Encyclopedia > Standard enthalpy change of combustion
The standard enthalpy of combustion is the enthalpy change when one mole of a substance completely reacts with oxygen under standard thermodynamic conditions (although experimental values are usually obtained under different conditions and subsequently adjusted). By definition, combustion reactions are generally strongly exothermic and so enthalpies of combustion are generally strongly negative. t In thermodynamics and molecular chemistry, the enthalpy or heat content (denoted as H or ΔH, or rarely as χ) is a quotient or description of thermodynamic potential of a system, which can be used to calculate the useful work obtainable from a closed thermodynamic system under constant pressure. ... The Three-Letter Acronym STP can have several meanings: Straight Through Processing is a banking term where a financial transaction is automatically completed without manual intervention. ... In thermodynamics, the word exothermic describes a process or reaction that releases energy in the form of heat. ...
It is commonly denoted as or
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). A calorimeter is a device used for calorimetry, the science of measuring the heat of chemical reactions or physical changes as well as heat capacity. ...
Enthalpy is a quantifiable state function, and the total enthalpy of a system cannot be measured directly; the enthalpychange of a system is measured instead.
Enthalpy is a thermodynamic potential, and is useful particularly for nearly-constant pressure processes, where any energy input to the system must go into internal energy or the mechanical work of expanding the system.
It is seen that the enthalpy for a general system will continuously increase to its minimum value, which it maintains at equilbrium.In a more general form, the first law describes the internal energy with additional terms involving the chemical potential and the number of particles of various types.
Second, if a process begins and ends at the same pressure, the amount of energy exchanged with the environment as heat is given by the change in the system's enthalpy.
An enthalpychange (ΔH) in chemistry describes the heat energy exchange with the surroundings at constant pressure.
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