the entropy, increases with an increase in volume, temperature, or motion of molecules, because any of these changes increases the possible motions and locations of the molecules. As a result, entropy generally increases when liquids or solutions are formed from solids, gases are formed from either solids or liquids, or the number of molecules of gas increases during a chemical reaction. The third law of thermodynamics states that the entropy of a pure crystalline solid at 0 K is zero.
Section 19.4 The third law allows us to assign entropy values for substances at different temperatures. Under standard conditions the entropy of a mole of a substance is called its standard molar entropy, denoted S°. From tabulated values of S°, we can calculate the entropy change for any process under standard conditions. For an isothermal process, the entropy change in the surroundings is equal to
Section 19.5 The Gibbs free energy (or just free energy), G, is a thermodynamic state function that combines the two state functions enthalpy and entropy: For processes that occur at constant temperature, For a process or reaction occurring at constant temperature and pressure, the sign of relates to the spontaneity of the process. When is negative, the process is spontaneous. When is positive, the process is nonspontaneous; the reverse process is spontaneous. At equilibrium the process is reversible and is zero. The free energy is also a measure of the maximum useful work that can be performed by a system in a spontaneous process. The standard free-energy change, for any process can be calculated from tabulations of standard free energies of formation, which are defined in a fashion analogous to standard enthalpies of formation, The value of for a pure element in its standard state is defined to be zero.
Sections 19.6 and 19.7 The values of and generally do not vary much with temperature. As a consequence, the dependence of with temperature is governed mainly by the value of T in the expression The entropy term has the greater effect on the temperature dependence of and, hence, on the spontaneity of the process. For example, a process for which and such as the melting of ice, can be nonspontaneous at low temperatures and spontaneous at higher temperatures. Under nonstandard conditions is related to and the value of the reaction quotient, Q: At equilibrium Thus, the standard free-energy change is directly related to the equilibrium constant for the reaction. This relationship can be used to explain the temperature dependence of equilibrium constants.