Principle of Calorimetry

We are concerned here with the problem of determining experimentally the enthalpy change A/f or the energy change Ai accompanying a given isothermal change in state of a system, normally one in which a chemical reaction occurs. We can write the reaction schematically in the form 

In practice we do not actually carry out the change in state isothermally this is not necessary because A/f and AE are independent of the path. In calorimetry we usually find it convenient to use a path composed of two steps  

Step 1. A change in state is carried out adiabatically in the calorimeter vessel to yield the desired products but in general at another temperature  

Step 11. The products of step I are brought to the initial temperature Tq by adding heat to (or taking it from) the system  

As we shall see, it is often unnecessary to carry out this step in actuality, since the associated change in energy or enthalpy can be calculated from the known temperature difference. 

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In the CSM laboratory, Rueff et al. (1988) used a Perkin-Elmer differential scanning calorimeter (DSC-2), with sample containers modified for high pressure, to obtain methane hydrate heat capacity (245-259 K) and heat of dissociation (285 K), which were accurate to within 20%. Rueff (1985) was able to analyze his data to account for the portion of the sample that was ice, in an extension of work done earlier (Rueff and Sloan, 1985) to measure the thermal properties of hydrates in sediments. At Rice University, Lievois (1987) developed a twin-cell heat flux calorimeter and made AH measurements at 278.15 and 283.15 K to within 2.6%. More recently, at CSM a method was developed using the Setaram high pressure (heat-flux) micro-DSC VII (Gupta, 2007) to determine the heat capacity and heats of dissociation of methane hydrate at 277-283 K and at pressures of 5-20 MPa to within 2%. See Section 6.3.2 for gas hydrate heat capacity and heats of dissociation data. Figure 6.6 shows a schematic of the heat flux DSC system. In heat flux DSC, the heat flow necessary to achieve a zero temperature difference between the reference and sample cells is measured through the thermocouples linked to each of the cells. For more details on the principles of calorimetry the reader is referred to Hohne et al. (2003) and Brown (1998). 

The principle of calorimetry is very interesting for biological applications. Calorimetric biosensors are based on the detection of the heat production of biological reactions which is caused by enthalpy changes. The micro calorimetric sensing principle is very versatile because of the exothermic nature of nearly all enzymatic reactions [8] and was introduced as a conventionally constructed device very early [9] ... 

Calorimetry. The procedures for the operation of the bomb calorimeter are, with minor exceptions noted below, those described in detail in Exp. 6. That experiment and the general discussion presented in the section Principles of Calorimetry should be studied carefully. 

A general discussion of calorimetric measurements is presented in the section Principles of Calorimetry, which should be reviewed in connection with this experiment. We shall not consider here the concentration dependence of these enthalpy changes. Such concentration dependence is generally a small effect, since the heats of dilution involved are usually much smaller than the heats of chemical reaction (indeed they are zero for perfect solutions). Since we are dealing here with solutions of moderate concentration, particularly in the case of the NaOH solution, it may be useful to make parallel determinations of heats of dilution of the solutions concerned by a procedure similar to that described here if time permits. 

Let an oven dry soil having mass im ), specific heat (s ) and initial temperature (9 ) be dropped all of a sudden into water contained in a calorimeter with mass, specific heat and initial temperature as m, and 02 respectively. If and Sg are the mass and specific heat of the calorimeter and if the resulting temperature of soil-water mixture at equilibrium is 9 then according to the principle of calorimetry (Heat gained = Heat lost) with all limitating it follows that... 

The techniques and equipment employed in calorimetry depend on the nature of the process being studied. For many reactions, such as those occurring in solution, it is easy to control pressure so that AH is measured directly. Although the calorimeters used for highly accurate work are precision instruments, a simple coffee-cup calorimeter ( FIGURE 5.18) is often used in general chemistry laboratories to illustrate the principles of calorimetry. Because the calorimeter is not sealed, the reaction occurs under the essentially constant pressure of the atmosphere. 

Lavoisier and Laplace (1784) say that heat may be regarded as a fluid penetrating all bodies, either free or combined, or alternatively as the result of insensible movements of the molecules of matter , oscillating in small voids in the body, and in both hypotheses the amount of heat is constant. This follows in the second hypothesis from the law of conservation of vis viva (mv i.e. of kinetic energy hnv ). The second hypothesis (which is almost certainly that held by Laplace) is favoured by many phenomena such as the production of heat by friction. Lavoisier and Laplace do not decide in favour of either hypothesis, since both lead to the conservation of free heat , which states that the quantity of free heat always remains the same in the simple mixture of bodies , and is the fundamental principle of calorimetry, and both hypotheses lead to the very general principle that ... 

These matters will be considered here. To begin with, the physical principles of calorimetry that were treated in detail in the first part of this book will be... 

Abstract Basic principles of calorimetry coupled with other techniques are introduced. These methods are used in heterogeneous catalysis for characterization of acidic, basic and red-ox properties of solid eatalysts. Estimation of these features is achieved by monitoring the interaction of various probe molecules with the surface of such materials. Overview of gas phase, as well as liquid phase techniques is given. Special attention is devoted to coupled calorimetry-volumetry method. Furthermore, the influence of different experimental parameters on the results of these techniques is discussed, since it is known that they can significantly influence the evaluation of catalytic properties of investigated materials. 

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