Understanding Entalpy and Combustion Enthalpy Change Standards

Enthalpy is a term in thermodynamics which states the amount of internal energy of a thermodynamic system plus the energy used to do work. From the review, the enthalpy can not be measured, but which can be calculated is the value changes. Mathematically, enthalpy change can be formulated as follows :

ΔH = ΔU + PΔV

Where :

H : System enthalpy (joules)
U : Internal energy (joules)
P : Pressure of the system (Pa)
V : System volume ( m³)

Enthalpy = H = Heat of reaction at constant pressure = Qp. Enthalpy change is the change in energy that accompanies the event of chemical changes at a constant pressure.

Termination of bonds requires energy (endothermic)
example : H₂ ®   2H - a kJ ; DH = + akJ 

Bond formation to give the energy (exothermic)
example : 2H ®   H₂ + a kJ ; DH = -a kJ

Standard combustion enthalpy change is the change in total enthalpy in a reaction system (where rekatan and the reaction product expressed as a thermodynamic system) that occurs when one molecule reacts completely with oxygen that occurs at 298K and atmospheric pressure of 1 atm. Generally, the value of the enthalpy of combustion is expressed in joules or kilojoules by one mole of reactants to react completely with oxygen.
Almost all combustion reactions will produce a negative value of enthalpy change

(ΔH_comb < 0).