Calorimetry

Considerable effort has been expended in studying heat effects in biological systems. These range from measuring the calorie count of foods on supermarket shelves to the heat release from humans and animals. Laplace (ca 1800) crudely measured the heat rejected by a guinea pig in the first effort to determine animal metabolism rates. Modern evaluations of human activity yield heat release rates ranging from ~ 100 W while sleeping to ~ 200 W during a mild activity such as walking. The input energy rates are about twice these values, the difference corresponding to the various work outputs, ranging from large-scale muscle contraction to microscopic motions of cilia on mobile cell components. Energy is supplied to the body by oxidation of glucose, which produces ~ 3000 kJ/mole, or ~ 16 kJ/g. About half of this energy produces work and the remainder is rejected from the body as heat. To supply the body's energy needs requires consumption of 50 - 100 grams of glucose per hour.

Scientifically-oriented calorimetry involves determination of: i) enthalpy changes of well-defined reactions, such as ligand binding to macromolecules or denaturation (unfolding) of proteins; ii) heat releases from functioning organs such as nerve fibers or muscles.

Figure 11.21 shows the types of calorimeters used in biological studies.

11.9.1 Bomb calorimeter

The so-called bomb calorimeter shown in Fig. 11.21 is used principally to measure heat released by total combustion of organic compounds, for which a generic reaction is:

CxHyOzNu + (2x + %y - z) O2 = xCO2 + % yH2O + % uN2

This is not an equilibrium reaction, as it completely consumes the reactants and produces a mixture of products. It is therefore a standard-state reaction, and the heat evolved is the standard enthalpy of the reaction. As shown in the figure, the substance and high-pressure oxygen are contained in a strong vessel with heat-transmitting walls. Combustion is initiated by a spark, and after completion of the reaction, the temperature rise of the water bath surrounding the vessel is measured. Even though the reactant oxygen and the product gases are not pure and at 1 atm pressure, the heat released is nonetheless the standard enthalpy of the reaction (do you know why?). This property and the measured rise in temperature of the system (water, vessel and reaction-product gases) is:

where n is the number of moles of the substance combusted and the subscripts W, V, and g refer to water, vessel and gas, respectively.

Fig. 11.21 Calorimeters used in biological studies
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Solar Panel Basics

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