Enthalpy

As stated previously, Enthalpy is the amount of heat content used or released in a system at constant pressure. Enthalpy is usually expressed as the change in enthalpy. The change in enthalpy is related to a change in internal energy (U) and a change in the volume (V), which is multiplied by the constant pressure of the system.

Introduction

Enthalpy (H) is the sum of the internal energy (U) and the product of pressure and volume (PV) given by the equation:

`H=U+PV`
When a process occurs at constant pressure, the heat evolved (either released or absorbed) is equal to the change in enthalpy. 

`H = U`

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 (`DeltaH`) for a process between initial and final states:

`DeltaH=DeltaU+-PV`
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:

`DeltaH=DeltaU+PDeltaV`
Also at constant pressure the heat flow (q) for the process is equal to the change in enthalpy defined by the equation:

`DeltaH=q`
By looking at whether q is exothermic or endothermic we can determine a relationship between `DeltaH` 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 `DeltaH` 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 `DeltaH` will also be negative.