Potential curves endothermic

A potential curve of an endothermically chemisorbed atom or molecule represents an excited state with respect to the normal state of the physically adsorbed atom or molecule. When cesium atoms are adsorbed on salt layers or on cesium oxide, they are adsorbed as atoms and not, as they would be on metal surfaces, as ions. Ionization can be brought about by absorption of light 172) or by thermal excitation (173). 

The potential curves of the adsorption of cesium on a CaF2 surface are given in Fig. 21, which shows that the curve for the ion represents an endothermic chemisorption. By the absorption of light of suitable wave length the system is transferred from minimum B to a point P of the upper curve and an electron is freed and may be drawn off as a photoelectron. The phenomenon of the selective photoelectric effect could be fully explained by this photoionization process (174). By thermal excitation the transfer can be effected at point electron emission of oxide cathodes. Point S is reached by taking up an amount of energy, which may be called the work function of the oxide cathode in this case but which is completely comparable with the energy of activation in chemisorption discussed in Sec. V,9 and subsequently. We shall not discuss these phenomena in this article but refer to a book of the author where these subjects are dealt with in detail (174) ... 

The ground state potential curve (state A, Fig. 7) shows a potential barrier in the vicinity of the excited state minimum. The dotted parts of branches b and c beyond point I correspond to the potential curves of the doubly excited configurations. Going from DHP to I requires an activation energy of 23 kcal/mole which is lowered to ca. 18 kcal/mole if the C.l. depression of state A is taken into account ) Going from cis-stilbene to 1 along the thermal path is very strongly endothermic... 

For highly exothermic reactions, -AI > Es> and a = 0 (activationless process), while for highly endothermic reactions, AI > Es> and a = 1 (barrierless process). As we have mentioned in Chapter 1, these extreme cases follow from the general phenomenological theory of an elementary act. A gradual transition from one extreme value of a to another is natural The specific form of the dependence described by Equation (3.9) follows from the parabolic shape of the potential curves, i.e. from the harmonic oscillator approximation. 

Sketch a potential energy diagram which might represent an endothermic reaction. (Label parts of curve representing activated complex, activation energy, net energy absorbed.)... 

Figure 1-2. Potential energy curve for an endothermic reaction.

Fig. 4.11 Schematic potential energy curves for activated and non-activated chemisorption of hydrogen on a clean metal surface and exothermic or endothermic solution in the bulk. A more pronounced minimum just below the surface allows for subsurface hydrogen (onedimensional Lennard-Jones potential, Somorjai (1987) Ref [33]).

Figure 1.5. Concentration dependence of the chemical potential of mixing Ap,f RT (a), the molar Gibbs potential of mixing AGm/RT (6) for regular mixtures with ajRT (the digits at the curves) = 3, 2, 2,1 (endothermic mixtures), 0 (ideal mixture), —I (exothermic mixture). stands for dApifdxj. State diagram (c) bi—binodal, sp—spinodal, C—critical point. ajRTc = 2 at T = Tc...

One problem inherent in any Marcus-type equations, based on parabolic potential energy curves, lies in their limiting behaviour at very high endo- or exo-thermicities for very endothermic reactions the theory leads to AG =AG and for very high exothermic processes, which avoid the so called "inverted region", there is a cut-off AG =0. Both forms of limiting behaviour have been criticized as being physically unrealistic [24,25]. However, this problem does not arises within ISM AGl for very exothermic reactions... 

The adsorption of H is conveniently described in terms of simplified one dimensional potential energy curves for an H2 molecule and for 2H atoms on a metal surface (Fig. 1). Far from the surface the two curves are separated by the heat of dissociation Ed = 218 kJ/mol H. The flat minimum in the H2 + M curve corresponds to physisorbed H2 (heat of physi-sorption Ep 10 kJ/mol H) and the deep minimum in the 2H + M curve describes chemisorbed H (heat of chemisorption E 50 kJ/mol H). If the two curves intersect above the zero energy level, the chemisorption requires an activation energy E. In further steps the chemisorbed H atoms penetrate the surface and are then dissolved exothermically or endothermically in the bulk where hydrides can be formed. There is now experimental and theoretical evidence that not all chemisorbed H necessarily stays on top of the first metal atom layer, but also below it as a so called subsurface H (two step chemisorption). 

Potential Energy Curves Showing Energy Changes Involved in Endothermic... 

POTENTIAL ENERGY CURVES SHOWING ENERGY CHANGES INVOLVED IN ENDOTHERMIC AND EXOTHERMIC REACTIONS... 

The above observations all indicate that substitution of iron for aluminum in the octahedral layer and of aluminum for silicon in the tetrahedral layer both cause dehydroxylation to commence at a lower temperature. This, combined with the fact that the binding energy of the hydroxyl groups also affects the results, explains the extreme complexity of the curves obtained for this subgroup. Yet all have a potential diagnostic feature in the size and configuration of the low-temperature endothermic peak. 

Fig. 1.9. Potential energy curves for strongly endothermic reactions. See text for notation.

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