Enthalpy constant-pressure calorimetry

Constant-pressure calorimetry Constant-pressure calorimetry directly measures an enthalpy change (A/ ) for a reaction because it monitors heat flow at constant pressure AH=qp. 

The measurement of heat using a simple calorimeter such as that shown in Fig. 9.7 is an example of constant-pressure calorimetry, since the pressure (atmospheric pressure) remains constant during the process. Constant-pressure calorimetry is used in determining the changes in enthalpy occurring in solution. Recall that under these conditions the change in enthalpy equals the heat. 

The enthalpy change, dH = T dS + V dp, can be described as dH = dq - -V dp, and for a constant-pressure process, c/p = 0, we have dH = dqp. For a finite state change at constant pressure, qp = AH, that is, the heat transferred is equal to the enthalpy change of the system. This relation is the basis of constant pressure calorimetry, the constant-pressure heat capacity being Cp = dqldT)p. The relationship qp = AH is valid only in the absence of external work, w. When the system does external work, the first law must include dw. Then, the heat transferred to the system under constant-pressure conditions is qp = AH -f w. Thus, if a given chemical reaction has an enthalpy change of -50 kJ mol and does 100 kJ mol" of electrical work, the heat transferred to the system is —50 + 100 = 50 kJ mol". ... 

Flame combustion calorimetry in oxygen is used to measure the enthalpies of combustion of gases and volatile liquids at constant pressure [54,90]. Some highly volatile liquids (e.g., n-pentane [91]) have also been successfully studied by static-bomb combustion calorimetry. In general, however, the latter technique is much more difficult to apply to these substances than flame combustion calorimetry. In bomb combustion calorimetry, the sample is burned in the liquid state and must be enclosed in a container prior to combustion. Encapsulation may be difficult, because it is necessary to minimize the amount of vaporized compound inside the container as much as possible. In addition, volatile liquids tend to burn violently under a pressure of 3.04 MPa of oxygen, which leads to incomplete combustion. These problems are avoided in flame combustion calorimetry, where the sample is carried to the combustion zone as a vapor and burned under controlled conditions at atmospheric pressure. 

It is common to equate the strength of interaction of an acid and a base with the enthalpy of reaction. In some cases this enthalpy may be measured by direct calorimetry AH q for an adiabatic process at constant pressure. 

Since enthalpy changes can be obtained directly from measurement of heat absorption at constant pressure, even small values of AH for chemical and biochemical reactions can be measured using a micro-calorimeter.1112 Using the technique of pulsed acoustic calorimetry, changes during biochemical processes can be followed on a timescale of fractions of a millisecond. An example is the laser-induced dissociation of a carbon monoxide-myoglobin complex.13... 

This section reviews calorimetry [69-71] the measurement for a "system" (=sample+container) of (1) the "latent" enthalpy AH, (2) the internal energy AE, (3) the heat capacity at either constant pressure, CP = dH/dT, or (4) the heat capacity at constant volume Cv = dE/dT. All these measurements require careful control of the initial and final states, along with reliable temperature measurements for the system relative to its surroundings. Around room... 

H) enthalpy change. Compare with heat. Enthalpy (H) is defined so that changes in enthalpy (H) are equal to the heat absorbed or released by a process running at constant pressure. While changes in enthalpy can be measured using calorimetry, absolute val-... 

The measurement of heat capacity and related quantities is known as calorimetry. Most often the constant-pressure heat capacity is measured some instruments measure the constant-volume heat capacity Cy. Often, what is actually measured is not the derivatives Cp and Cy but an energy change divided by a small but finite temperature change. In some cases, the original enthalpy increment data may be more useful than the approximate heat capacities derived from them. In addition to the lUPAC books referenced in Section 1.8.1, the monograph of Flemminger and Flohne... 

