Enthalpy calorimeter

Calorimetry is the basic experimental method employed in thennochemistry and thennal physics which enables the measurement of the difference in the energy U or enthalpy //of a system as a result of some process being done on the system. The instrument that is used to measure this energy or enthalpy difference (At/ or AH) is called a calorimeter. In the first section the relationships between the thennodynamic fiinctions and calorunetry are established. The second section gives a general classification of calorimeters in tenns of the principle of operation. The third section describes selected calorimeters used to measure thennodynamic properties such as heat capacity, enthalpies of phase change, reaction, solution and adsorption. 

Combustion or bomb calorimetry is used primary to derive enthalpy of fonuation values and measurements are usually made at 298.15 K. Bomb calorimeters can be subdivided into tluee types (1) static, where the bomb or entire calorimeter (together with the bomb) remains motionless during the experiment (2) rotating-... 

Accurate enthalpies of solid-solid transitions and solid-liquid transitions (fiision) are usually detennined in an adiabatic heat capacity calorimeter. Measurements of lower precision can be made with a differential scaiming calorimeter (see later). Enthalpies of vaporization are usually detennined by the measurement of the amount of energy required to vaporize a known mass of sample. The various measurement methods have been critically reviewed by Majer and Svoboda [9]. The actual teclmique used depends on the vapour pressure of the material. Methods based on... 

Batch calorimeters are instmments where there is no flow of matter in or out of the calorimeter during the time the energy change is being measured. Batch calorimeters differ in the way the reactants are mixed and in the method used to detennine the enthalpy change. Enthalpy changes can be measured by the various methods... 

Reviews of batch calorimeters for a variety of applications are published in the volume on Solution Calorimetry [8] cryogenic conditions by Zollweg [22], high temperature molten metals and alloys by Colinet andPasturel [19], enthalpies of reaction of inorganic substances by Cordfunke and Ouweltjes [16], electrolyte... 

Figure Bl.27.6. A calorimeter for enthalpies of mixing in the absence of a vapour space. (Reproduced with pennission from Larkin J A and McGlashan ML 1961 J. Chem. Soc. 3245.)...

Various flow calorimeters are available connnercially. Flow calorimeters have been used to measure heat capacities, enthalpies of mixing of liquids, enthalpy of solution of gases in liquids and reaction enthalpies. Detailed descriptions of a variety of flow calorimeters are given in Solution Calorimetry by Grolier [17], by Albert and Archer [18], by Ott and Womiald [H], by Simonson and Mesmer [24] and by Wadso [25]. 

By allowing compounds to react in a calorimeter, it is possible to measure the heat evolved in an exothermic reaction or the heat absorbed in an endothermic reaction. Thousands of reactions have been studied to produce a rich library of thermochemical data. These data take the form of heats of reaction and correspond to the value of the enthalpy change A/-/° for a particular reaction of a particular substance. 

Enthalpy changes for biochemical processes can be determined experimentally by measuring the heat absorbed (or given off) by the process in a calorimeter (Figure 3.2). Alternatively, for any process B at equilibrium, the standard-state enthalpy change for the process can be determined from the temperature dependence of the equilibrium constant ... 

Thermochemistry is concerned with the study of thermal effects associated with phase changes, formation of chemical compouncls or solutions, and chemical reactions in general. The amount of heat (Q) liberated (or absorbed) is usually measured either in a batch-type bomb calorimeter at fixed volume or in a steady-flow calorimeter at constant pressure. Under these operating conditions, Q= Q, = AU (net change in the internal energy of the system) for the bomb calorimeter, while Q Qp = AH (net change in the enthalpy of the system) for the flow calorimeter. For a pure substance. 

As noted earlier, for a reaction at constant pressure, such as that taking place in an open coffee-cup calorimeter, the heat flow is equal to the change in enthalpy. If a reaction is carried out at constant volume (as is the case in a sealed bomb calorimeter) and there is no mechanical or electrical work involved, no work is done. Under these conditions, with w = 0, the heat flow is equal to the change in energy, AE. Hence we have... 

Coffee-cup calorimeter Bomb calorimeter Standard enthalpy change First law of thermodynamics AH versus AE... 

