The bomb calorimeter

A bomb calorimeter typically is used to carry out the complete combustion of a solid or liquid substance in the presence of excess oxygen. The combustion reaction is initiated with electrical ignition. In addition to the main combustion reaction, there may be unavoidable side reactions, such as the formation of nitrogen oxides if N2 is not purged from the gas phase. Sometimes auxiliary reactions are deliberately carried out to complete or moderate the main reaction. 

From the measured heating curve and known properties of the calorimeter, reactants, and products, it is possible to evaluate the standard molar enthalpy of combustion, AcH°, of the substance of interest at a particular temperature called the reference temperature, Tref. (Tref is often chosen to be 298.15 K, which is 25.00 °C.) With careful work, using temperature measurements with a resolution of 1 x 10 K or better and detailed corrections, the precision of Ac//° can be of the order of 0.01 percent. 

Bomb calorimetry is the principal means by which standard molar enthalpies of combustion of individual elements and of compounds of these elements are evaluated. From these values, using Hess s law, we can calculate the standard molar enthalpies of formation of the compounds as described in Sec. 11.3.2. From the formation values of only a few compounds, the standard molar reaction enthalpies of innumerable reactions can be calculated with Hess s law (Eq. 11.3.3 on page 320). 

Because of their importance, the experimental procedure and the analysis of the data it provides will now be described in some detail. A comprehensive problem (Prob. 11.7) based on this material is included at the end of the chapter. 

There are five main steps in the proeedure of evaluating a standard molar enthalpy of combustion  

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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. 

Isooctane is a primary component of gasoline and gives gasoline its octane rating. Burning 1.00 mL of isooctane (d = 0.688 g/mL) releases 33.0 kj ofheat. When 10.00 mL of isooctane is burned in a bomb calorimeter, the temperature in the bomb rises from 232°C to 66.5°C. What is the heat capacity of the bomb calorimeter ... 

Salicylic acid, C7H603, is one of the starting materials in the manufacture of aspirin. When 1.00 g of salicylic acid burns in a bomb calorimeter, the temperature rises from 23.11°C to 28.91°C. The temperature in the bomb calorimeter increases by 2.48°C when the calorimeter absorbs 9.37 kj. How much heat is given off when one mole of salicylic acid is burned ... 

Aniline, C6H5NH2(1), is a derivative of benzene in which a hydrogen atom has been replaced by an NH2 group, (a) Write the balanced equation for the combustion of aniline, (b) What is the mass of each product when 0.1754 g of aniline is burned in excess oxygen (c) If the bomb calorimeter in which this reaction was carried out had a volume of 355 mL, what minimum pressure of oxygen at 23°C must have been used to ensure complete combustion Assume that the volume of the aniline is negligible. 

A micro-bomb calorimeter exploded when the wrong proportions of sample and oxidants were used. Instead of 4 g of peroxide and 0.2 g of nitrate for 0.2 g of the sugar sample, 0.35 g of peroxide and 2.6 g of dextrose were used. The deficiency of peroxide to absorb the decomposition gases and excess of organic matter led to a rapid rise in temperature and pressure, which burst the bomb calorimeter. 

Figure 3.6 Schematic representation of the bomb calorimeter for measuring the changes in internal energy that occur during combustion. The whole apparatus approximates to an adiabatic chamber, so we enclose it within a vacuum jacket (like a Dewar flask)...

The heat released in the bomb calorimeter is not always the same as that released in the body (see footnotes to Table 2.3). 

As was mentioned already, heats of combustion can be determined either experimentally or by calculation, the latter method being slightly less accurate than the former. The experimental methods may be subdivided into those employing the "universal burner (or gas) calorimeter or the "bomb calorimeter . The former method is applicable only to gases or to substances easily volatile at room temperature, whereas the latter method is applicable to liquids and solids... 

The bomb calorimeter provides the most suitable and accurate apparatus for determination of the calorific values of solid and liquid fuels. Since the combustion takes place in a closed system, heat transfer from the calorimeter to the water is complete, and since the reaction is one between the fuel and gaseous oxygen, no corrections are necessary for the heat absorbed during the reduction of the oxidizing agent. In addition, the losses due to radiation can be reduced to comparatively small quantities, and more important, can be determined with a considerable degree of accuracy. Corrections due to the heat evolved in the formation of nitric and sulfuric acids under the conditions existing in the bomb can be determined accurately. 

