The Heat Output of Reactions

Self-Tfst 6.4B A calorimeter was calibrated by mixing two aqueous solutions together, each of volume 0.100 L. The heat output of the reaction that took place was known to be 4.16 kj, and the temperature of the calorimeter rose by 3.24°C. 

A reaction known to release 1.78 kj of heat takes place in a calorimeter containing 0.100 L of solution, like that shown in Fig. 6.13. The temperature rose by 3.65°C. Next, 50 mL of hydrochloric acid and 50 mL of aqueous sodium hydroxide were mixed in the same calorimeter and the temperature rose by 1.26°C. What is the heat output of the neutralization reaction ... 

Then use the value of that heat capacity to convert the temperature rise caused by the neutralization reaction into the heat output of the reaction. For this step, use the same equation rearranged to qca = Cca AT, but with AT now the change in temperature observed during the reaction, and then convert the value of qQai to q for the reaction mixture, q = —qci. So, overall, we use... 

A very important application of the heats of reaction is the calcn of the reaction temp of propints which determine in part the ballistic performance. Heats of reaction also permit the calcn of the adiabatic reaction temp of pyrot reactions which are of value in judging the luminous yield of flares as well as the heat output of thermite reactions... 

In some cases, the heat source can include both the heat of polymerization and the heat output of crystallization of the newly formed products. This is the case in anionic activated e-caprolactam polymerization. This dual heat source must be included in the energy balance equation. As was discussed above, the temperature dependence of the crystallization rate is somewhat complicated. Nevertheless, the propagation of the heat wave is analogous to other well-known cases of wave propagation from consecutive reactions. 

For example, some microcalorimetric data are represented in Figure 3. The exact process that gave rise to these data is not important, but it shall be assumed that they represent the heat output of a partially completed reaction. These data have been fitted to the exponential model shown above to determine the equation parameters that describe them. Once these values have been determined, it is possible to extend the data to such a time as the observed heat flow (power) equals 0 (shown by the dotted line). The area under the dotted line represents the total heat, Q, which would be generated by the reaction if it went to completion. From this information, it is a simple matter to determine the extent of reaction at any time, t. Hence, some useful information can be derived by using nonspecific models. 

The reaction, which involves nuclear fission, also produces more neutrons and enormous amounts of energy. Some nuclear power plants use fuel rods made of natural U, unenriched in (e.g., Canadian CANDU fuel cf. Kathren 1984). In most countries the I uel rods are fabricated from U enriched in to 1.8 to 3.7% of total U (Adloff and GuiMaumont 1993). This increases the radioactivity and, therefore, the heat output of the fuel, which is derived from the kinetic energy of the fission fragments and radioactivity of fission products. The heat is used to produce steam to run turbines for generating electricity,... 

The neutron carries much of the energy released by this reaction. The neutrons have no electric charge so they are not contained by the magnetic field. They must be captured in a blanket material. When the blanket material absorbs them, their energy is transformed into heat. The heat from the neutrons and the radiant energy emitted from the hot plasma and directly adsorbed by the walls of the reactor are the heat output of the fusion reactor. This heat can be used to produce steam for the generation of electric power by conventional steam turbines. 

It is not possible to produce nitrous oxide (NjO) in the pyrotechnic manner. The heat output of the reaction... 

The heat output of this reaction per gram of reactants is 0.87 kcal/g or 3-7 kcal/cm (theoretical) and must be called moderate, both on a weight or actual volume basis (the density of the unconsolidated mixture, the form in which the material is used, is about 2 g/cm ). The heat output is a little higher for the reaction... 

When the addition of diethyl propylmalonate is started, the heat output of the reaction is compensated by the jacket cooling shown by the positive heat flow (A) in Figure 4.23(a). As the reaction proceeds a sharp decrease (B) in Figure 4.23(a) occurs coincidental with nucleate boiling which is shown by the increase (B) in the reflux heat release curve in Figure 4.23(b). The heat flow... 

Combustion. The primary reaction carried out in the gas turbine combustion chamber is oxidation of a fuel to release its heat content at constant pressure. Atomized fuel mixed with enough air to form a close-to-stoichiometric mixture is continuously fed into a primary zone. There its heat of formation is released at flame temperatures deterruined by the pressure. The heat content of the fuel is therefore a primary measure of the attainable efficiency of the overall system in terms of fuel consumed per unit of work output. Table 6 fists the net heat content of a number of typical gas turbine fuels. Net rather than gross heat content is a more significant measure because heat of vaporization of the water formed in combustion cannot be recovered in aircraft exhaust. The most desirable gas turbine fuels for use in aircraft, after hydrogen, are hydrocarbons. Fuels that are liquid at normal atmospheric pressure and temperature are the most practical and widely used aircraft fuels kerosene, with a distillation range from 150 to 300 °C, is the best compromise to combine maximum mass —heat content with other desirable properties. For ground turbines, a wide variety of gaseous and heavy fuels are acceptable. 

Impurities or the delayed addition of a catalyst causes inhibition or delayed initiation resulting in accumulation in the reactors. The major hazard from accumulation of the reactants is due to a potentially rapid reaction and consequent high heat output that occurs when the reaction finally starts. If the heat output is greater than the cooling capacity of the plant, the reaction will run away. The reaction might commence if an agitator is restarted after it has stopped, a catalyst is added suddenly, or because the desired reaction is slow to start. 

Standard enthalpies of combustion are listed in Table 6.4 and Appendix 2A. We have seen in Toolbox 6.1 how to use enthalpies of combustion to obtain the standard enthalpies of reactions. Here we consider another practical application— the choice of a fuel. For example, suppose we want to know the heat output from the combustion of 150. g of methane. The thermochemical equation allows us to write the following relation... 

Summarizing, the output of the reactor is an integral over time and over the entire reaction space with all interconnections between different zones of the reactor. Mixing and heat- and mass-transfer conditions are usually different in various zones and the pattern of these differences as well as proportions between size of zones vary with scale. Obviously, the histories of concentrations and temperatures in the zones differ. Whether the integral outputs of laboratory and full-scale reactors differ from each other, depends on the sensitivity of the process to mixing and heat- and mass-transfer conditions. If the sensitivity is low only minor... 

When assessing the energy output of an exothermic reaction it is advantageous to determine the rate of heat release directly, e.g., by using reaction calorimetry. Alternatively, it is possible to... 

Propane gas, C3H8, is sometimes used as a fuel. In order to measure its energy output as a fuel a 1.860 g sample was combined with an excess of 02 and ignited in a bomb calorimeter. After the reaction, it was found that the temperature of the calorimeter had increased from 25.000°C to 26.061°C. The calorimeter contained 1.000 kg of water. The heat capacity of the calorimeter was 4.643 kJ/°C. Determine the heat of reaction, in kJ/mol propane. The reaction was ... 

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