Thermal, Mechanical, and Chemical Equilibria

Besides thermal equilibrium, we can expand this discussion to mechanical and chemical equilibria, as follows. Assume that the separation wall between the two systems Ai and A2 is a movable piston. We know empirically that the piston will move until the systems reach mechanical equilibrium. Although the volumes of each of the two... 

The thermodynamic stability of heterogeneous systems in the condensed state requires the existence of mechanical, thermal, and chemical equilibria. Chemical equilibrium implies that the chemical potential of each component in the system is the same in both the droplet interphase and the massive phase. Therefore, the thermodynamic equilibrium condition imposes a continuous exchange of matter within the system. 

In inert systems, spreading terminates in a minimum energy state marked by mechanical and thermal equilibrium. Equilibrium is typically achieved on the order of seconds for low-viscosity liquids such as liquid metals. However, in reactive systems, spreading terminates in a minimum energy state marked by mechanical, thermal, and chemical equilibria. Attainment of chemical equilibrium in metal-metal systems can take dramatically longer than mechanical and thermal equilibria. Chemical equilibrium is marked by the equilibration of chemical potentials of a given species among all phases within the system. 

The equations developed in preceding sections are for PVT systems in states of internal equilibrium. The criteria for internal thermal and mechanical equilibrium are well known, and need not be discussed in detail. They simply require uniformity of temperature and pressure throughout the system. The criteria for phase and chemical-reaction equilibria are less obvious. 

Classical thermodynamics deals with macroscopic thermal-mechanical properties and their relationships for massive assemblages of atoms or molecules (i.e., 10 fundamental particles) in terms of energy conversion and transformation. Studies of phase/chemical changes and equilibria involving nanoparticles are important areas where the classical thermodynamic approach is effective. Because quantum mechanical effects may be marked (e g., the energy of a nanoparticle may not be continuous) where there are only several hundred (or even only tens) of atoms in a nanoparticle, one may ask, Is classical thermodynamics still valid for nanoparticle systems ... 

It is a derived thermodynamic property, unlike the measured thermodynamic properties, temperature and pressure, that provide the criteria for thermal and mechanical equilibrium, respectively Although the chemical potential is an abstract concept, it is useful since it provides a simple criterion for chemical equilibria of each species i. 

Between two systems there can be a variety of interactions. Thennodynamic equilibrium of a system implies themial, chemical and mechanical equilibria. It is therefore logical to consider, in sequence, the following interactions between two systems thermal contact, which enables the two systems to share energy material contact, which enables exchange of particles between them and pressure transmitting contact, which allows an exchange of volume between the two systems. In each of the cases, the combined composite system is... 

This is a short but critically important section. When a system is at equilibrium, it has no tendency to change in either direction (forward or reverse) and will remain in its state until it is disturbed from outside the system. For example, when a block of metal is at the same temperature as its surroundings, it is in thermal equilibrium with them, and energy has no tendency to flow into or out of the block as heat. When a gas confined to a cylinder by a piston has the same pressure as the surroundings, the system is in mechanical equilibrium with the surroundings, and the gas has no tendency to expand or contract (Fig. 7.21). When a solid, such as ice, is in contact with its liquid form, such as water, at certain conditions of temperature and pressure (at 0°C and 1 atm for water), the two states of matter are in physical equilibrium with each other, and there is no tendency for one form of matter to change into the other form. Physical equilibria, which include vaporization as well as melting, are dealt with in detail in Chapter 8. When a chemical reaction mixture reaches a certain composition, it seems to come to a halt. A mixture of substances at chemical equilibrium has no tendency either to produce... 

Return now to the questions surrounding the actual sequence of events leading to substitution following population of the reactive state. As in thermal substitution mechanisms it is appropriate to determine whether a dissociative or an associative mechanism obtains. Certainly, this point is the one most often clarified, but other aspects also deserve some scrutiny. These include the possibility of acid-base equilibria in the excited state, isomerization of potentially ambidentate ligands, the extent to which the extruded ligand is electronically or vibrationally excited, the degree of molecular distortion upon population of the reactive state and the possibility of competing chemical processes which may be influenced by the environment or by structural modifications of the molecule. 

Others have defined physical chemistry as that field of science that applies the laws of physics to elucidate the properties of chemical substances and clarify the characteristics of chemical phenomena. The term physical chemistry is usually applied to the study of the physical properties of substances, such as vapor pressure, surface tension, viscosity, refractive index, density, and crystallography, as well as to the study of the so-called classical aspects of the behavior of chemical systems, such as thermal properties, equilibria, rates of reactions, mechanisms of reactions, and ionization phenomena. In its more theoretical aspects, physical chemistry attempts to explain spectral properties of substances in terms of fundamental quantum theory, the interaction of energy with matter, the nature of chemical bonding, the relationships correlating the number of energy states of electrons in atoms and molecules with the observable properties shown by these systems, and the electrical, thermal, and mechanical effects of individual electrons and protons on solids and liquids. ... 

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