Enthalpy is the measurement of energy in a thermodynamic system. The quantity of enthalpy equals to the total content of heat of a system, equivalent to the system’s internal energy plus the product of volume and pressure.
Technically, enthalpy describes the internal energy that is required to generate a system and the amount of energy that is required to make room for it by establishing its pressure and volume and displacing its environment.
When a process begins at constant pressure, the evolved heat (either absorbed or released) equals to the change in enthalpy. Enthalpy is the sum of internal energy denoted by U and product of volume and Pressure, denoted by PV, expressed in the following manner.
Enthalpy is also described as a state function completely based on state functions P, T and U. It is normally shown by the change in enthalpy (ΔH) of a process between the beginning and final states.
If the pressure and temperature don’t change throughout the process and the task is limited to pressure and volume, the change in enthalpy is given by,
The flow of heat (q) at constant pressure in a process equals to the change in enthalpy based on the following equation,
A relationship between q and ΔH can be defined knowing whether q is endothermic or exothermic. An endothermic reaction is the one that absorbs heat and reveals that heat is consumed in the reaction from the surroundings, hence q>0 (positive). If q is positive, then ΔH is also positive, at constant pressure and temperature for the above equation. Similarly, if the heat is released being an exothermic reaction, the heat is given to the surroundings. Hence, q<0 (negative). Therefore, ΔH will be negative if q is negative.
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