Acetylene standard enthalpy

B) Calculate the standard enthalpy change, AH°, for the combustion of acetylene. 

Calculate the standard enthalpy of formation of acetylene (C2H2) from its elements ... 

Experimental values for the standard enthalpy of formation of this ion, as reported in Table 7, are in fair agreement. They originate in (1) the proton affinity of acetylene " obtained in SIFT experiments and (2) the adiabatic ionization energy of the vinyl radical combined with its standard enthalpy of formation. ... 

The value of AH is an approximate measure of the stability of a substance relative to the elements from which it is made. The standard enthalpies of formation of graphite, diamond, water, ethyne (acetylene, C2H2), ammonia and sodium chloride are shown in Fig. 13.6. The reference states of elements define an energy baseline or sea level . Compounds such as ethyne, for which AHf is positive, and which therefore possess a greater enthalpy than their constituent elements, appear above sea level and are called endothermic compounds. Compounds such as water, ammonia and sodium chloride, for which AHf is negative and which therefore possess a lower enthalpy than their constituent elements, appear below sea level and are called exothermic compounds. 

The hydrocarbons acetylene (C2H2) and benzene (Cydg) have the same empirical formula. Benzene is an aromatic hydrocarbon, one that is unusually stable because of its structure, (a) By using the data in Appendix C, determine the standard enthalpy change for the reaction 3 C2H2(g) -------- CgHg(l). 

Calculate the standard enthalpy of formation of acetylene (C2H2) from its elements 2C(graphite) + Hjl. ) ---------------------------> C2H2(g)... 

There are many compounds in existence which have a considerable positive enthalpy of formation. They are not made by direct union of the constituent elements in their standard states, but by some process in which the necessary energy is provided indirectly. Many known covalent hydrides (Chapter 5) are made by indirect methods (for example from other hydrides) or by supplying energy (in the form of heat or an electric discharge) to the direct reaction to dissociate the hydrogen molecules and also possibly vaporise the other element. Other known endothermic compounds include nitrogen oxide and ethyne (acetylene) all these compounds have considerable kinetic stability. 

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