Electrical Calorimeters

A number of other calorimeters which have been constructed for various special purposes will be described later on. 

Conduction and radiation of heat mean and true specific heat. 

The heat content of a body can be increased in two ways. Heat may either be produced in the interior of the body itself (e.g. by chemical change or by the passage of an electric current) or it may be caused to pass into the body from a hotter body in its neighbourhood. In the latter case heat is carried by radiation or by conduction from the places of higher temperature to those of lower temperature the hotter body acts as a source of heat. If the two bodies are only separated by a gas or by a liquid, the flow of heat may be aided by convection that is to say, by currents which carry portions of the fluid, heated by contact with the hot body, into the neighbourhood of the colder body. 

All substances in all states of aggregation have in a varying degree this power of conducting heat from places of higher temperature to places of lower temperature. 

This fundamental property of matter must have its origin in the nature of heat itself. The fluid theory of heat assumed that the caloric fluid tended like a gas to distribute itself uniformly over the whole of the available space, and hence to travel from places of higher to places of lower density. The discovery that heat could be converted into mechanical work, and vice versa, led to the abandonment of the material theory of heat and to the acceptance of the kinetic theory, which looks upon heat as the kinetic energy of the ultimate particles of which 

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Elba reaction org chem The formation of anthracene derivatives by dehydration and cycllzation of diaryl ketone compounds which have a methyl group or methylene group heating to an elevated temperature is usually required. elbs re.ak shan ( ELDOR See electron electron double resonance. ( el,dor or e el de o ar ( electrical calorimeter analy chem Device to measure heat evolved (from fusion or vaporization, for example) measured quantities of heat are added electrically to the sample, and the temperature rise is noted. a lek tra kal kal a rlm ad ar)... 

A calorimeter is a device used to measure the work that would have to be done under adiabatic conditions to bring about a change from state 1 to state 2 for which we wish to measure AU= U -U This work is generally done by passing a known constant electric current 3 for a known time t through a known resistance R embedded in the calorimeter, and is denoted by where... 

Values of COT) can be derived from a constant volume calorimeter by measuring AU for small values of Tj - TO and evaluating AU/(T2 - T ) as a fiinction of temperature. The energy change AU can be derived from a knowledge of tlie amount of electrical energy required to change the temperature of the sample + container... 

With most non-isothemial calorimeters, it is necessary to relate the temperature rise to the quantity of energy released in the process by determining the calorimeter constant, which is the amount of energy required to increase the temperature of the calorimeter by one degree. This value can be detemiined by electrical calibration using a resistance heater or by measurements on well-defined reference materials [1], For example, in bomb calorimetry, the calorimeter constant is often detemiined from the temperature rise that occurs when a known mass of a highly pure standard sample of, for example, benzoic acid is burnt in oxygen. 

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... 

Now if the chemical reaction had been allowed to proceed without the performance of any external electrical work, say in a calorimeter, so that the initial and final temperatures of the system are both T, the change of intrinsic energy would have been the same as that occurring in the process described above, as we know from the First Law. Thus the heat of reaction, Q will be equal to the increase of intrinsic energy ... 

FIGURE 6.12 A bomb calorimeter is used to measure heat transfers at constant volume. The sample in the central rigid container called the bomb is ignited electrically with a fuse wire. Once combustion has begun, energy released as heat spreads through the walls of the bomb into the water. The heat released is proportional to the temperature change of the entire assembly. 

A calorimeter was calibrated with an electric heater, which supplied 22.5 kj of energy to the calorimeter and increased the temperature of the calorimeter and its water bath from 22.45°C to 23.97°C. What is the heat capacity of the calorimeter ... 

A calorimeter is calibrated with an electrical heater. Before the heater is turned on, the calorimeter temperature is 23.6 °C. The addition of 2.02 X 10 J of electrical energy from the heater raises the temperature to 27.6 °C. Determine the total heat capacity of this calorimeter. 

A sketch of the process helps to identify what takes place. The electrical heater converts electrical energy into heat that flows into the calorimeter and raises the temperature of the water bath. 

Muscle activity is accompanied by cellular pumping of sodium ions. The energy requirements of the sodium pump have been studied on an individual cardiac muscle mounted inside a tiny differential calorimeter and stimulated by electrical impulses. The heat evolved was different in the presence and absence of a known inhibitor of the sodium pump. 

C06-0017.A coffee-cup calorimeter is calibrated using a small electrical heater. The addition of 3.45 kJ of electrical energy raises the calorimeter temperature from 21.65 °C to 28.25 °C. Calculate the heat capacity of the calorimeter. 

C06-0018. Adding 1.530X 10 Jof electrical energy to a constant-pressure calorimeter changes the water temperature from 20.50 °C to 21.85 °C. When 1.75 g of a solid salt is dissolved in the water, the temperature falls from 21.85 °C to 21.44 °C. Find the value of gp for the solution process. 

C06-0069. Constant-pressure calorimeters can be calibrated by electrical heating. When a calorimeter... 

C06-0071. An electric heater adds 19.75 kJ of heat to a constant-volume calorimeter. The temperature of the calorimeter increases by 4.22 °C. When 1.75 g of methanol is burned in the same calorimeter, the temperature increases by 8.47 °C. Calculate the molar energy of combustion of methanol. 

A 4.9 g sample of the liquid siloxane in a glass dish was put into a bomb calorimeter (on an open bench) containing 5 ml of sodium hydroxide solution to absorb combustion gases. The electric igniter system consisted of a metal wire in contact with a cotton-wool wick which dipped into the siloxane sample. The bomb was sealed, pressured up to 39-44 bar with oxygen, and the igniter was fired. A violent explosion blew the lid off the bomb (rated at 190 bar working, 250 bar test), and examination of the deformed bomb indicated that a maximum detonation pressure of around 900 bar had been attained. 

Fig. 4. Vertical cross section of a high-temperature Calvet calorimeter (16) cell guides (A) thermal insulation (B) top (C) and bottom (N) electrical heaters thermostat consisting of several metal canisters (D, G, and H) switch (E) electrical heater (F) thermometers (I, J, and K) microcalorimetric element (L) and heat sink (M).

In this microcalorimeter, the heat sink is not a massive metal block but is divided into several parts which are mobile with respect to each other. Each thermoelectric element (E) and a cell guide (D) are affixed to a fluxmeter holder (C). The holder (C) is mobile with respect to a massive arm (B) which, in turn, rotates around a vertical axle (A). All parts of the heat sink are made of brass. Surfaces in contact are lubricated by silicone grease. Four thermoelectric elements (E) are mounted in this fashion. They enclose two parallelepipedic calorimetric cells, which can be made of glass (cells for the spectrography of liquids are particularly convenient) or of metal (in this case, the electrical insulation is provided by a very thin sheet of mica). The thermoelectric elements surrounding both cells are connected differentially, the Petit microcalorimeter being thus a twin differential calorimeter. 

The basic principle of heat-flow calorimetry is certainly to be found in the linear equations of Onsager which relate the temperature or potential gradients across the thermoelements to the resulting flux of heat or electricity (16). Experimental verifications have been made (89-41) and they have shown that the Calvet microcalorimeter, for instance, behaves, within 0.2%, as a linear system at 25°C (41)-A. heat-flow calorimeter may be therefore considered as a transducer which produces the linear transformation of any function of time f(t), the input, i.e., the thermal phenomenon under investigation]] into another function of time ig(t), the response, i.e., the thermogram]. The problem is evidently to define the corresponding linear operator. 

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). 

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