The Meaning of Enthalpy

For reactions at constant pressure, a thermodynamic variable called enthalpy H) eliminates the need to consider PV work separately. The enthalpy of a system is defined as the internal energy plus the product of the pressure and volume  

The change in enthalpy (AH) is the change in internal energy plus the product of the constant pressure and the change in volume  

Notice the right side of this equation is identical to the right side of Equation 6.5  

the change in enthalpy equals the heat gained or lost at constant pressure. With most changes occurring at constant pressure, AH is more relevant than AE and easier to find to find AH, measure qp. 

We discuss the laboratory method for measuring this heat in Section 6.3. 

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What is the meaning of enthalpy of a reaction Explain with a formula. 

Explain the meaning of enthalpy and enthalpy change in chemical reactions and processes. 

Enthalpy is the thermodynamic parameter that is useful for describing the thermal events of a material under constant pressure, for example, one atmosphere. We can understand the meaning of enthalpy and enthalpy change by reviewing the first law of thermodynamics. 

The meaning of the word aromaticity has evolved as understanding of the special properties of benzene and other aromatic molecules has deepened. Originally, aromaticity was associated with a special chemical reactivity. The aromatic hydrocarbons were considered to be those unsaturated systems that underwent substitution reactions in preference to addition. Later, the idea of special stability became more important. Benzene can be shown to be much lower in enthalpy than predicted by summation of the normal bond energies for the C=C, C—C, and C—H bonds in the Kekule representation of benzene. Aromaticity is now generally associated with this property of special stability of certain completely conjugated cyclic molecules. A major contribution to the stability of aromatic systems results from the delocalization of electrons in these molecules. 

The bond enthalpy in NO is 632 kj-mol 1 and that of each N—O bond in N02 is 469 kj-mol. Using Lewis structures and the mean bond enthalpies given in Table 6.8, explain (a) the difference in bond enthalpies between the two molecules (b) the fact that the bond enthalpies of the two bonds in N02 are the same. 

Benzene is more stable and less reactive than would be predicted from its Kekule structures. Use the mean bond enthalpies in Table 6.8 to calculate the lowering in molar energy when resonance is allowed between the Kekule structures of benzene. 

As described earlier, the reaction enthalpy is a very important factor that influences the reactivity of alcohol in free radical abstraction reactions. The IPM model helps to estimate the increment of activation energy AEn which characterizes the influence of enthalpy on the activation energy (see Equation [6.20] in Chapter 6). The parameters bre and values ACH for reactions of different peroxyl radicals with alcohols are given in Table 7.8. The mean value... 

The meaning of this observation is seen by considering Scheme 1. AH° (3) and AH0(4) are the enthalpies of hypothetical reactions of a family of compounds ML , where reactants and products are in the standard reference states and in the gas phase, respectively, and AHy are vaporization or sublimation enthalpies. 

An increase in the number of methylene units in alkyl benzene did not significantly affect the 7t-energy effect on their retention, but the enthalpy effect increased dramatically [80]. This means that a hydro-phobic compound can be adsorbed directly onto an octadecyl-bonded silica gel. The value of enthalpy effect of a methylene unit in alkyl benzene was calculated to be 500 cal/mol. 

These data for D (M-L) offer some basis for making predictions about the enthalpies of metal-carbon bonds involving other metals in these groups. It is important to bear in mind that all of the data in Table 6 concern the metals in their highest formal oxidation state. It is usually true that the mean bond enthalpy increases as the formal oxidation state of the metal decreases. This is exemplified by the values of >(M-Cl) (M = Nb, Ta, Mo, W) in various oxidation states of M (Table 7a), and... 

Table 8. First dissociation enthalpy Dj (Cl iM-Cl) kJ mol-1 compared with the mean dissociation enthalpy D (M-Cl) kJ mol-1 of MCln...

Equations 2.39 and 2.40 lead to Avap//°(C2l I5OH) = 42.4 0.5 kJ mol-1 [40], which agrees with the mean of the calorimetric results for the same liquid, 42.30 0.04 kJ mol-1 [39]. Note that the less sophisticated approach (equation 2.33) apparently underestimates the vaporization enthalpy by 0.6 kJ mol-1. However, this is not true because AvapH = 41.8 kJ mol-1 refers to the mean temperature, 326 K. A temperature correction is possible in this case, because the molar heat capacities of liquid and gaseous ethanol are available as a function of T [40]. That correction can be obtained as ... 

The observed flame features indicated that changing the atomization gas (normal or preheated air) to steam has a dramatic effect on the entire spray characteristics, including the near-nozzle exit region. Results were obtained for the droplet Sauter mean diameter (D32), number density, and velocity as a function of the radial position (from the burner centerline) with steam as the atomization fluid, under burning conditions, and are shown in Figs. 16.3 and 16.4, respectively, at axial positions of z = 10 mm, 20, 30, 40, 50, and 60 mm downstream of the nozzle exit. Results are also included for preheated and normal air at z = 10 and 50 mm to determine the effect of enthalpy associated with the preheated air on fuel atomization in near and far regions of the nozzle exit. Smaller droplet sizes were obtained with steam than with both air cases, near to the nozzle exit at all radial positions see Fig. 16.3. Droplet mean size with steam at z = 10 mm on the central axis of the spray was found to be about 58 /xm as compared to 81 pm with preheated air and 96 pm with normal unheated air. Near the spray boundary the mean droplet sizes were 42, 53, and 73 pm for steam, preheated air, and normal air, respectively. The enthalpy associated with preheated air, therefore, provides smaller droplet sizes as compared to the normal (unheated) air case near the nozzle exit. Smallest droplet mean size (with steam) is attributed to decreased viscosity of the fuel and increased viscosity of the gas. 

Three peroxides with aromatic substituents have reported enthalpy of vaporization data, all from the same source". The enthalpies of vaporization of cumyl hydroperoxide and ferf-butyl cumyl peroxide are the same, which makes us skeptical of at least one of these values. The calculated b value for cumyl hydroperoxide is 31.5, consistent with the alkyl hydroperoxides. The calculated b value for tert-butyl cumyl peroxide is 15.4 and more than twice that for the mean of the dialkyl peroxides. The structurally related tert-butyl p-isopropylcumyl peroxide has a b value of 8.8 and so is consistent with the other disubstituted peroxides. 

In the calculations of enthalpies of formation of ionic compounds, the differences from the accepted experimental values (which are very accurate) are probably due to the essential simplicity of the model, rather than having any other significance. If large discrepancies are found between experimental and calculated quantities, this probably means that the model is in error and that the compounds have a considerable covalent character. 

The standard enthalpies of formation of BH3(g) and diborane are +100 kj-mol 1 and +36 kj-mol, respectively, and the enthalpies of formation of B(g) and H(g) are +563 kj-mol-1 and +218 kj-mol"1, respectively, (a) Use these values to calculate the mean bond enthalpies of the B—H bonds in each case (b) Assume that terminal B—H bonds have the same strengths in each compound and estimate the bond enthalpy of the three-center B—H—B bonds in diborane. (c) Which bonds would you expect to be longer, the terminal B—H bonds or the three-center bonds Explain your answer. 

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