Entropy standard values

Always check the signs of entropy values. Standard entropies are almost always positive, while standard entropies of formation are positive and negative. 

A lustrous metal has the heat capacities as a function of temperature shown in Table 1-4 where the integers are temperatures and the floating point numbers (numbers with decimal points) are heat capacities. Print the curve of Cp vs. T and Cp/T vs. T and determine the entropy of the metal at 298 K assuming no phase changes over the interval [0, 298]. Use as many of the methods described above as feasible. If you do not have a plotting program, draw the curves by hand. Scan a table of standard entropy values and decide what the metal might he. 

The contribution of this lack of regularity to the entropy of ice is thus R In 3/2 = 0.805 E. U. The observed entropy discrepancy of ice at low temperatures is 0.87 E. U., obtained by subtracting the entropy difference of ice at very low temperatures and water vapor at standard conditions, for which the value 44.23 E. U. has been calculated from thermal data by Giauque and Ashley,7 from the spectroscopic value 45.101 E. U. for the entropy of water vapor given by Gordon.8 The agreement in the experimental and theoretical entropy values provides strong support of the postulated structure of ice.9... 

Thermochemical data from the compilation of Stull et at., 1969. Entropy values are based on a 1 M standard state. The asterisk denotes symmetry-corrected quantities. Symmetry numbers were chosen as follows 18 for the n-alkanes, cis-3-hexene, dibuthyl sulphide, diethyl ether, and diethyl amine 2n for the cycloalkanes and 2 for all of the remaining ring compounds 3 for the alkanols, alkanethiols and alkyl amines 9 for the methyl alkyl sulphides... 

The need for entropy values is bypassed when the van t Hoff equation (d In K/dT) =AH/RT2 is used. This can be integrated, either assuming AH is temperature-independent, or by incorporating a specific heat-temperature variation. This is the so-called second law method which contrasts with the third law method. In the latter method, the standard enthalpy is obtained from each equilibrium constant using free-energy functions of all the species present, for example... 

To begin the problem, we need the standard entropy values for each of the reactants and products. These are the values we ll be using to complete this problem ... 

A Unlike heats of formation and Gibbs free energies of formation, the standard entropy values for elements are not zero. 

Standard entropies for many substances are available in tables such as Tables 11.2 through 11.6. Generally, the values listed are for 298.15 K, but many of the original sources, such as the tables of the Thermodynamics Research Center, the JANAF tables, or the Geological Survey tables, give values for other temperatures also. If heat capacity data are available, entropy values for one temperature can be converted to those for another temperature by the methods discussed in Section 11.4. 

The entropy of a substance is a measure of the amount of disorder within that system -the larger the value of the entropy, the greater the amount of disorder. Entropy is given the symbol S and the standard entropy of a substance, S°, is the entropy of 1 mol of the substance at a pressure of 1 atm and (usually) a temperature of 298 K. Standard entropy values, S°, for some selected substances are given on p. 17 of the SQA Data Booklet. Notice that the units of entropy are J K" mol". ... 

We have seen in chapter 2 that the heat capacity at constant P is of fundamental importance in the calculation of the Gibbs free energy, performed by starting from the standard state enthalpy and entropy values... 

Standard state entropy values and Maier-Kelley coefficients of heat capacity at constant P, with respective T limits of validity, are listed for the same components in table 5.13. The adopted polynomial expansion is the Haas-Fisher form ... 

Entropy is a thermodynamic quantity that is a measure of disorder or randomness in a system. When a crystalline structure breaks down and a less ordered liquid structure results, entropy increases. For example, the entropy (disorder) increases when ice melts to water. The total entropy of a system and its surroundings always increases for a spontaneous process. The standard entropies, S° are entropy values for the standard states of substances. 

The corrected standard entropy values at 298.15 K for the monoclinic lanthanide(III) chlorides (in J K 1 -mol 1) as derived from the work of Tolmach et al. (Tolmach et al., 1987, 1990a, 1990b, 1990c Gorbunov et al., 1986) 7mjn is... 

Carnall, 1982). The experimental standard entropy values thus obtained for the monoclinic lanthanide trichlorides are listed in table 4. 

Table 5.1. Standard chemical potentials pi , standard molar enthalpy h , and standard molar absolute entropy values s of substances in the standard state of 298 K and...

In contrast to AfG° and Afl° which are relative values (representing differences between values for the compound and the elemental reference states, arbitrarily assigned to be zero), standard entropy values, S° (Frame 16, section 16.2) are absolute values. This arises because the entropy of a perfectly crystalline solid at the absolute zero of temperature has a value of zero (Frames 16 and 17), i.e. ... 

Absolute standard molar entropy values, >9 (298 K) can be provided. They are tabulated at 25 °C and for P° = 1 bar pressure. It should be noted from equation (16.4) that the values are absolute entropy values (in contrast to values of AfH° and AfG° values which are quoted as differences (i.e. relative values) in thermochemical tables (Frame 11, section 11.2)). 

Like enthalpy, entropy can t be measured directly. It is possible to measure changes in the enthalpy of a system, which allows you to better understand the entropy of a system under specific conditions. The entropy values for one mole of a substance are known as standard molar entropies, 5°. The entropy change in a chemical reaction can be calculated using the equation ... 

While absolute entropy values can now be determined absolute values of Internal Energy and Enthalpy cannot be conceived. For ease of calculation, related especially to metallurgical reactions (constant pressure processes), a suitable reference point of enthalpy is conventionally chosen and that is - for pure elements, the enthalpy is zero when in Standard State . Standard... 

The steam/water enthalpy and entropy values for calculation of AG came from the steam tables and Mollier chart. See also Kotas (1995), p. 239, for moist standard atmosphere analysis. 

The standard entropy values represent the increase in entropy that occurs when a substance is heated from 0 K to 298 K at 1 atm. 

The standard entropy values (S°) of many common substances at 298 K and 1 atm are listed in Appendix 4. From these values you will see that the entropy of a substance does indeed increase in going from solid to liquid to gas. 

The value of AS° can be calculated by using the standard entropy values and the equation discussed in Section 10.8 ... 

Standard sorption entropy values, that refer to a gas-phase pressure,... 

VThere the superscript denotes the entropy value relative to a Standard state (the "absolute entropy ) and A and B are empirical constants. The new defined entropy value is different from the conventional thermodynamic scale value where Sjj4- is zero at all temperatures. The absolute entropy (S ) and conventional entropy (S ) of a species i are related by ... 

---