Phase transitions heat transfer during

A pure component undergoes a first-order phase transition or discontinuous change when it changes from one phase to another, for example, the gas-to-liquid transition. Essentially, during the phase transition, one phase increases at the expense of the other and this usually follows the direction of heat transfer. ... 

As an example, let the system contain a fixed amount n of a pure substance divided into liquid and gas phases, at a temperature and pressure at which these phases can coexist in equilibrium. When heat is transferred into the system at this T and p, some of the liquid vaporizes by a liquid-gas phase transition and V increases withdrawal of heat at this T and p causes gas to condense and V to decrease. The molar volumes and other intensive properties of the individual liquid and gas phases remain constant during these changes at constant T and p. On the pressure-volume phase diagram of Fig. 8.9 on page 208, the volume changes correspond to movement of the system point to the right or left along the tie line AB. 

The effect of heat transfer rates on the type of reaction may be considered by way of the following models. Crystallization and reaction are principal exothermic occurrences. Gaseous product formation, phase transitions, and melting represent the major endothermic reactions. The surface of each sample will be considered to be instantaneously heated by a constant heat source. Only the effects during a heating cycle are discussed for the homogeneous specimen. 

The dynamic properties of reaction systems ultimately depend on the nature of interactions between molecules. The nonhnearity of macroscopic rate laws is due to the participation of more than one species in an elementary act and the complex cooperative interaction of adsorbed atoms and molecules with each other and with the catalyst surface. The nonlinearity of macroscopic rate laws is also due to phase transitions in the adsorption layer, surface restructuring during the reaction, catalyst surface (energetic) nonuniformity, and the influence of mass, heat and pulse transfer processes on the reaction rate due to the delay in the feedback. 

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