Endothermic reaction constant

FIGURE 6.19 If an endothermic reaction absorbs 100 k of heat at constant pressure, the height of the enthalpy "reservoir" rises by 100 k and AH = +100 kj. 

Self-Test 6.7B In an endothermic reaction at constant pressure, 30. kj of energy entered the system as heat. The products took up less volume than the reactants, and 40. kj of energy entered the system as work as the outside atmosphere pressed down on it. What are the values of (a) AH and (b) At/ for this process ... 

We can see from Table 9.2 that the equilibrium constant depends on the temperature. For an exothermic reaction, the formation of products is found experimentally to be favored by lowering the temperature. Conversely, for an endothermic reaction, the products are favored by an increase in temperature. 

Reactions of D with D20 and of 0 with 02, N20, and N02 have been studied with a magnetic sector mass spectrometer. Competition between electron transfer and ion-atom interchange has been observed in the production of 02 by reaction of 0 with 02, an endothermic reaction. The negative ion of the reacting neutral molecule is formed in 02, N2Of and N02 but not in D20. Rate constants have been estimated as a function of repeller potential. 

Experimental studies on how temperature affects equilibria reveal a consistent pattern. The equilibrium constant of any exothermic reaction decreases with increasing temperature, whereas the equilibrium constant of any endothermic reaction increases with increasing temperature. We can use two equations for A G °, Equations and, to provide a thermod3mamic explanation for this behavior AG = -RT x Teq AG° — AH°-TAS°... 

A constant volume batch reactor is used to convert reactant. A, to product, B, via an endothermic reaction, with simple stoichiometry, A —> B. The reaction kinetics are second-order with respect to A, thus... 

Figure 6.4a shows the behavior of an endothermic reaction as a plot of equilibrium conversion against temperature. The plot can be obtained from values of AG° over a range of temperatures and the equilibrium conversion calculated as illustrated in Examples 6.1 and 6.2. If it is assumed that the reactor is operated adiabatically, a heat balance can be carried out to show the change in temperature with reaction conversion. If the mean molar heat capacity of the reactants and products are assumed constant, then for a given starting temperature for the reaction Ttn, the temperature of the reaction mixture will be proportional to the reactor conversion X for adiabatic operation, Figure 6.4a. As the conversion increases, the temperature decreases because of the reaction endotherm. If the reaction could proceed as far as equilibrium, then it would reach the equilibrium temperature TE. Figure 6.4b shows how equilibrium conversion can be increased by dividing the reaction into stages and reheating the reactants...

For cases where AH0 is essentially independent of temperature, plots of in Ka versus 1/T are linear with slope —(AH°/R). For cases where the heat capacity term in equation 2.2.7 is appreciable, this equation must be substituted in either equation 2.5.2 or equation 2.5.3 in order to determine the temperature dependence of the equilibrium constant. For exothermic reactions (AH0 negative) the equilibrium constant decreases with increasing temperature, while for endothermic reactions the equilibrium constant increases with increasing temperature. 

This behavior can be shown graphically by constructing the rD-7 -/A relation from equation 5.3-16, in which kp kr, and Keq depend on T. This is a surface in three-dimensional space, but Figure 5.2 shows the relation in two-dimensional contour form, both for an exothermic reaction and an endothermic reaction, with /A as a function of T and ( rA) (as a parameter). The full line in each case represents equilibrium conversion. Two constant-rate ( -rA) contours are shown in each case (note the direction of increase in (- rA) in each case). As expected, each rate contour exhibits a maximum for the exothermic case, but not for the endothermic case. 

There is an important difference in this behavior between an exothermic reaction and an endothermic reaction. Fran equation 3.1-5, the van t Hoff equator, the equilibrium constant (Keq) decreases with increasing T for an exothermic reaction, and increases for an endothermic reaction. The behavior of f eq(T) corresponds to this. 

For adiabatic operation, with constant cP and (—AHRA), equation 21.5-9 is a linear relation for fA(T), with a positive slope for an exothermic reaction and a negative slope for an endothermic reaction. It may be regarded as an operating line, since each point (/a. T) actually exists at some position x in the reactor. 

Heat effects accompanying chemical reaction influence equilibrium constants and compositions as well as rates of reaction. The enthalpy change of reaction, AHr, is the difference between the enthalpies of formation of the participants. It is positive for endothermic reactions and negative for exothermic ones. This convention is the opposite of that for heats of reaction, so care should be exercised in applications of this quantity. Enthalpies of formation are empirical data, most often known at a standard temperature, frequently at 298 K. The Gibbs energies of formation, AGfl likewise are empirical data. 

The reaction enthalpy, AHr, is the quantity of heat that is either absorbed by the system (endothermic reaction) or released by the system (exothermic reaction), at constant pressure, as determined by the reaction equation. The reaction enthalpy AHr depends both on the chemical nature of the individual reactants and their physical states. 

A change in temperature, however, does force a change in the equilibrium constant. Most chemical reactions exchange heat with the surroundings. A reaction that gives offbeat is classified as exothermic, whereas a reaction that requires the input of heat is said to be endothermic. (See Table 13-2.) A simple example of an endothermic reaction is the vaporization of water ... 

The first law of thermodynamics also tells you that if no work is done on or by the sample, that is, pressure and volume are held constant, any heat flow is counterbalanced by a change in internal energy. An exothermic reaction releasing heat to the surroundings, therefore, is accompanied by a decrease in internal energy, whereas an endothermic reaction has a concomitant increase in internal energy. 

Endothermic Reaction A reaction in which heat is absorbed. To maintain a constant temperature during reaction, heat must be added to reactants and products. 

Alternatively, for many classes of compounds the heats of formation can be estimated through additivity of bond properties or group additivity rules [32], Let s take a simple example using the additivity of bond properties to estimate the heat of formation of some species A. Suppose we know (1) the heat of formation of a related compound ABR, where B is the atom to which A is bonded and R is the rest of the molecule, (2) the heat of formation of BR, and (3) that for a series of other molecules in which a A-B bond occurs the A-B bond-dissociation-energy is nearly constant, and we assign it the value B.D.E.(A-B) J. Now consider the endothermic reaction... 

The equilibrium constant of an endothermic reaction (AH° =+) increases if the temperature is raised. 

The equilibrium constant for the reaction between methanol on surface sites and internal sites, K, is the most complex in its temperature and acetylation dependence. In some coals temperature dependences shift about from exothermic to endothermic reactions, and no overall pattern for high rank and low rank coals seems to exist. 

The principle ofLe Chatelier summarizes the conclusions that may be drawn from the illustrative examples in this chapter "Whenever a stress is placed on a system at equilibrium, the equilibrium position shifts in such a way as to relieve that stress." If the stress is an increase in the partial pressure (concentration) of one component, the equilibrium shifts toward the opposite side in order to use up part of the increase. If the stress is an increase in the total pressure, the stress may be partially relieved by a shift toward the side with the smaller number of gaseous moles if there are the same number of gaseous moles on each side, no shift will occur, and no stress will be relieved. If the stress is an increase in temperature, the stress is partly relieved because, for an endothermic reaction, the equilibrium constant increases and the equilibrium shifts to the right for an exothermic reaction, the equilibrium constant decreases and the equilibrium shifts to the left. A catalyst places no stress on the system and causes no shift in the equilibrium position. 

Self-Test 6.9B In a certain endothermic reaction at constant pressure,... 

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