Enthalpy change products favored

From the chemical point of view, the solvent in which the CL experiment is carried out can have a dramatic influence on the efficiency of the CL reaction as solvation can alter the shapes, the depths, and the densities of the vibrational states of the potential surfaces representing the ground states of products and reactants and the lowest excited singlet state of the potential fluorophore. The alteration of the intersections of these potential energy surfaces can affect the enthalpies of reaction and the enthalpies of activation for dark and lumigenic reactions. In some cases, these changes will favor CL (if AH decreases relative to AHa) and in some cases, they will make it thermodynamically unfavorable for CL to occur. 

The change in enthalpy (A H°) is the heat of reaction—the amount of heat evolved or consumed in the course of a reaction, usually given in kilojoules (or kilocalories) per mole. The enthalpy change is a measure of the relative strength of bonding in the products and reactants. Reactions tend to favor products with the lowest enthalpy (those with the strongest bonds). 

The enthalpy change for the reaction is favorable because (1) electrostatic repulsion between the negative charges in ATP exceeds that in the reaction products, (2) the reaction products are resonance stabilized, and (3) the enthalpies of solvation of the products are larger than that for ATP. The entropy change for the reaction is favorable because of the release of a phosphate group. Note that this implies that ATP hydrolysis is strongly temperature-dependent [cf. Eq. 10.7)]. 

NaCl(s). What does it mean when there is a double arrow in a chemical equation Here is an equation with a double arrow A + B <—> C + D. Reactions with a double arrow mean that the reaction is a reversible reaction. As substances A and B react to form the products C and D (the forward reaction), the products C and D can form the original reactants (the reverse reaction). It sounds counterproductive, but it happens. Later you will see how conditions can be manipulated to make a reaction favor the products, but for now the focus is only on what a reversible reaction means for the enthalpy changes during the reaction. 

Chemical processes can be thermodynamically favored, or spontaneous, too. By spontaneous, however, we do not mean that the reaction will form products without any intervention. That can be the case, but often some energy must be imparted to get the process started. The enthalpy change in a reaction gives one indication as to whether the reaction is likely to be spontaneous. The combustion of 2(5 ) and 02( ), for example, is highly exothermic ... 

What could be the cause of such a large difference in thermodynamic stability After all, the number of Ni -N coordinate-covalent bonds is six in both the products of these two reactions, so the enthalpy changes (Ai/) involved when these bonds are formed should be fairly similar. That seems to leave entropy as the major explanation for the effect. Indeed, the rationale for the chelate effect can be understood in two ways, both related to the relative probabilities that the two reactions will occur. First, consider the number of reactants and products in the two cases. As written more explicitly in Equations (6.11) and (6.12), it is apparent that the number of ions and molecules scattered throughout the water structure in the first reaction stays the same (seven in both the reactants and the products). In the second reaction, however, three ethylenediamine molecules replace six water molecules in the coordination sphere, and the number of particles scattered at random throughout the aqueous solution increases from four to seven. The larger number of particles distributed randomly in the solution represents a state of higher probability or higher entropy for the products of the second reaction. Therefore, the second reaction is favored over the first due to this entropy effect. 

This result is the van t Hoff equation (Jacobus Henricus van t Hoff, Holland, 1852-1911). For a reaction with an enthalpy change independent of temperature. Equation 5.49 indicates that In K diminishes with increasing temperature if the reaction enthalpy change is negative. This means that increasing the temperature pushes the equilibrium toward the reactants for an exofhermic reaction. Increasing the temperature favors the products for an endothermic reaction. 

In a reaction in which the number of product molecules is equal to the number of reactant molecules, (e.g., A + B —> C + D), entropy effects are usually small, but if the number of molecules is increased (e.g., A —> B + C), there is a large gain in entropy because more arrangements in space are possible when more molecules are present. Reactions in which a molecule is cleaved into two or more parts are therefore thermodynamically favored by the entropy factor. Conversely, reactions in which the number of product molecules is less than the number of reactant molecules show entropy decreases, and in such cases there must be a sizable decrease in enthalpy to overcome the unfavorable entropy change. 

AH = enthalpy. This is the net amount of energy available from changes in bonding between reactants and products. If heat is given off, the reaction is favorable and AH < 0. 

The thennodynainics of complexation between hard cations and hard (O, N donor) hgands often are characterized by positive values of both the enthalpy and entropy changes. A positive AH value indicates that the products are more stable than the reactants, i.e., destabilizes the reaction, while a positive entropy favors it. If TAS > AH°, AG° will be negative and thus log(3 positive, i.e., the reaction occurs spontaneously. Such reactions are termed entropy driven since the favorable entropy overcomes the unfavorable enthalpy. 

The enthalpy value of Eq. (3.23) is very small as might be expected if two Cd-N bonds in Cd(NH3) 2 are replaced by two Cd-N bonds in Cd(en). The favorable equilibrium constants for reactions [Eqs. (3.22) and (3.23)] are due to the positive entropy change. Note that in reaction, Eq. (3.23), two reactant molecules form three product molecules so chelation increases the net disorder (i.e., increase the degrees of freedom) of the system, which contributes a positive AS° change. In reaction Eq. (3.23), the AH is more negative but, again, it is the large, positive entropy that causes the chelation to be so favored. 

Both U02(TTA)2 and Th(TTA)4 have two molecules of hydrate water when extracted in benzene, and these are released when TBP is added in reactions Eqs. (4.11) and (4.12). The release of water means that two reactant molecules (e.g., U02(TTA)2 2H2O and TBP) formed three product molecules (e.g., U02(TTA)2 TBP and 2H2O). Therefore, AS is positive. Since TBP is more basic than H2O, it forms stronger adduct bonds, and, as a consequence, the enthalpy is exothermic. Hence, both the enthalpy and entropy changes favor the reaction, resulting in large values of log K. 

Based on the above arguments, we expect reaction (4) to be much more favorable than reaction (3) because (3) involves attack on a stronger bond. However, there is a second equally important factor involved in the difference in reaction enthalpies for (3) and (4). The extra Mo=0 bond of 2 would appear to be a spectator to reaction (4), but in fact it helps promote the reaction. The reason is that in the product, 5, this spectator group is free to utilize two Mo d7T orbitals to form a super double bond, whereas in the reactant, 2, the second Mo=0 bond (the one involved directly in the reaction) requires one of these drr orbitals. Thus the spectator Mo=0 bond changes from a double bond to a super double bond when the aUyl reacts with the other Mo=0 bond. The net result then is that reaction (4) is more favored than (3) by 33 kcal. 

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