Enthalpy change, calculation using bond energies

Compare your answers from parts a and b of Exercise 31 with AH values calculated for each reaction using standard enthalpies of formation in Appendix 4. Do enthalpy changes calculated from bond energies give a reasonable estimate of the actual values ... 

The enthalpy change for each of the following reactions was calculated using bond energies. The bond energies of X—O, Y—O, andZ—O are all equal. 

Calculating enthalpy changes using bond energies... 

A more useful quantity for comparison with experiment is the heat of formation, which is defined as the enthalpy change when one mole of a compound is formed from its constituent elements in their standard states. The heat of formation can thus be calculated by subtracting the heats of atomisation of the elements and the atomic ionisation energies from the total energy. Unfortunately, ab initio calculations that do not include electron correlation (which we will discuss in Chapter 3) provide uniformly poor estimates of heats of formation w ith errors in bond dissociation energies of 25-40 kcal/mol, even at the Hartree-Fock limit for diatomic molecules. 

Poisoning is caused by chemisorption of compounds in the process stream these compounds block or modify active sites on the catalyst. The poison may cause changes in the surface morphology of the catalyst, either by surface reconstruction or surface relaxation, or may modify the bond between the metal catalyst and the support. The toxicity of a poison (P) depends upon the enthalpy of adsorption for the poison, and the free energy for the adsorption process, which controls the equilibrium constant for chemisorption of the poison (KP). The fraction of sites blocked by a reversibly adsorbed poison (0P) can be calculated using a Langmuir isotherm (equation 8.4-23a) ... 

The experimental activation energy calculated on the basis of a %-order reaction is found to be 46 Kcal. If we use the simple scheme, this should be equal [Eq. (XIII.14.6)] to E2 + HiEi — E ). The value of E2 has been measured at 7.5 Kcal (Table XII.6), while Eb = 0. If the bond energy of the H in the HCO radical is assigned the value 15 Kcal, current thermal data give a value of 82.4 Kcal to the enthalpy change of reaction 1, so that if there is no activation energy for the back reaction, this can be taken as the minimum value of Ei and the calculated value of the chain decomposition is then 48.7 Kcal — 22T/2 48 Kcal at 800°K, which may be considered to be in excellent agreement with the data. 

Bond dissociation energies are also used to calculate the enthalpy change (AH°) in a reaction in which several bonds are broken and formed. Aff ° indicates the relative strength of bonds broken and formed in a reaction. 

Tables of AH° for compounds are the most important data source for thermochemistry. From them it is easy to calculate AH° for reactions of the compounds, and thereby systematically compare the energy changes due to bond rearrangements in different reactions. Appendix D gives a short table of standard enthalpies of formation at 25°C. The following example shows how they can be used to determine enthalpy changes for reactions performed at 25°C and 1 atm pressure.

Stepwise formation constants and free energy, enthalpy, and entropy changes have been calculated for the 1 1, 1 2, and 1 3 complexes of AP" with malonate and succinate ions. Table 8 gives the calculated values for the stability constants of aluminium complexes with propionate and lactate ions in aqueous solutions, with I = 1. Solution calorimetry was used to determine values for the enthalpies of formation of tris(tropolonato)Al " and tris(4-methyltropolonato)Al " (Table 9) they were used to derive gas-phase enthalpies of formation, and hence the A1—O bond energies. ... 

Write a solution plan (without actual numbers, but including the bond energies you would use and how you would combine them algebraically) for calculating the total enthalpy change of the following reaction ... 

We know that heat is given out during this reaction. But where exactly does the heat energy come from We will now calculate the enthalpy change for this reaction using mean bond enthalpies. To do this we carry out the following operations ... 

Whereas the calculated enthalpy change for reaction (13.18) is —816kJmol , the experimentally measured value is AHT = —802kJmol . The error would be much greater if we had used the mean bond energy if C=0 in all common compounds (743 kJ mol ) rather than the mean bond energy for CO2 (816 kJ mol ) -see Exercise 130. This example serves to remind us that the choice of bond enthalpies in such calculations should be carefully reviewed beforehand. 

We can understand why bromination is more selective than chlorination by using bond dissociation enthalpies (Table 4.3) to calculate the energy changes for the propagation step in which each halogen atom abstracts a hydrogen from ethane. 

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