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Since the pressure of the standard formation reaction is fixed at 1 bar, the standard formation enthalpy or reaction heat is a function of temperature. For tabulation purposes, standard formation enthalpies are all given at a single temperature: 298 K, represented by the symbol Δ f H ⦵ 298 K.
Starting with simple linear and branched alkanes and alkenes the method works by collecting a large number of experimental heat of formation data (see: Heat of Formation table) and then divide each molecule up into distinct groups each consisting of a central atom with multiple ligands: X-(A)i(B)j(C)k(D)l
Standard enthalpy of formation is the enthalpy change when one mole of any compound is formed from its constituent elements in their standard states. The enthalpy of formation of one mole of ethane gas refers to the reaction 2 C (graphite) + 3 H 2 (g) → C 2 H 6 (g).
Enthalpies and enthalpy changes for reactions vary as a function of temperature, [5] but tables generally list the standard heats of formation of substances at 25 °C (298 K). For endothermic (heat-absorbing) processes, the change ΔH is a positive value; for exothermic (heat-releasing) processes it is negative.
Benson group-increment theory (BGIT), group-increment theory, or Benson group additivity uses the experimentally calculated heat of formation for individual groups of atoms to calculate the entire heat of formation for a molecule under investigation. This can be a quick and convenient way to determine theoretical heats of formation without ...
Most computerized databases will create a table of thermodynamic values using the values from the datafile. For MgCl 2 (c,l,g) at 1 atm pressure: Thermodynamic properties table for MgCl 2 (c,l,g), from the FREED datafile. Some values have truncated significant figures for display purposes. The table format is a common way to display ...
Low heat values are calculated from high heat value test data. They may also be calculated as the difference between the heat of formation ΔH ⦵ f of the products and reactants (though this approach is somewhat artificial since most heats of formation are typically calculated from measured heats of combustion).. [1]
The standard Gibbs free energy of formation (G f °) of a compound is the change of Gibbs free energy that accompanies the formation of 1 mole of a substance in its standard state from its constituent elements in their standard states (the most stable form of the element at 1 bar of pressure and the specified temperature, usually 298.15 K or 25 °C).