The standard enthalpy of combustion is the enthalpy change when one mole of a reactant completely burns in oxygen under standard thermodynamic conditions (although experimental values are usually obtained under different conditions and subsequently adjusted). By definition, the combustion reactions are always particularly exothermic and so enthalpies of combustion are always negative, although the values for individual combustions may vary.
The most common way of calculating the enthalpy change of combustion (or formation) is by using a Hess cycle or by using numerical based bond enthalpies. It is commonly denoted as or . When the enthalpy required is not a combustion, it can be denoted as . Enthalpies of combustion are typically measured using bomb calorimetry, and have units of energy (typically kJ); strictly speaking, the enthalpy change per mole of substance combusted is the standard molar enthalpy of combustion (which typically would have units of kJ mol−1).
Famous quotes containing the words standard, change and/or combustion:
“An indirect quotation we can usually expect to rate only as better or worse, more or less faithful, and we cannot even hope for a strict standard of more and less; what is involved is evaluation, relative to special purposes, of an essentially dramatic act.”
—Willard Van Orman Quine (b. 1908)
“The attitude is we live and let live. This is actually an amazing change in values in a rather short time and it’s an example of freedom from religion.”
—Tom Wolfe (b. 1931)
“Him the Almighty Power
Hurld headlong flaming from th’ Ethereal Skie
With hideous ruine and combustion down
To bottomless perdition, there to dwell
In Adamantine Chains and penal Fire,
Who durst defie th’ Omnipotent to Arms.
Nine times the Space that measures Day and Night
To mortal men, he with his horrid crew
Lay vanquisht, rowling in the fiery Gulfe”
—John Milton (1608–1674)