Entropy changes, standard

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. 

Table 17.1 can be used to calculate the standard entropy change, AS°, for reactions, using the relation... 

Key Terms enthalpy, H free energy of formation, AG standard entropy change, AS° entropy, S spontaneous process standard free energy change, AG° free energy, G... 

The standard entropy of vaporization of benzene is approximately 85 J-K -mol-1 at its boiling point, (a) Estimate the standard enthalpy of vaporization of benzene at its normal boiling point of 80.°C. (b) What is the standard entropy change of the surroundings when 10. g of benzene, CfiHfi, vaporizes at its normal boiling point ... 

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 ... 

C14-0087. Calculate the standard entropy change at 298 K of each of the following reactions, which are important in the chemistry of coal. Assume that coal has the same thermodynamic properties as graphite. 

In much the same fashion as the AH° was tabulated, the standard molar entropies (S°) of elements and compounds are tabulated. This is the entropy associated with 1 mol of a substance in its standard state. Unlike the enthalpies, the entropies of elements are not zero. For a reaction, it is possible to calculate the standard entropy change in the same fashion as the enthalpies of reaction ... 

In a reaction such as that represented by Equation (12.23), the standard entropy change at the temperature T is given by the expression... 

Consider the following example. Using information from the SQA Data Booklet and the fact that silver(l) nitrate and nitrogen dioxide have S° values of 142 and 241J K mol respectively, calculate the standard entropy change, AS°, for the decomposition of silver(l) nitrate ... 

It can be calculated that the standard entropy change for this reaction Is... 

Note that reaction 2.16 is endothermic (the plus sign for AH° means that heat is taken into the system), whereas reactions 2.14 and 2.15 are exothermic (the reactions give out heat to the surroundings). Many heats of formation or of reaction can be measured by calorimetry (i.e., by recording the temperature rise of a thermally insulated apparatus of known heat capacity when the reaction of interest is carried out in it) or can be obtained from other AH data, as shown for the water-gas reaction. If we know AH° and also know the standard entropy change (AS°) for a given reaction, we can calculate its equilibrium constant (K°) from a combination of Eqs. 2.9... 

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