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There are two kinds of enthalpy of combustion, called high(er) and low(er) heat(ing) value, depending on how much the products are allowed to cool and whether compounds like H 2 O are allowed to condense. The high heat values are conventionally measured with a bomb calorimeter. Low heat values are calculated from high heat value test data.
Standard enthalpy of combustion is the enthalpy change when one mole of an organic compound reacts with molecular oxygen (O 2) to form carbon dioxide and liquid water. For example, the standard enthalpy of combustion of ethane gas refers to the reaction C 2 H 6 (g) + (7/2) O 2 (g) → 2 CO 2 (g) + 3 H 2 O (l).
Enthalpy of formation is defined as the enthalpy change observed in a constituent of a thermodynamic system when one mole of a compound is formed from its elementary antecedents. Enthalpy of combustion is defined as the enthalpy change observed in a constituent of a thermodynamic system when one mole of a substance burns completely with oxygen.
For instance, carbon and hydrogen will not directly react to form methane (CH 4), so that the standard enthalpy of formation cannot be measured directly. However the standard enthalpy of combustion is readily measurable using bomb calorimetry. The standard enthalpy of formation is then determined using Hess's law. The combustion of methane:
Std enthalpy change of formation, Δ f H o liquid: −238.4 kJ/mol Standard molar entropy, S o liquid: 127.2 J/(mol K) Enthalpy of combustion Δ c H o: −715.0 kJ/mol Heat capacity, c p: 70.8–90.5 J/(mol K) (at −97.6 to 64.7 °C) [5] 79.9 J/(mol K) at 20 °C Gas properties Std enthalpy change of formation, Δ f H o gas: −201.3 kJ/mol ...
The law states that the total enthalpy change during the complete course of a chemical reaction is independent of the sequence of steps taken. [2] [3] Hess's law is now understood as an expression of the fact that the enthalpy of a chemical process is independent of the path taken from the initial to the final state (i.e. enthalpy is a state ...
Std enthalpy change of formation, Δ f H o liquid –198.7 kJ/mol Standard molar entropy, S o liquid: 295 J/(mol K) Enthalpy of combustion, Δ c H o –4163 kJ/mol Heat capacity, c p: 197.66 J/(mol K) Gas properties Std enthalpy change of formation, Δ f H o gas –167.2 kJ/mol Standard molar entropy, S o gas: 388.82 J/(mol K) Heat capacity, c ...
Std enthalpy change of formation, Δ f H o solid-763 kJ/mol Standard molar entropy, S o solid: 112.7 J/(mol K) Heat capacity, c p: 106.3 J/(mol K) at –124 °C Liquid properties Std enthalpy change of formation, Δ f H o liquid –303.0 kJ/mol Standard molar entropy, S o liquid: 192.8 J/(mol K) Enthalpy of combustion, Δ c H o –2021 kJ/mol ...