If a calorimetry experiment is carried out under a constant pressure, the heat transferred provides a direct measure of the enthalpy change of the reaction. Constant-volume calorimetry is carried out in a vessel of fixed volume called a bomb calorimeter. Bomb calorimeters are used to measure the heat evolved in combustion reactions. The heat transferred under constant-volume conditions is equal to A Corrections can be applied to A values to yield enthalpies of combustion. 

Thermal properties are measured by some form of calorimetry, an exacting experimental procedure in which some kind of reaction is carried out, such as dissolution of a solid phase, and the heat (q) released or absorbed is measured. If the reaction occurred at constant pressure, the measured is a AH, and if not, it is fairly easily converted into a AH. Entropy can also be measured by calorimetry, though of a different type, and combining the enthalpy and entropy measurements gives AG numbers. Values of AG° can also be obtained by other methods, to be discussed in later chapters. All these quantities are related to the heat capacity, which turns out to be a very fundamental and important parameter. If pressure changes are important, then the volume or density is also required. 

As the temperature of a crystalline polymer increases, the phase transition is achieved when the crystallinity is dissolved. This process is known as fusion and is investigated by X-ray diffraction, specific volume, double refraction or calorimetry measurements. Using this last test, it is quite easy to identify the melting point, because this process is accompanied by an increase in enthalpy (heat absorption at constant pressure) (Fig. 2.22). 

When hydrocarbons or other organic compounds are burnt in a flame calorimeter it is often necessary to pre-mix the gas with oxygen to increase flame stability and obtain complete reaction. However, if the optimum proportion of oxygen is exceeded the flame temperature becomes too high and thermal decomposition with deposition of carbon occurs below the jet. Since the combustion takes place at constant pressure close to 1 bar, the enthalpy of combustion is measured under conditions near to those of the standard states. Flame calorimetry has the advantage that enthalpies of combustion are obtained for the gaseous state without the necessity of measuring enthalpies of vaporization in separate experiments and, moreover, completeness of combustion can be established by determination of both the water and the carbon dioxide produced. 

Flow calorimetry is now the commonly used method for the determination of the heat capacities of gases and vapours at essentially constant pressure, Cp. Many methods have been described for the determination of Cp and pieces of apparatus and techniques have been developed so that, particularly in the last twenty years, accuracies of 0.1 per cent or better have been claimed for the results. Enthalpies of vaporization to a slightly higher level of accuracy are often determined during the course of the heat capacity measurements. 

The goal of thermodynamics is to establish basic functions of state, the most important of which (for differential scanning calorimetry) are U, internal energy H, enthalpy p, pressure V, volume S, entropy and Cp, heat capacity at constant pressure. 

Immersion calorimetry is a useful technique for the characterization of porous materials. The heat evolved in the immersion process is directly related to the integral enthalpy of adsorption if the experiment is carried out at constant pressure and temperature [1]. The experimental data, which are obtained by immersion measurements, are normally used to determine the textural characteristics of the adsorbent, i.e. the micropore volinne or the surface areas accessible to the wetting liquid. In relation to the former Stoeekli et al [2] consider that a thermodynamic consequence of the Dubinin theory is the equation 1. 

Among various thermodynamic measurements heat capacity calorimetry is an extremely useful tool with which to investigate thermal properties of liquid crystals [1,2]. The heat capacity is usually measured under constant pressure and designated as Cp. It is defined as the enthalpy, H, required to raise the temperature of one mole of a given substance by 1 K. From this definition, Cp = (5H/5T)p, the enthalpy increment is determined by integration of Cp with respect to temperature, that is... 

Since enthalpy change is measurable by direct calorimetry, a balance can be set-up and verified with the experimentally determined reaction rates. At constant pressure (which is the case of bioreactors) and if no work is performed, the energy balance reads ... 

I continue to feel that the study of the volume changes in protein reactions is sorely neglected. They may be determined by dilatometry and by the effects of pressure on protein equilibrium constants. The results complement the results of the determination of enthalpy changes as measured by calorimetry and the effects of temperature on equilibrium constants. Much useful insight at the molecular level can be obtained from a knowledge of volume changes... 

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