The implication of this equation is that, because chemical reactions typically take place at constant pressure in vessels open to the atmosphere, the heat that they release or require can be equated to the change in enthalpy of the system. It follows that if we study a reaction in a calorimeter that is open to the atmosphere (such as that depicted in Fig. 6.11), then the measurement of its temperature rise gives us the enthalpy change that accompanies the reaction. For instance, if a reaction releases 1.25 kj of heat in this kind of calorimeter, then we can write AH = q — —1.25 kj. 

STRATEGY The heat released by the reaction at constant pressure is calculated from the temperature change multiplied by the heat capacity of the calorimeter. Use the molar mass of one species to convert the heat released into the reaction enthalpy corresponding to the thermochemical equation as written. If the temperature rises, the... 

We have seen that a constant-pressure calorimeter and a constant-volume bomb calorimeter measure changes in different state functions at constant volume, the heat transfer is interpreted as A U at constant pressure, it is interpreted as AH. However, it is sometimes necessary to convert the measured value of AU into AH. For example, it is easy to measure the heat released by the combustion of glucose in a bomb calorimeter, but to use that information in assessing energy changes in metabolism, which take place at constant pressure, we need the enthalpy of reaction. 

As Example shows, 20.0 g of NH4 NO3 absorbs 5.28 kJ of energy when the compound dissolves in a constant-pressure calorimeter. This information lets us calculate the molar enthalpy change for the dissolving... 

Table II shows the "heats of formation" of the conjugate phases, that is, the excess enthalpies for mixing the appropriate amounts of water and amphiphile (at the same initial temperature and pressure as the final system) to make a unit amount of the conjugate phase. Values labeled "calorimeter" and "phase volume," respectively, are based on the same set of calorimetric titrations. In the former case the phase composition was taken from the calorimetric measurements, and in the latter case the composition was taken from our phase-volume compositions. Literature values for the heats of formation are based on data from references 13-16.

Now, it is necessary to calibrate the calorimeter in order to analyze quantitatively the recorded thermograms and determine the amount of heat evolved by the interaction of a dose of gas with the adsorbent surface. The use of a standard substance or of a standard reaction is certainly the most simple and reliable method, though indirect, for calibrating a calorimeter, since it does not require any modification of the inner cell arrangement. [For a recent review on calibration procedures, see 72).3 No standard adsorbent-adsorbate system has been defined, however, and the direct electrical calibration must therefore be used. It should be remarked, moreover, that the comparison of the experimental heat of a catalytic reaction with the known change of enthalpy associated with the reaction at the same temperature provides, in some favorable cases, a direct control of the electrical calibration (see Section VII.C). 

Rate of heat release measurements have been attempted since the late 1950 s. A prominent example of instrument design for the direct measurement of the sensible enthalpy of combustion products is the Ohio State University (OSU) calorimeter. This has been standardized by ASTM and a test method employing this technique (ASTM-E-906) is part of a FAA specification for evaluation of large interior surface materials. 

The calculation of other bond enthalpy terms, such as E (Ge—Ge), E (Ge—O), E (Ge—N) and E(Ge—S), can be made from data in Table 1. However, due to the above-mentioned controversy involving most of the data obtained with static-bomb combustion calorimeters, we refrain from tabulating those terms. 

Proper calibration of the DSC instruments is crucial. The basis of the enthalpy calibration is generally the enthalpy of fusion of a standard material [21,22], but electrical calibration is an alternative. A resistor is placed in or attached to the calorimeter cell and heat peaks are produced by electrical means just before and after a comparable effect caused by the sample. The different heat transfer conditions during calibration and measurement put limits on the improvement. DSCs are usually limited to temperatures from liquid nitrogen to 873 K, but recent instrumentation with maximum temperatures close to 1800 K is now commercially available. The accuracy of these instruments depends heavily on the instrumentation, on the calibration procedures, on the type of measurements to be performed, on the temperature regime and on the... 

Heat capacities at high temperatures, T > 1000 K, are most accurately determined by drop calorimetry [23, 24], Here a sample is heated to a known temperature and is then dropped into a receiving calorimeter, which is usually operated around room temperature. The calorimeter measures the heat evolved in cooling the sample to the calorimeter temperature. The main sources of error relate to temperature measurement and the attainment of equilibrium in the furnace, to evaluation of heat losses during drop, to the measurements of the heat release in the calorimeter, and to the reproducibility of the initial and final states of the sample. This type of calorimeter is in principle unsurpassed for enthalpy increment determinations of substances with negligible intrinsic or extrinsic defect concentrations... 

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