When a process occurs at constant pressure, the heat evolved (either released or absorbed) is equal to the change in enthalpy. Enthalpy (H) is the sum of the internal energy (U) and the product of pressure and volume (P⁢V) given by the equation: (1)H=U+P⁢V When a process occurs at constant pressure, the heat evolved (either released or absorbed) is equal to the change in enthalpy. Enthalpy is a state function which depends entirely on the state functions T, P and U. Enthalpy is usually expressed as the change in enthalpy (Δ⁢H) for a process between initial and final states: (2)Δ⁢H=Δ⁢U+Δ⁢P⁢V If temperature and pressure remain constant through the process and the work is limited to pressure-volume work, then the enthalpy change is given by the equation: (3)Δ⁢H=Δ⁢U+P⁢Δ⁢V Also at constant pressure the heat flow (q) for the process is equal to the change in enthalpy defined by the equation: (4)Δ⁢H=q By looking at whether q is exothermic or endothermic we can determine a relationship between Δ⁢H and q. If the reaction absorbs heat it is endothermic meaning the reaction consumes heat from the surroundings so q>0 (positive). Therefore, at constant temperature and pressure, by the equation above, if q is positive then Δ⁢H is also positive. And the same goes for if the reaction releases heat, then it is exothermic, meaning the system gives off heat to its surroundings, so q<0 (negative). If q is negative, then Δ⁢H will also be negative.

•   Enthalpy Definition: Enthalpy (H) is the sum of internal energy (U) and the product of pressure and volume (PV).
•   Enthalpy Change at Constant Pressure: At constant pressure, the change in enthalpy (ΔH) is equal to the heat flow (q) for the process.
•   Relationship Between ΔH and q: If a reaction is endothermic (absorbs heat), ΔH is positive. If a reaction is exothermic (releases heat), ΔH is negative.