Exothermic, generally enthalpy

The thermolyses presented in this chapter are one example of a series of analogous reactions. The Bell-Evans-Polanyi relationship of Equation 1.3 also holds for many other series of analogous reactions. The general principle that can be extracted from Equation 1.3 is that, at least for a reaction series, the more exothermic the enthalpy of reaction, the faster it will be. But this doesn t mean that all reactions that are exothermic are fast, so be careful. 

Having made its way to the interior surface of the porous particle molecule A is now ready for the first chemical step, adsorption on (he surface. In catalysis, adsorption is almost always chemisorption. Chemisorption results from chemical bonds between the molecule (adsorbate) and the solid surface (adsorbent). It is therefore very specific/ and receptive sites for chemisorption must exist. Physical adsorption comes from general van der Waals forces, which are physical in origin, weaker than chemisorption, and not specific. Chemisorption stops when a monolayer of adsorbed molecules is formed. It is activated with energies around lOkcal mole, is exothermic with enthalpy changes of -IS to -40kcal mole, is slowly reversible or even irreversible, and is the key step in activation of reaction intermediates. 

Figure 6.12 a General enthalpy level diagram for uncatalysed and catalysed reactions of an exothermic reaction, b The mountain pass analogy for the mechanism of catalytic action stressing the idea of the creation of an alternative reaction pathway... 

Using bond enthalpies, it is possible to explain why certain gas phase reactions are endothermic and others are exothermic. In general, a reaction is expected to be endothermic (Le., heat must be absorbed) if—... 

The exact enthalpy of polymerization for a particular monomer will depend on the steric and electronic effects imposed by the substituents attached to the E=E double bond. For olefins, resonance stabihzation of the double bond and increased strain in the polymer due to substituent interactions are the most important factors governing AHp For example, propylene has a calculated AH of -94.0 kJ moT, whereas the polymerization of the bulkier 2-methylpropene is less exothermic (-78.2 kJ moT ) [63]. Due to resonance effects, the experimentally determined AH of styrene (-72.8 kJ mol ) is less exothermic than that for propylene, while that for bulkier a-methylstyrene is even less favorable (-33.5 kJ moT ) [63]. In general, bulky 1,2-disubstituted olefins (i.e., PhHC= CHPh) are either very difficult or impossible to polymerize. 

Two types of situation may generally arise in respect of this equation. In the first, the enthalpy of the products exceeds that of the reactants (AH is positive), while in the second the converse happens (AH is negative). A reaction that conforms to the former situation is called an exothermic reaction and a reaction that corresponds to the latter situation is called an endothermic reaction. An exothermic reaction is accompanied by evolution of heat. An endothermic reaction, in contrast, occurs with absorption of heat. Enthalpy changes are... 

If a weak acid or weak alkali is used (or both are weak), the standard enthalpy of neutralisation is generally less exothermic. This is because heat energy is required to ionise a weak acid or base. 

If die semiempirical equations proposed in Reference 4 are correct, then die exothermicity of die reaction RX+R1 Y —RY+R1 X would be identical in the gaseous and liquid phases. Experience shows die assumption generally results in much poorer agreement for all-solid reactions. No such semiempirical equations for enthalpy of sublimation and its estimation appear to be available, although some cancellation is expected. 

The qualitative trend predicted by this equation is that, when the heat of solution is negative (the dissolution is exothermic, i.e., heat is evolved, the enthalpy of solvation is more negative than the lattice enthalpy is positive), the solubility diminishes with increasing temperatures. The opposite trend is observed for endothermic dissolution. An analogue of Eq. (2.58), with H replacing G, and the same tables [12] can be used to obtain the required standard enthalpies of solution of ionic solutes. No general analogues to Eqs. (2.53)-(2.55) are known as yet. 

Another class of compounds deals with iV,iV-dialkoxylamines, species of the general type R N(0R )2. The reaction of 2-nitroisoxazoline-iV-oxide in equation 16 has been observed and found to be exothermic by 126 4 kJmoR. From archival values for the gas phase enthalpy of formation of both reagents, we deduce the enthalpy of formation of the product, 5-nitro-l-aza-2,8-dioxabicyclo[3.3.0]octane, to be —53 10 kJmol . ... 

Interestingly, the standard entropies (and in turn heat capacities) of both phases were found to be rather similar [69,70]. Considering the difference in standard entropy between F2(gas) and the mixture 02(gas) + H2(gas) taken in their standard states (which can be extracted from general thermodynamic tables), the difference between the entropy terms of the Gibbs function relative to HA and FA, around room temperature, is about 6.5 times lower than the difference between enthalpy terms (close to 125 kJ/mol as estimated from Tacker and Stormer [69]). This indicates that FA higher stability is mostly due to the lower enthalpy of formation of FA (more exothermic than for HA), and that it is not greatly affected by entropic factors. Jemal et al. [71] have studied some of the thermodynamic properties of FA and HA with varying cationic substitutions, and these authors linked the lower enthalpy of formation of FA compared to HA to the decrease in lattice volume in FA. 

K. N. Houk, N. G. Rondan, and J. Mareda, Theoretical Studies of Halocarbene Cycloaddition Selectivities. A New Interpretation of Negative Activation Energies and Entropy Control of Selectivities, Tetrahedron 1985, 41, 1555. Calculations on carbene addition reactions led to a general explanation of why it is possible for very exothermic, bimolecular reactions to have negative activation enthalpies. 

One difference in behavior between the hydrophilic alkali halides and hydrophobic solutes like the larger tetraalkylammonium halides in water is expressed by the enthalpy. The enthalpies of solution of the larger tetraalkylammonium halides in water are more exothermic than those of the corresponding alkali halides but in other solvents, e.g., several amides, propylene carbonate (PC), and dimethylsulfoxide (DMSO), the reverse is true. Generally, this phenomenon is attributed to an enhanced hydrogen bonding in the highly structured solvent water in the vicinity of the tetraalkylammonium ions (hydrophobic hydration) (i). This idea is substantiated by the absence of the effect in solvents like N,N-dimethylformamide (DMF), PC, and DMSO (2), where specific structural effects are not present in the pure solvents. 

Several alkene isomers vary structurally in the position of a methyl branch on their parent 1-alkene chain. Generally, moving the methyl group from C3 to positions further from the double bond results in an exothermic enthalpy of isomerization. That is, the isoalkyl-1-alkenes are the most stable isomers and the 3-methyl-1-alkenes are (presumably) the least stable. Because of the problematic 5-methyl-1-hexene data and the lack of data for 3-methyl-1-heptene, nothing more quantitative can be said other than each methyl re-positioning down the chain results in about 1-2 kJmol-1 stabilization. A similar change in branching position from 3-methyl-n-alkanes to 2-methyl-n-alkanes releases about 3 kJmol-1. 

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