Enthalpy favourable changes

Now recall the reaction between mercury and oxygen. It favours the formation of HgO below about 400°C, but the decomposition of HgO above 400°C. This reaction highlights the importance of temperature to favourable change. Enthalpy, entropy, and temperature are linked in a concept called free energy. 

In Chapter 7, you learned that three factors—change in enthalpy (AH), change in entropy (AS), and temperature (T)—determine whether or not a change is favoured. The same three factors are important for determining how much of a salt will dissolve in a certain volume of water. These factors are combined in the following equation, where AG is the change in free energy of the system. 

The modem process for manufacturing nitric acid depends on the catalytic oxidation of NH3 over heated Pt to give NO in preference to other thermodynamically more favour products (p. 423). The reaction was first systematically studied in 1901 by W. Ostwald (Nobel Prize 1909) and by 1908 a commercial plant near Bochum. Germany, was producing 3 tonnes/day. However, significant expansion in production depended on the economical availability of synthetic ammonia by the Haber-Bosch process (p. 421). The reactions occurring, and the enthalpy changes per mole of N atoms at 25 C are ... 

The reaction favours the formation of ozone with a significant equilibrium constant. Appendix C also lists the enthalpies of formation and the standard enthalpy of the reaction ArH° can be calculated. The answer for the enthalpy calculation is ArH° = —106.47 kJ mol, showing this to be an exothermic reaction, liberating heat. The entropy change at 298 K can also be calculated because ArG° = ArH° — T ArS°, so ArS° = 25.4 Jmol-1 K-1, indicating an increase in the entropy of the reaction as it proceeds by creating one molecule from two. 

The overall enthalpy change of the insertion process contains contributions from four bonds (M-CO, M-COR, M-R and CO-R). As there is no significant difference between (Mn-R) and Zs(Mn-COR) then, at least in the case of manganese and hydrocarbon groups, R, the dominant factor will be the difference between T (Mn-CO) and E R-COX) [for R = CH3, E = 339 kJ mop1 (X = H), 370 kJ mol"1 (X = Cl) (Ref.23 )] which suggests that the insertion reaction is thermodynamically favoured with respect to decarbonylation. Kinetic studies of the carbonyl insertion reaction in solution have shown87) that the enthalpy of activation is 62 kj mol-1 for inser-... 

As seen in Table 2, A//yS = 9.42 kcal mol-1 and AAxS = 13.9 e.u., and so the free energy of transition state stabilization (approximately 5 kcal mol-1) results from a favourable enthalpy change, partly offset by an unfavourable entropy change. A similar situation pertains to binding of the substrate also (Table 2). Thus, the similarity between transition state binding and substrate binding, pointed out above from the correlation of p/fTS with pKs, is evident in thermodynamic parameters as well. 

For soft cations, such as Ag+ and Pb2+, covalent contributions are much more important, and consequently the observed order of complex stabilities is quite different from that for alkali cations NH > O > S for Pb2+ and NH, S > O for Ag+. Dissection of the overall effect into enthalpy and entropy contributions (Table 15) reveals the complicated nature of the heteroatom effect. For K+ and Ba2+, the more favourable entropy contribution for N and S ligands is more than offset by the unfavourable change in enthalpy of binding. 

What conditions determine whether or not a change is favourable How are different conditions related to equilibrium, where forward and reverse changes occur at the same rate The answers to these questions are linked to two important concepts in thermodynamics enthalpy and entropy. 

From left to right, the reaction is exothermic. Therefore, the enthalpy change, AH, is negative. If the enthalpy change was the only condition that determined whether a reaction is favourable, then the synthesis reaction would take place. The synthesis reaction does take place—but only at relatively moderate temperatures. Above 400 C, the reverse reaction is favourable. The decomposition of HgO(s) occurs. Thus, the direction in which this reaction proceeds depends on temperature. 

Temperature and enthalpy are not the only conditions that determine whether a change is favourable. Consider the process shown in Figure 7.5. A closed valve links two flasks together. The left flask contains an ideal gas. The right flask is evacuated. When the valve is opened, you expect the gas to diffuse into the evacuated flask until the pressure in both flasks is equal. You do not expect to see the reverse process—with all the gas molecules ending up in one of the flasks—unless work is done on the system. 

O O Write a short paragraph, or use a graphic organizer, to show the relationship among the following concepts favourable chemical change, temperature, enthalpy, entropy, free energy. 

Based on the change in enthalpy, you would expect that water would always freeze. Use the concepts of entropy and free energy to explain why this phase change is favourable only below 0°C. 

A wide range of ligands of the type RXCH2C02H where R = alkyl, alkenyl or aryl and X = S, Se or Te have been reacted with silver ions and thermodynamic data for some representative examples are given in Table 49.358,359 The silver-selenium complexes were found to be more stable than their silver-sulfur analogues as a result of both larger favourable enthalpy and smaller unfavourable entropy changes. 

---