# Enthalpy

Enthalpy is defined as the heat of reaction at constant pressure. So it is equivalent to heat as long as pressure remains unchanged.

The symbol for enthalpy is “H” and the units remain in Joules (J) or kilojoules (kJ) just like heat.

THUS: H= qp or enthalpy= heat @ constant pressure.

Notation of Enthalpy:

The enthalpy for a reaction, or ∆H, tells whether the reaction was exothermic (- ∆H) or endothermic (+ ∆H).

• Enthalpy Notation at End of Equations

The ∆H can be written at the end of the chemical equation to signify the amount of heat AND whether it was absorbed (+∆H) or released (-∆H).

CH4 (g) + 2O2   —> CO2 (g) + 2H2O (l)       ΔH= -890.4 kJ

H2O (s) —->  H2O (l)           ΔH= +6.01 kJ

• Enthalpy Notation Within the Equation
∆H can also be written as a numerical value within the chemical equation itself.

A (+) ∆H would be written as a reactant as it would signify a necessary component for the reaction to occur, much like a chemical reactant.

H2O (s) + 6.01 kJ —> H2O (l)

A (-) ∆H would be written in the product half of the chemical equation, as it would justify heat as a “product” of the reaction.

CH4 (g) + 2O2 —> CO2 (g) + 2H2O (l) + 890.4 kJ