Molar standard entropies

The heats of formation, the standard molar entropies at 298 K, are given below. 

The entropy of a substance, unlike its enthalpy, can be evaluated directly. The details of how this is done are beyond the level of this text, but Figure 17.4 shows the results for one substance, ammonia. From such a plot you can read off the standard molar entropy at 1 atm pressure and any given temperature, most often 25°C. This quantity is given the symbol S° and has the units of joules per mole per kelvin (J/mol-K). From Figure 17.4, it appears that... 

Standard molar entropies of elements, compounds, and aqueous ions are listed in Table 17.1 (p. 456). Notice that—... 

In taking these sums, the standard molar entropies are multiplied by the number of moles specified in the balanced chemical equation. 

TABLE 7.2 Standard Molar Entropy of Water at Various Temperatures... 

Standard molar entropies increase as the complexity of a substance increases. 

The standard molar entropies of gases are higher than those of comparable... 

To calculate the change in entropy that accompanies a reaction, we need to know the molar entropies of all the substances taking part then we calculate the difference between the entropies of the products and those of the reactants. More specifically, the standard reaction entropy, AS°, is the difference between the standard molar entropies of the products and those of the reactants, taking into account their stoichiometric coefficients ... 

The standard reaction entropy is the difference between the standard molar entropy of the products and that of the reactants weighted by the amounts of each species taking part in the reaction. It is positive (an increase in entropy) if there is a net production of gas in a reaction it is negative (a decrease) if there is a net consumption of gas. 

Calculate the standard Gibbs free energy of formation of HI(g) at 25°C from its standard molar entropy and standard enthalpy of formation. 

Calculate the standard reaction entropy from standard molar entropies (Example 7.9). 

Which substance in each of the following pairs would you expect to have the higher standard molar entropy at 298 K Explain your reasoning, (a) Iodine vapor or bromine vapor ... 

Predict which of the hydrocarbons below has the greater standard molar entropy at 25°C. Explain your reasoning. 

Predict which of the organic compounds (a) dimethyl ether, (CH3)20, or (b) diethyl ether (CH3CH2)20, has the greater standard molar entropy at 25°C. Explain your reasoning. 

Explain why the standard molar entropy of liquid benzene is less than that of liquid cyclohexane. 

The third law of thermodynamics establishes a starting point for entropies. At 0 K, any pure perfect crystal is completely constrained and has S = 0 J / K. At any higher temperature, the substance has a positive entropy that depends on the conditions. The molar entropies of many pure substances have been measured at standard thermodynamic conditions, P ° = 1 bar. The same thermodynamic tables that list standard enthalpies of formation usually also list standard molar entropies, designated S °, fbr T — 298 K. Table 14-2 lists representative values of S to give you an idea of the magnitudes of absolute entropies. Appendix D contains a more extensive list. 

Table 14-2 Standard Molar Entropies (S°) of Selected Substances at 298 K...

Molar entropies increase as the size and complexity of the molecule increases. Compare, for example, the standard molar entropies of the three two-carbon hydrocarbons ... 

If a2/ax = gim2lgxmx m2/mx (a = activity, g = molal activity coefficient, m = molality) if Henry s law is obeyed and a = gm, mh and m2 are the molal solubilities of the polymorphs, and if the standard molar enthalpies and standard molar entropies of solution are, respectively,... 

It follows that the standard molar entropy of the transition can be derived from the measured standard molar enthalpy of transition through the relationship... 

Here /g,hq and y ,ss are the activity coefficients of component B in the liquid and solid solutions at infinite dilution with pure solid and liquid taken as reference states. A fus A" is the standard molar entropy of fusion of component A at its fusion temperature Tfus A and AfusGg is the standard molar Gibbs energy of fusion of component B with the same crystal structure as component A at the melting temperature of component A. 

Given the following standard molar entropies measured at 25°C and 1 atm pressure, calculate AS° in (J/K) for the reaction... 

Restatement What is the standard molar entropy of HCOOH(s) ... 

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 ... 

Whether obtained from an actual experimentally feasible process or from a thought process, As i Gg, which is obtained from Eq. (2.9) by re-arrangement, pertains to the solvation of the solute and expresses the totality of the solute-solvent interactions. It is a thermodynamic function of state, and so are its derivatives with respect to the temperature (the standard molar entropy of solvation) or pressure. This means that it is immaterial how the process is carried out, and only the initial state (the ideal gaseous solute B and the pure liquid solvent) and the final state (the dilute solution of B in the liquid) must be specified. 

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