Enthalpy

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

Enthalpy

A measure of the total heat content within a given sample of air. It is typically used to determine the amount of fresh outside air that can be added to recirculated air for the lowest heating/cooling cost.

enthalpy

[en′thal·pē]

(thermodynamics)

The sum of the internal energy of a system plus the product of the system's volume multiplied by the pressure exerted on the system by its surroundings. Also known as heat content; sensible heat; total heat.

The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.

Enthalpy

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, x_i), where N is the number of particles in the system and x_i 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 x_i, 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 c_p = (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

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References in periodicals archive

Based on the observations made from the intercalation curves (Figure 4), OCV curves versus lithium mole fraction (Figure 5), entropy (Figure 6), enthalpy (Figure 6), and Monte Carlo model (Figure 7), a model is proposed for lithium intercalation in the structure of ROHG as shown in Figure 8.

Thermodynamics of Lithium Intercalation in Randomly Oriented High Graphene Carbon

We have selected the gas phase enthalpy of formation for H[O.sub.2] from the Active Thermochemical Tables (ATcT) work of Ruscic et al.

An Evaluation of Gas Phase Enthalpies of Formation for Hydrogen-Oxygen ([H.sub.x][O.sub.y]) Species

This side of enthalpy will likely work in your favor.

A lesson in aquaponics: Part 2: meet your new friend, enthalpy

In this work, drying air enthalpy, as a function of air temperature and humidity ratio, is also included in that model to better use the adiabatic behavior of the drying process.

Fixed-bed drying simulation with constant enthalpy, using the improved Michigan State University model/Simulacao de secagem em camadas-fixas com entalpia constante, usando o modelo melhorado da Michigan State University

Enthalpy changes experienced by the drug in contact with these diluents were therefore computed taking the peaks of diluents and composite system located at the temperature above 210[degrees]C into an account, while rejecting any thermal event-taking place at the temperature exceeding ofloxacin-melting process.

Influence of sample preparation method on isothermal studies during compatibility screening of ofloxacin; correlation with DSC data

The denaturation temperature (Td) and enthalpy change (IHd) were determined from the maximal peak temperature and the area of the peak, respectively.

STUDY OF THERMAL AND HYDROLYTIC DENATURATION OF CASEIN ([alpha]S1) USING DIFFERENTIAL SCANNING CALORIMETRY (DSC)

4 showed that the melting enthalpy and crystallization enthalpy of nylon 12 6 were characterized by DSC thermograms.

Synthesis and Characterization of the Novel Nylon 12 6 Based on 1,12-Diaminododecane

Enthalpy is an indication of the strength of the water vapor molecules interconnection with the active sorption sites (Oliveira et al., 2014b); and entropy is associated with the degree of disorder of water molecules (Goneli et al., 2010; Majd et al., 2013).

Thermodynamic properties of water desorption in Buchenavia capitata (Vahl) Eichler/Propriedades termodinamicas de dessorcao da agua em sementes de Buchenavia capitata (Vahl) Eichler