Calorimeter substance

As noted above, heat that is released or consumed by a process cannot be measured directly. Unlike material quantities such as amount of substance, which can be determined with a balance, or the Volume of a liquid, which can be established with a liter measure, the quantity heat must be measured indirectly through its effect on a substance whose temperature it either raises or lowers. The fundamental equation for all of calorimetry describes the relationship between heat exchanged with a calorimeter substance and a corresponding temperature change ... 

For heats (and therefore temperature changes) that are not too large, the temperature change is directly proportional to the exchanged heat. The proportionality constant is the heat capacity of the calorimeter substance (previously referred to as the water value"). However, if the temperature change exceeds a few Kelvin, the temperature dependence of heat capacity stands in the way of a linear relationship, and a knowledge of the temperature function of the particular heat capacity in question is required in order to determine heat on the basis of a measured temperature difference. 

In this alternative method of heat measurement, which is also indirect, a measured temperature difference is used to calculate the amount of heat exchanged. A distinction can be made between temporal and spatial temperature-difference measurements. In temporal temperature-difference measurement the temperature of a calorimeter substance is measured before and after a process, and a corresponding heat is calculated on the basis of Equation (1) (which presupposes an accurate knowledge of the heat capacity). In the spatial method a temperature difference between two points within the calorimeter (or between the calorimeter substance and the surroundings) is the quantity of interest. The basis for interpretation in this case is from Fourier s law the equation for stationary heat conduction (Newton s law of cooling) ... 

An adiabatic calorimeter is equipped with an electronic control system designed to ensure to the greatest extent possible that the surroundings of the calorimeter cell remain at all times at precisely the same temperature as the substance under examination. The object is to prevent virtually all heat transport to the surroundings, thereby making it possible to establish the total heat effect from the change in temperature of the calorimeter substance. Good adiabatic calorimeters are difficult to construct, and they are not available commercially. Nevertheless, precision instruments of this type permit quantities such as specific heat capacities. as well as associated anomalies, to be meas-... 

Many combustion calorimeters are available commercially. Instruments utilizing water as the calorimeter substance generally operate adiabati-cally. thereby avoiding the somewhat complicated Regnault-Pfaundler procedure (cf. [29]) for determining and interpreting prior and subsequent temperature patterns. "Dry combustion calorimeters that avoid the use of water or other liquids have also recently come onto the market. 

Calorimeters with a liquid calorimeter substance can be made in a variety of designs (see Chapter 7). Using a version known as a combustion calorimeter, Crawford (1788) also measured the heats of combustion of various substances and compared these with the heat generated by a guinea pig. 

The calorimeter substance used for this purpose does not have to be water. Other liquid and even solid substances are also suitable. If the calorimeter substance is a solid (usually a metal), the calorimeter is referred to as aneroid (nonliquid). 

In all calorimeters based on this principle (Figure 1.4), the heat exchange between the sample and the calorimeter substance alters the temperature of the latter from T(tim) = Ti to T(tfi ) = Tfi. The change of temperature AT = Tfi — Tin, is measured. The quantity of heat to be determined is AQ = Ccai AT The heat capacity Ccai of the calorimeter, which represents the sum of the heat capacity of the calorimeter substance and the heat capacities of other instrument components (stirrer, thermometer, vessel) involved to a greater or lesser extent in the temperature change AT must be known. This heat capacity is an instrument-... 

Figure 1.4 Calorimeter for the measurement ofa time-dependent temperature difference (classical calorimeter with liquid calorimeter substance).

Of course, the compensation of an exothermic or endothermic thermal effect can be achieved by phase transition of a large choice of pure calorimeter substances. Among the substances that have been used for this purpose are water (melting point, 273.15 K = 0°C), diphenyl ether (melting point, 300.05 K = 26.90°C), and nitrr en (boiling point, 77.35 K = 195.80 °C at 1013 mbar, for exothermic effects only). 

Heat capacity of the sample Heat capacity of the calorimeter Initial temperature of furnace and sample Initial temperature of the calorimeter substance Final temperature of the calorimeter with sample... 

D Furnace, sample, calorimeter substance Figure 7.13 Drop calorimeter. 

Nernst, Koref, and Lindematm (1910) described an aneroid drop calorimeter for the measurement of specific heat capacities. Figure 7.14 shows the design of this instrument. The entire system is located in isothermal surroundings such as melting ice. Of particular interest is the measurement of the temperature change of the calorimeter substance by means of 10 iron-constantan thermocouples mounted between the calorimeter substance and the isothermal Ud. 

It follows from these considerations that in calorimeters with large thermal resistance between sample vessel and surroundings, the measured temperature signal is proportional to the heat exchanged with the calorimeter substance. In... 

Figure 7.36 "Mixing calorimeter with reacting calorimeter substance.

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