Forward reaction entropy changes

Here AH is the standard enthalpy change of reaction AS the standard reaction entropy change R the gas constant T the temperature /(eq the equilibrium constant for the reaction, given simply by the product of concentrations (activities in reality) of all the products to the power of their stoichiometric coefficients over the same product for reactants m the number of products / / the forward rate constant kr the reverse rate constant n, the stoichiometric coefficient of species i and 1 the number of reactants. A AG value below zero indicates a reaction with an equilibrium point where there is an excess of products over reactants, a... 

Positive entropy changes are favourable negative enthalpy changes are favourable and hence help to drive the reaction forwards. Negative entropy changes are unfavourable positive enthalpy changes are unfavourable and hence help to drive the reaction backwards. 

Since the equilibrium is largely in favour of the octadiene, kj > k, and hence these Arrhenius parameters must be very close to the values for the forward reaction. The normal value for the A factor is to be expected in this case since the reactant has a rigid structure with no possibility of free internal rotations, and hence there is httle entropy change on going to the transition complex. 

The reactions of the thiophene derivatives in both forward and reverse directions are characterized by lower enthalpies and entropies of activation than the reactions of the selenophene analogs. In the forward reactions, enthalpy and entropy changes compensate nearly exactly and result in slightly greater rates of adduct formation for the selenophene derivatives despite the higher enthalpies of activation. The higher entropies of activation for the selenophene derivatives have been attributed to less solvated transition states as compared to the reactions of the thiophene analogs (Table XXVIII). 

From measured forward and reverse rate constants of the reaction of a solvated electron with water Baxendale (I) estimated the standard potential for the solvated electron. Use of a more recent rate constant (3) and correction (8) for a certain omitted entropy change yields a value, E° e = + 2.7 volts, for the standard oxidation potential of the solvated electron. To calculate AF0,int for a reaction from a difference of the standard oxidation potentials of the two reactants, E° e — E°, the AF°f must be corrected for the AF0. of about 5 kcal./mole. This correction can be made (8) by taking the effective E° e, E° eff, for a solvated electron to be 2.9 volts. 

Chemical reactions are reversible. As reactants are converted to products, the concentration of the reactants decreases, and the concentration of the products increases. Since rates are related to concentrations, the rate of the forward reaction begins to slow, and the rate of the reverse reaction quickens as a reaction proceeds. Eventually, the two rates become equal. This condition, where the forward reaction rate equals the reverse reaction rate, is called chemical equilibrium. At chemical equilibrium, there is no change in the concentration of the products or reactants. Equilibrium will be reached from either direction, beginning with predominantly reactants or predominantly products. Equilibrium is the point of greatest entropy. 

C. The entropy change of a forward reaction is exactly opposite to the entropy of the reverse reaction. 

You have read that both heat and entropy play a role in determining spontaneity. In general, the direction of a chemical reaction is determined by the magnitude and direction of the heat energy and entropy changes. For example, a reaction will proceed in the forward direction, toward formation of products, if that direction results in both a release of heat and an increase in entropy. As an example, consider the combustion of butane, C4H10. 

It was also indicated that the compact nature of the transition state could arise, in part, from a reduction in volume as a precursor contact pair is formed, with this factor being greater for the reverse than for the forward reaction. Markedly negative entropies of activation in both directions, determined earlier, [138] indicated that electrostriction increase is important particularly for the back reaction. Intrinsic volume changes were not considered to contribute... 

For the reverse change (condensation), A5 niv also equals zero, but AS°ys and ASsun have signs opposite those for vaporization. A similar treatment of a chemical change shows the same result the entropy change of the forward reaction is equal in magnitude but opposite in sign to the entropy change of the reverse reaction. Thus, when a system reaches equilibrium, neither the forward nor the reverse reaction is spontaneous, and so there is no net reaction in either direction. 

Here, AH is the enthalpy change for the polymerization, which equals the difference in activation energies for the forward and reverse reaction, and AS is the standard entropy change. 

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