Endothermic reaction definition

Heat accompanying a chemical change. Thermochemistry, exothermic reaction, endothermic reaction. Definition of heat of reaction. Heat content. Heat of formation. Heat of combustion. Heat values of foods. Heat of neutralization. Heats of formation and relative electronegativity of atoms. The production of high temperatures and low temperatures. 

In order to exemplify the potential of micro-channel reactors for thermal control, consider the oxidation of citraconic anhydride, which, for a specific catalyst material, has a pseudo-homogeneous reaction rate of 1.62 s at a temperature of 300 °C, corresponding to a reaction time-scale of 0.61 s. In a micro channel of 300 pm diameter filled with a mixture composed of N2/02/anhydride (79.9 20 0.1), the characteristic time-scale for heat exchange is 1.4 lO" s. In spite of an adiabatic temperature rise of 60 K related to such a reaction, the temperature increases by less than 0.5 K in the micro channel. Examples such as this show that micro reactors allow one to define temperature conditions very precisely due to fast removal and, in the case of endothermic reactions, addition of heat. On the one hand, this results in an increase in process safety, as discussed above. On the other hand, it allows a better definition of reaction conditions than with macroscopic equipment, thus allowing for a higher selectivity in chemical processes. 

Exothermic Reaction. A reaction in which heat is liberated. It usually proceeds rapidly sometimes explosively. In an endothermic reaction a chemical change proceeds slowly with absorption of a definite number of calories Ref Hackh s (1944), 328-R (Exothermic) 305 R (Endothermic)... 

When the enthalpies of reaction between branched ketones and the corresponding 1,1-disubstituted alkenes are calculated using the multiple enthalpies of formation available for the latter, the following ranges are obtained Me/i-Pr, 196.6 to 200.5 Et/i-Pr, 201.2 to 206.6 and Me/t-Bu, 200.5 to 205.1 kJmol-1. Perhaps it is reasonable to conclude that the reaction enthalpies for the branched compounds either will be approximately constant, as for the unbranched ketone/alkene conversions, or will be more endothermic with branching, as in the branched aldehyde/alkene conversions. In either case, the least endothermic reaction enthalpy for the Me/i-Pr conversion above seems inconsistent and therefore the enthalpies of formation for 2,3-dimethyl-l-butene from References 16 or 26, which are essentially identical, should be selected. These enthalpies were also selected in a previous section. However, there is too much inconstancy, as well as too much uncertainty, in the replacement reactions of carbonyls and olefins to be more definitive in our conclusions. 

The normalized temperature t, given by equation (40), is a convenient independent variable only if it varies monotonically with distance through the flame. Although it is conceivable that nonmonotonic behavior of t could occur—for example, for flames in which endothermic reactions become dominant at the highest temperatures—no practical examples of adiabatic flames with temperature peaks or valleys are known the use of t seems appropriate from the viewpoint of monotonicity. The main limitation to the formulation given is assumption 7 to the effect that Cp i = Cp = constant. This assumption can be removed, with t = (T — Tq)I(T — Tq), merely at the expense of complicating the definition of / in equation (82), namely, /=Z7= 1 oo)/Cp(T — To), where each hj is a known function... 

Conversion efficiency is definitely affected by the large void fraction, which is apparent in the results from changes in the total throughput, or space velocity (0.56 versus 1.11 sec ), shown in Fig. 7. In this comparison, the concentration of unconverted hexane increased tenfold when the flow rate was doubled. The impact of improvements in conductive heat transfer, combined with the mass transfer limitations associated with the cell size and honeycomb design, and a catalyst loading that was nearly one-half Chat of commercial pellet catalysts (average, 11.5% versus 19.2%) suggested that both carbon formation and steam/hydrocarbon reactions were better controlled with monolithic supports under the conditions employed. This comparison was made where the extent of the endothermic reaction is equal between the pellet bed and the hybrid cordierite/metal monolith bed. 

The definition for an exothermic reaction is one where energy is given off. An endothermic reaction is where energy is required. 

You should be able to interpret chemical equations, and appreciate that they represent definite quantities of materiab reacting together. You should be familiar with the concepts of exothermic and endothermic reactions. 

However, more recent work has shown this idea to be untenable. There are many spontaneous endothermic reactions—for example, the solution in water of salts such as lithium chloride, where there is a cooling of the system. Furthermore there are many examples in which a reaction can be made to proceed in either direction to a definite state of equilibrium. Therefore heat evolved is not the sole driving force, although it does make an important contribution to the driving force. 

The relative importance of vibrational and translational energy in promoting chemical reactions is of both theoretical and practical interest. In reactions of diatomic molecules with atoms it has been substantiated both experimentally and theoretically that for endothermic reactions vibrational energy is more important, while for exothermic reactions the opposite is true. For polyatomic molecules, however, there is insufficient experimental and theoretical evidence to draw conclusions. The major work on laser-excited polyatomic reactions has involved the vibrational excitation of ozone in its exothermic reaction with nitric oxide. Although the vibrational energy increased the reaction rate, comparison with statistical models and the temperature dependence of the thermal reaction indicate about equal importance for vibrational and translational energy. On the other hand, a molecular beam study of the temperature dependence of the reaction of potassium with sulfur hexafluoride" has shown a definite preference for vibrational energy of the SF. ... 

From the definition of the periodic steady state it follows, that on one side the oxygen removed from the solid by the reduction must be replaced during the oxidation step. On the other side the heat released by the exothermic reaction must be used by the endothermic reaction or leave the reactor by convection. Since equation 8 is a combination of heat and mass balance and temperature is a measure for heat it can be used for the... 

The reactants are mixed at time t seconds and rapidly stirred so that reaction is complete within 30 s of mixing. As the reaction mixture can never be truly insulated, the container and its contents immediately start to lose heat (points C to D in an exothermic reaction. Fig. 13.5(a)) or gain heat (points E to F in an endothermic reaction. Fig. 13.5(b)). The temperature of the mixture changes too rapidly for a single definitive measurement to be taken immediately after mixing. Instead, tempera-... 

Because this is an endothermic reaction, the energy of the products must, by definition, be higher in energy than the energy of the starting materials. 

In a fuel cell electrochemical reactions take place leading to the direct conversion of chemical energy to electricity. The type of reaction that takes place plays a crucial role and, depending on its course, the fuel cell can achieve higher or lower efficiency. Exothermic reactions are characterized by the fact that the heat generated is put into the environment, whereas endothermic reactions absorb heat from the environment. Fuel cell efficiency is often based on only the chemical energy of the fuel delivered with such a definition, the heat supplied from the environment may result in the achievement of efficiency values above unity. Fig. 5.96 shows the theoretical efficiency of fuel cells from the supply of various compounds. 

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