Standard enthalpy entropy

Standard Enthalpies, Entropies and Gibbs Free Energies of Sublimation of... 

Tkble 6.13 standard Enthalpies, Entropies, Free Energies, and Ceiling Temperatures for Polymerization of Various Monomer-Polymer Systems at 25°C... 

Calculate the standard enthalpy, entropy, and Gibbs free-energy changes accompanying the reaction at 573 K. 

Thermodynamic data that are suitable for tabulation include standard enthalpies, entropies, and free energies and can be regarded as universally applicable for systems at specified temperature when all participants are at thermal equilibrium. Though such data can also be obtained without thermal equilibrium, compensating experiments, or mathematical corrections are required, sometimes creating difficulties in practice and/or interpretation. A chemical system in the gas phase can reach thermal equilibrium, at a defined temperature, when a sufficient number of intermolecular collisions produce a Boltzmann distribution of energies in all modes, electronic, vibrational, rotational, and translational. In measurements made with an ion trap instrument or Fourier Transform Ion Cyclotron Resonance (FT-ICR) spectrometer at low pressure, hot ions must be cooled, commonly with a pulse of buffer... 

The relationship between the immersion and surface excess thermodynamic quantities has been further discussed by Schay, Everett [2-5], and Woodbury and colleagues [29,30]. The standard enthalpy entropy Ajoisf , and free enthalpy of component 1 in component 2 at... 

Calcium carbonate is insoluble in water. Yef it dissolves in an acidic solution. Calculate the standard enthalpy, entropy, and Gibbs free energy change for the reaction between sohd calcium carbonate and hydrochloric acid. What drives the reaction, the enthalpy change, or the entropy change ... 

The importance of enthalpy scales in chemical praxis is emphasized by the ubiquitous presence of standard enthalpy, entropy and free energy tables in every book of chemical thermodynamics and many reference books. 

Themiodynamic tables usually report at least tln-ee quantities almost invariably the standard enthalpy of fomration at 298 K, Af (298 K) usually the standard entropy at 298 K,, S (298 K) (not AS y298 K), but the... 

In a fiormal analogy to the expressions for the thenuodynamical quantities one can now defiine the standard enthalpy // and entropy ofiactivation. This leads to the second Eyring equation. ... 

Sodium Chlorite. The standard enthalpy, Gibbs free energy of formation, and standard entropy for aqueous chlorite ions ate AH° = —66.5 kJ/mol ( — 15.9 kcal/mol), AG = 17.2 kJ/mol (4.1 kcal/mol), and S° = 0.1883 kJ/(molK) (0.045 kcal/(molK)), respectively (107). The thermal decomposition products of NaClO, in the 175—200°C temperature range ate sodium chlorate and sodium chloride (102,109) ... 

Now, classical thermodynamics gives another expression for the standard free energy which separates it into two parts, the standard free enthalpy and the standard free entropy. 

Equation (1) can be viewed in an over-simplistic manner and it might be assumed that it would be relatively easy to calculate the retention volume of a solute from the distribution coefficient, which, in turn, could be calculated from a knowledge of the standard enthalpy and standard entropy of distribution. Unfortunately, these properties of a distribution system are bulk properties. They represent, in a single measurement, the net effect of a large number of different types of molecular interactions which, individually, are almost impossible to separately identify and assess quantitatively. 

It is clear that a graph of ln(V j-) or In(k ) against 1/T will give straight line. This line will provide actual values for the standard enthalpy (AH ), which can be calculated from the slope of the graph and the standard entropy (AS ), which can be calculated from the intercept of the graph. These types of curves are called van t Hoff curves and their important characteristic is that they will always give a linear relationship between In(V r) and (1/T). However, it is crucial to understand that the distribution... 

Summarizing, the greater the forces between the molecules, the greater the energy (enthalpy) contribution, the larger the distribution coefficient, and the greater the retention. Conversely, any reduction in the random nature of the molecules or any increase in the amount of order in the system reduces the distribution coefficient and attenuates the retention. In chromatography, the standard enthalpy and standard entropy oppose one another in their effects on solute retention. Experimentally it has... 

Different portions of the standard free energy of distribution can he allotted to different parts of a molecule and, thus, their contribution to solute retention can be disclosed. In addition, from the relative values of the standard enthalpy and standard entropy of each portion or group, the nianner in which the different groups interact with the stationary phase may also be revealed. 

Finally, another important and interesting fact is established from the data treated in this manner. All the examples given confirm that the standard entropy term tends to increase with the standard enthalpy term. Consequently, the increase in retention is not as great as that which would be expected from the increase in standard enthalpy alone. 

Figure 9. Graph of Standard Free Entropy against Standard Free Enthalpy for an Ether, Thioether and Amine...

Introducing the functions for standard free enthalpy and standard free entropy. 

It is seen from equation (22) that there will, indeed, be a temperature at which the separation ratio of the two solutes will be independent of the solvent composition. The temperature is determined by the relative values of the standard free enthalpies of the two solutes between each solvent and the stationary phase, together with their standard free entropies. If the separation ratio is very large, there will be a considerable difference between the respective standard enthalpies and entropies of the two solutes. As a consequence, the temperature at which the separation ratio becomes independent of solvent composition may well be outside the practical chromatography range. However, if the solutes are similar in nature and are eluted with relatively small separation ratios (for example in the separation of enantiomers) then the standard enthalpies and entropies will be comparable, and the temperature/solvent-composition independence is likely be in a range that can be experimentally observed. 

Finally, it is necessary to select values for the thermodynamic constants that are to be used in equation (9). The data selected were that published by Beesley and Scott [2], for the two enantiomers, (S) and (R) 4-benzyl-2-oxazolidinone. The values for the standard free enthalpy and standard free entropy for the (R) isomer were... 

In their original air standard cycle analysis, using constant specific heats, Hawthorne and Davis 4 considered the dry [CBTIiXr cycle. They assumed a perfect heat exchanger, with the specific heats of gas and air constant and identical, so that Ty becomes equal to Tj in Fig. 6.6. From their examination of the enthalpy-entropy diagram of this... 

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... 

The solid product, BaO, was apparently amorphous and porous. Decomposition rate measurements were made between the phase transformation at 1422 K and 1550 K (the salt melts at 1620 K). The enthalpy and entropy of activation at 1500 K (575 13 kJ mole-1 and 200 8 J K"1 mole-1) are very similar to the standard enthalpy and entropy of decomposition at the same temperature (588 7 kJ and 257 5 J K-1, respectively, referred to 1 mole of BaS04). The simplest mechanistic explanation of the observations is that all steps in the reaction are in equilibrium except for desorption of the gaseous products, S02 and 02. Desorption occurs over an area equivalent to about 1.4% of the total exposed crystal surface. Other possible models are discussed. 

The equilibrium concentrations of many disubstituted benzenes (containing alkyl and halogen substituents) show that the meta isomer is in nearly all cases the most thermodynamically stable. It is not obvious why this should be so. Shine182 had discussed this problem in terms of the relative sizes of the standard enthalpy and entropy changes between any pair of isomers. 

Here, AHfu° is the standard enthalpy of fusion and T, is the melting point. All entropies of fusion are positive, and so they are normally reported without their positive sign. 

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