The state of the compound is specified by the following symbols Δh = δu +δn grt These tables include heat of formation data gathered from a variety of sources, including the primary and secondary literature, as well as the nist chemistry webbook
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Note that the table for alkanes contains δh fo values in kcal, and the table for miscellaneous compounds and elements contains these values in kj/mol.
For tabulation purposes, standard formation enthalpies are all given at a single temperature
298 k, represented by the symbol δfh⦵ For many substances, the formation reaction may be considered as the sum of a number of simpler reactions, either real or fictitious. Since the usual (but not technically standard) temperature is 298.15 k, we will use a subscripted “298” to designate this temperature Thus, the symbol (δ h 298 ∘) is used to indicate an enthalpy change for a process occurring under these conditions.
The standard enthalpy of formation at 25°c (298,15 k) for 1 mol of the substance in its given state (g= gas and l= liquide) from its elements in their standard state (stable forms at 1 bar and 25°c) We do this by defining the enthalpy of the standard state of the elements, which is only about 100 definitions and should cover all possible matter since all matter is made of elements The enthalpy of an element in its standard state at 298k is defined as zero. Standard enthalpy of formation at 298 k
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Before explaining the concepts involved, here is an important idea The standard enthalpy of formation for an element in its standard state is zero Now i know you don't know exactly what that means, but please remember it. To calculate the change in internal energy (δu°) for the given reaction at 298 k, we can use the relationship between the change in enthalpy (δh°) and the change in internal energy (δu°)
The formula we will use is
