Entropy change reactants favored

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. 

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. 

This is a strongly endothermic process, but it becomes possible at high temperature due to a favorable entropy change - formation of the random vapor state from solid reactants. Such reactions provide another reason for the lower flame temperatures achieved when organic binders are added to oxidizer/metal mixtures [3]. 

Entropy is often described as randomness, disorder, or freedom of motion. Reactions tend to favor products with the greatest entropy. Notice the negative sign in the entropy term (—TAS°) of the free-energy expression. A positive value of the entropy change (AS°), indicating that the products have more freedom of motion than the reactants, makes a favorable (negative) contribution to AG°. 

Dissociation of reactant A is an endothermic process in which the entropy change is positive. Consequently, the equilibrium constant increases at higher temperature via Le Chatelier s principle, which shifts the reaction to the right in favor of... 

Due to the negative entropy change calculated above, we expect that AG° will become positive at some temperature higher than 300°C. We need to find the temperature at which AG° becomes zero. This is the temperature at which reactants and products are equally favored (Kp = 1). 

For many reactions in which the number of molecules of products equals the number of molecules of reactants (e.g., when two molecules react to produce two molecules), the entropy change will be small. This means that except at high temperatures (where the term T AS° becomes large even if A5° is small) the value of AH° will largely determine whether or not the formation of products will be favored. If AH° is large and negative (if the reaction is exothermic), then the reaction will favor the formation of products at equilibrium. If AH° is positive (if the reaction is endothermic), then the formation of products will be unfavorable. 

If all three reactions have the same entropy change between the reactant and product, which reaction has the largest favorable AG . 

Similarly, reactants are favored for endothermic reactions (positive AH°) with negative entropy change (A5°< 0)... 

We noted earher that the reaction of an aldehyde with an alcohol to give an acetal is not favorable entropically. However, the simple expedient of using a diol to make the acetal ehminates this unfavorable entropy change. The most common alcohol used is for making cychc acetals 1,2-ethanediol (ethylene glycol). In this reaction, two molecules of reactant yield two molecules of product, and the entropy change is approximately zero. The acid catalyst is tolunesulfonic acid (TsOH), and benzene is the solvent. 

During a chemical reaction there is a change in the identity of substances as reactants transform to products. If the sum of the standard entropies of the products is greater than the sum of the standard entropies of the reactants, then there is a net increase in entropy and the reaction is favored. The difference in entropies can be calculated using data such as is given in Table 9.2 by the same... 

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