Standard entropy change calculating

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

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) +... 

C14-0087. 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 ... 

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... 

As an example, let s calculate the standard entropy change for the reaction... 

Because the free-energy change for any process at constant temperature and pressure is AG = AH — TAS, we can calculate the standard free-energy change AG° for a reaction from the standard enthalpy change AH0 and the standard entropy change AS°. Consider again the Haber synthesis of ammonia ... 

In these calculations, the electron affinity of the methyl radical has been taken1 as 27 kcal.mole-1. The other enthalpy terms are all well-known quantities the enthalpies of hydration of individual ions have been assigned as done by Valis ev (see ref. 2) and the enthalpy of hydration of the gaseous methyl anion has been taken as that of the bromide ion. It can be seen from Table 1 that not only is the formation of the methyl anion energetically very unfavoured in the gas phase, but it is also endothermic to the extent of 54 kcal.mole-1 in aqueous solution. A check on this final result can be made by consideration of the standard entropy change for the reaction... 

Bywater and Roberts have made a detailed comparison of such reactions for a series of compounds from which the changes in entropy of activation can be considered in some detail. Table XII.3 lists the calculated standard (300°K, 1 atm) molar entropies of translation and rotation of complexes made of H or CH3 and RH and the standard entropy changes in the reactions H + HR H2R, CII3 + HR = CH4R (neglecting vibration) calculated by these authors. 

We use the equation for standard entropy change to calculate A5j from the tabulated values of standard molar entropies, 298, for the substances in the reaction. 

Standard entropy changes cannot be measured directly in the laboratory. They are calculated from experimentally obtained values of AG° and Ai7 . From the data given here, calculate A5° at 298 K for each of the following reactions. 

C) Calculate the standard entropy change, A5°, for the combustion of acetylene gas. 

We can also calculate the standard entropy change associated with the reaction as AG° - AH0 - TAS°. 

From the absolute entropy values in Appendix 3, calculate the standard entropy changes for the following reactions at 25°C. 

A least square plot of log Ccsc as a function of number of carbon atoms in the alkyl chain is given in Fig. 4.13. obtained from the slope of this line is —0.95kT and is comparable to the free energy of micellization measured for similar surfactants in solution (Fig. 4.14). Heat and standard entropy changes associated with the adsorption process can be calculated by considering the adsorption of the long-chain molecules, X, as follows (Somasundaran and Fuerstenau, 1972). 

The standard entropy change of the reaction is calculated from the values of the absolute entropy of the products and reactants at the standard temperature Ts — 25 C. 

The first step is to calculate K. The method involving the standard heat of reaction and the standard entropy change is usually used because these data are available for most substances. In the tables giving standard heats of formation (or standard heats of combustion) and absolute ejn-tropies, the standard state is usually specified as unit fugacity for all ga es and is usually stated as th pure gas at 1 atm pressure. The calculation must be based upon the standard state specified by these tables and is not arbitrary, as many people are led to believe. If the standard state for each component is that of unit fugacity, the equilibrium equation becomes... 

Strategy To calculate the standard entropy change of a reaction, we look up the standard entropies of... 

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