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For many substances, the formation reaction may be considered as the sum of a number of simpler reactions, either real or fictitious. The enthalpy of reaction can then be analyzed by applying Hess' law, which states that the sum of the enthalpy changes for a number of individual reaction steps equals the enthalpy change of the overall reaction.
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).
Enthalpy of combustion, Δ c H o –2726.3 kJ/mol Heat capacity, c p: 172.0 J/(mol K) Gas properties Std enthalpy change of formation, Δ f H o gas –252.7 kJ/mol Standard molar entropy, S o gas: 342.2 J/(mol K) Heat capacity, c p: 119.46 J/(mol K) at 25 °C van der Waals' constants [2] a = 1761 L 2 kPa/mol 2 b = 0.1344 liter per mole
For the special case of the formation of a compound from the elements, the change is designated ΔH form and is a weak function of temperature. Values of Δ H form are usually given where the elements and compound are in their normal standard states, and as such are designated standard heats of formation, as designated by a superscript °.
The standard state of a material (pure substance, mixture or solution) is a reference point used to calculate its properties under different conditions.A degree sign (°) or a superscript Plimsoll symbol (⦵) is used to designate a thermodynamic quantity in the standard state, such as change in enthalpy (ΔH°), change in entropy (ΔS°), or change in Gibbs free energy (ΔG°).
where ln denotes the natural logarithm, is the thermodynamic equilibrium constant, and R is the ideal gas constant.This equation is exact at any one temperature and all pressures, derived from the requirement that the Gibbs free energy of reaction be stationary in a state of chemical equilibrium.
As determined by the enthalpies below the corresponding molecules, the enthalpy of reaction for 2-methyl-1-butene going to 2-methyl-butane is −29.07 kcal/mol, which is in great agreement with the value calculated from NIST, [15] −28.31 kcal/mol. For 2-butanone going to 2-butanol, enthalpy of reaction is −13.75 kcal/mol, which again is in ...
Std enthalpy change of formation, Δ f H o solid? kJ/mol Standard molar entropy, S o solid? J/(mol K) Heat capacity, c p? J/(mol K) Liquid properties Std enthalpy change of formation, Δ f H o liquid: −80.882 ± 0.053 kJ/mol [2] Standard molar entropy, S o liquid? J/(mol K) Heat capacity, c p: 80.80 J/(mol K) Gas properties Std enthalpy ...