The Beckmann thermometer used with the bomb calorimeter should be calibrated for the normal depth of immersion with which it is used. To cover the normal range of laboratory temperatures, this calibration should be obtained for three settings of the zero on the scale, convenient values being 10, 15, and 20°C. Such a series of calibrations allows automatically for emergent stem corrections and variations in the value of the degree on the thermometer scale with different quantities of mercury in the bulb, in addition to those arising from inherent variations in the diameter of the capillary bore. 

In some instances, an adiabatic bomb calorimeter may not be available or the sample may be too small for accurate use. To combat such problems, there is evidence that differential thermal analysis (DTA) is applicable to the determination of the calorific value of coal. Data obtained by use of the DTA method are in good agreement with those data obtained by use of the bomb calorimeter (Munoz-Guillena et al., 1992). 

A cycloalkane can be represented by the molecular formula (CH2) , so the general reaction in the bomb calorimeter is... 

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. 

The bomb calorimeter in Exercise 73 is filled with 987 g of water. The initial temperature of the calorimeter contents is 23.32°C. A 1.056-g sample of benzoic acid (AEcomb = -26.42 kj/g) is combusted in the calorimeter. What is the final temperature of the calorimeter contents ... 

We now discuss the probable accuracy of measurements of heats of combustion in the bomb calorimeter. In general, we shall not be concerned with details of the calorimetric technique, which are to be found in the references cited, but in the accuracy actually obtained. We shall deduce this chiefly from a comparison of... 

Tanaka, Takahashi, Okawara, and Watase 508 give the heats of combustion of five silicon-containing compounds without mentioning any particular difficulty involved in the measurement. This is not in accord with experience in the author s laboratory, where it was found that although a silicone ignited easily in the bomb calorimeter, combustion was usually incomplete. Part of the carbonaceous matter appeared to be entrapped in the ash and could not be burnt off except at high temperature. Combustions carried out on a thin film of silicone gave results with a spread of =b 3 per cent. 

The method of determining heats of combustion has been described above, for carbon. This method, with use of a bomb calorimeter, is the customary basis for determining the value of a fuel, such as coal or oil. A weighed sample of the fuel is placed in the bomb calorimeter, the bomb is filled with oxygen, and the fuel is burned. The fuel value or calorific value of the fuel is considered to be measured by its heat of combustion, and when large amounts of fuel are purchased the price may be determined by the result of tests in a bomb calorimeter. 

How is the rate of oxygen consumption and the KQ used to calculate the energy used by the body The connection between complete combustion of glucose (or fat) to carbon dioxide and energy is determined with a bomb calorimeter. The bomb calorimeter is a machine used for the combustion of various foods in on atmosphere of pure oxygen and, consequently, measurement of the heat produced. The amount of heat produced is expressed in joules. Any fuel — such as paraffin, methane, starch, pork, or beans — can be placed in the bomb calorimeter. A spark... 

FIGURE 5.18 Bomb calorimeter. The cartoon depicts ignition of a dried food sample in the bomb calorimeter... 

The energy expenditure of an animal or human may also be determined by the method of direct calorimetry. Direct calorimetry requires the use of an insulated room, chamber, or suit for the human or animal. The enclosure contains a water jacket. The water passes from one end of the jacket to the other, maintaining the room, chamber, or suit at a constant temperature. The temperature of the water leaving the jacket is used to calculate the energy expended by the subject. The principles behind the use of the chamber are identical to those behind the use of the bomb calorimeter. The major difference is that in bomb calorimetry combustion is catalyzed by a small spark. In addition, in the bomb calorimeter oxygen is present at a high pressure to facilitate combustion. With direct calorimetry, combustion is catalyzed by enzymes. This combustion proceeds more slowly than that catalyzed by a spark, and the temperature of the subject does not increase much over the normal resting body temperature with the various activities. 

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