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enthalpy(ĕn`thălpē), measure of the heatheat,
nonmechanical energy in transit, associated with differences in temperature between a system and its surroundings or between parts of the same system. Measures of Heat
..... Click the link for more information. content of a chemical or physical system; it is a quantity derived from the heat and work relations studied in thermodynamicsthermodynamics,
branch of science concerned with the nature of heat and its conversion to mechanical, electric, and chemical energy. Historically, it grew out of efforts to construct more efficient heat engines—devices for extracting useful work from expanding hot gases.
..... Click the link for more information. . As a system changes from one state to another the enthalpy change, ΔH, is equal to the enthalpy of the products minus the enthalpy of the reactants. If heat is given off during a transformation from one state to another, then the final state will have a lower heat content than the initial state, the enthalpy change ΔH will be negative, and the process is said to be exothermic. If heat is absorbed during the transformation, then the final state will have a higher heat content, ΔH will be positive, and the process is said to be endothermic. The enthalpy change accompanying a chemical reaction is called the heat of the reaction. For a reaction in which a compound is formed from its composite elements, the enthalpy increase or decrease is called the heat of formation of the compound. Changes of state, or phase, of matter are also accompanied by enthalpy changes; the change associated with the solid-liquid transition is called the heat of fusion and the change associated with the liquid-gas transition is called the heat of vaporization (see latent heatlatent heat,
heat change associated with a change of state or phase (see states of matter). Latent heat, also called heat of transformation, is the heat given up or absorbed by a unit mass of a substance as it changes from a solid to a liquid, from a liquid to a gas, or the
..... Click the link for more information. ). The enthalpy change for a given reaction often may be used to tell how favorable the reaction is; an exothermic reaction involves a loss of heat and a consequent lower final energy and thus tends to be favorable, while an endothermic reaction tends to be unfavorable because it involves an increase in energy. However, there are other factors, such as entropyentropy
, quantity specifying the amount of disorder or randomness in a system bearing energy or information. Originally defined in thermodynamics in terms of heat and temperature, entropy indicates the degree to which a given quantity of thermal energy is available for doing
..... Click the link for more information. changes, which must also be taken into account in determining whether or not a given process can occur.
For any system, that is, the volume of substance under discussion, enthalpy is the sum of the internal energy of the system plus the system's volume multiplied by the pressure exerted by the system on its surroundings. The sum is given the special symbol H primarily as a matter of convenience because this sum appears repeatedly in thermodynamic discussion. Previously, enthalpy was referred to as total heat or heat content, but these terms are misleading and should be avoided. Enthalpy is, from the viewpoint of mathematics, a point function, as contrasted with heat and work, which are path functions. Point functions depend only on the initial and final states of the system undergoing a change; they are independent of the paths or character of the change. For change in enthalpy with pressure or temperature See Thermodynamic principles, Entropy, Thermodynamic processes
the thermodynamic potential that characterizes the state of a thermodynamic system upon the selection of entropy 5 and pressure p as the principal independent variables (seePOTENTIALS, THERMODYNAMIC and ). Enthalpy is represented as H(S, p, N, xi), where N is the number of particles in the system and xi is the system’s other macroscopic parameter. It is an additive function; that is, the enthalpy of the entire system is equal to the sum of the enthalpies of its component parts. Enthalpy is related to the internal energy U of the system by the equation
(1) H = U + pV
where V is the volume of the system. The total enthalpy differential, given fixed N and xi, has the form
(2) dH = TdS + vdp
From formula (2) it is possible to determine the temperature T and the volume of the system: T = (αH/αS)p and V = (αH/αp)s. At constant pressure (p = const), the heat capacity of the system cp = (αH/αT)p (seeHEAT CAPACITY). These properties of enthalpy, at p = const, are similar to the properties of internal energy at constant volume:
A minimum enthalpy value corresponds to the equilibrium state of a system given the constancy of S and p. A change in enthalpy (ΔH) is equal to the amount of heat imparted to the system or removed from it at constant pressure, therefore the values of ΔH characterize the heat effects of phase transitions (melting, boiling), chemical reactions, and other processes that occur at constant pressure. Enthalpy is conserved upon the thermal insulation of bodies (at p = const), and therefore it is sometimes called heat content or total heat. The condition for enthalpy conservation underlies, in particular, the theory of the Joule-Thomson effect, which has found important practical applications in the liquation of gases. The term “enthalpy” was proposed by H. KamerlinghOnnes.
D. N. ZUBAREV