Entropy change, for reactions

A The cycle of Figure 6.1 suggests that the equation giving the j entropy change for reaction (6.7) is, when combined with the result of Worked Problem 6.3 ... 

The entropy change for reaction 1 may be estimated at about 35 e.u., so that the rate constant for the reverse reaction (the recombination of CHs + CHO to reform acetaldehyde) would have a preexponential factor of 10 - to liters/mole-sec,... 

Because the entropy change for reaction (b) is negative (the product is much more ordered than the reactants), the free energy of reaction (b) becomes more positive as temperature increases. The reverse is true for reaction (c). Therefore, the overall reaction (c) — (b) has a substantial decrease of free energy with temperature, and above the... 

Markowitz [66] also notes that the entropy changes for reactions (2.13) and (2.14) are similar for the majority of metal perchlorates. Hence the differences in AG for reactions (2.13) and (2.14) arise from the differences in A// for these two reactions. As discussed above, this difference will then be determined by the differences in the enthalpies of formation of equivalent amounts of the chloride and the oxide. Such values are often more accessible than values of AG fo .son- The validity of this approach is confirmed [66] by a consideration of available data for the decomposition products of metal perchlorates. 

Table lX-3 gives the experimental data and the derived enthalpy and entropy changes for Reactions (IX. 15) and (IX. 16), using the auxiliary data for HSO and SO from Table IV-1. 

From the data of Table 9.1, the standard free energy, enthalpy and entropy changes for Reaction [9.3] are ... 

Standard entropy change for Reaction (13.32), J/K mol A at T Electrolyte thickness, m Dimensionless temperature Electrolyte resistivity, Q... 

The standard entropy change for the atom-molecule reactions is in the range 5-20 mole and the halogen molecule dissociation has an eiiU opy change of about 105 e.u. The halogen molecule dissociation energy decreases from chlorine to iodine, but the atom-molecule reactions become more endothermic from chlorine to iodine, and this latter effect probably influences the relative contributions to the mechanism from chain reaction and biinolecular reaction. 

This expression states that there will be energy free to do work when Q exceeds AE. Expressed in another way work ean be done, that is an action can proceed, if AE - 0 is negative. If the difference between AE and Q is given the symbol AA, then it can be said that a reaction will proceed if the value of AA is negative. Since the heat term is the product of temperature T and change of entropy AS, for reactions at constant temperature then... 

It can be shown - p- that if an LFER is observed over a range of temperatures, and if the enthalpy and entropy changes are temperature independent, then the enthalpy changes must be directly proportional to the entropy changes for the reaction series. Let us start with the proposition that a real effect of this type has been demonstrated for a reaction series we write this as... 

An important question for chemists, and particularly for biochemists, is, Will the reaction proceed in the direction written J. Willard Gibbs, one of the founders of thermodynamics, realized that the answer to this question lay in a comparison of the enthalpy change and the entropy change for a reaction at a given temperature. The Gibbs free energy, G, is defined as... 

The ease of dissociation of the X2 molecules follows closely the values of the enthalpy of dissociation since the entropy change for the reaction is almost independent of X. Thus F2 at 1 atm pressure is 1% dissociated into atoms at 765°C but a temperature of 975°C is required to achieve the same degree of dissociation for CI2 thereafter, the required temperature drops to 775°C for Br2 and 575°C for I2 (see also next section for atomic halogens). 

In principle, the second law can be used to determine whether a reaction is spontaneous. To do that, however, requires calculating the entropy change for the surroundings, which is not easy. We follow a conceptually simpler approach (Section 17.3), which deals only with the thermodynamic properties of chemical systems. 

Free energy changes for reactions, like enthalpy or entropy changes, are additive. That is, if Reaction 3 = Reaction 1 + Reaction 2 then AG3 — AG] + A G2... 

Self-Test 7.12A Without doing any calculations, estimate the sign of the entropy change for the reaction N2(g) + 3 H2(g) - 2 NH,(g) and explain your answer. 

Use data in Table 7.3 or Appendix 2A to calculate the standard entropy change for each of the following reactions at 25°C. For each reaction, interpret the sign and magnitude of the reaction entropy, (a) The synthesis of carbon disulfide from natural gas (methane) CH4(g) + 4 S(s, rhombic) - CS2(1) +... 

C14-0015. Compute the standard entropy change for the following reaction ... 

The entropy of any chemical substance increases as temperature increases. These changes in entropy as a function of temperature can be calculated, but the techniques require calculus. Fortunately, temperature affects the entropies of reactants and products similarly. The absolute entropy of every substance increases with temperature, but the entropy of the reactants often changes with temperature by almost the same amount as the entropy of the products. This means that the temperature effect on the entropy change for a reaction is usually small enough that we can consider A Sj-eaction he independent of temperature. 

The entropy change for the reaction is the difference in entropy between products and reactants ... 

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