Ethylene standard enthalpy

Find the standard enthalpy of formation for ethylene, C2H4(g), given the following data. 

The trans -> cis isomerisation of octafluorobut-2-ene is claimed by Schlag and Kaiser " to be a clean unimolecular reversible reaction. Studies by the static method using a seasoned quartz reaction vessel at 430-477 °C yielded the Arrhenius parameters, A = 3.4x 10 sec E = 56.4 kcal.mole L The E value is 6.4 kcal.mole lower than for the corresponding hydrocarbon - . Craig and Entemann have compared the standard enthalpy change for the cis trans isomerisation of 1,2-difluoroethylene with those for related halogenated ethylenes. The reported values are 4-928 cal.mole for difluoroethylene, 4-500 cal.mole for dichloroethylene, nearly zero for dibromoethylene, and —2000 cal.mole for diiodoethylene. 

While we know of no experimental thermochemical data for 123, Roth informs us that the enthalpy of formation of 124 is 259 kJmol-1. There are no experimental thermochemical data for 125 either, but it is easy to estimate the desired enthalpy of formation. We may either use the standard olefin approach with ethylene, 1,3-butadiene and (E)-l,3,5-hexatriene (i.e. with CH2=CH2, 33 and 79) or linearly extrapolate these three unsaturated hydrocarbons. From either of these approaches, we find a value of ca 225 kJ mol-1. Cross-conjugation costs some 35 kJ mol-1 in the current case. Interestingly, the directly measured cross-conjugated 1,1-diphenylethylene (126) is only ca 10 kJmol-1 less stable than its directly measured conjugated (E)- 1,2-isomer (40) despite the expected strain effects that would additionally destabilize the former species. 

These values of A Hr are standard state enthalpies of reaction (aU gases in ideal-gas states) evaluated at 1 atm and 298 K. 7VU values of A are in kilojoules per mole of the first species in the equation. When A Hr is negative, the reaction hberates heat, and we say it is exothermic, while, when A Hr is positive, the reaction absorbs heat, and we say it is endothermic. Tks Table 2-2 indicates, some reactions such as isomerizations do not absorb or liberate much heat, while dehydrogenation reactions are fairly endothermic and oxidation reactions are fairly exothermic. Note, for example, that combustion or total oxidation of ethane is highly exothermic, while partial oxidation of methane to synthesis gas (CO + H2) or ethylene (C2H4) are only slightly exothermic. 

The above data show that three pathways exist for the reaction of ozone with ethylene eoordinated and non-eoordinated addition in the singlet state and a reaction in the triplet state. The latter pathway is hardly possible for the thermal reaction the former two pathways exist in eompetition. To assess the effieieney of each pathway, we calculated the corresponding reaction rate eonstants. For this pmpose, a MOLTRAN program and results of the quantum ehemieal ealeulations were used to calculate the enthalpies (AH") and entropies (AS") of activation of the two reaction pathways. From the data obtained, the eorresponding rate constants k were ealeulated in terms of the standard transient state theoiy ... 

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