Enthalpy.
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.
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. 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 (ΔH ) for a process between initial and final states.
DH =DU + DPV.
ΔH=ΔU+ΔPV 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:
DH = DU +P DV
Also at constant pressure the heat flow (q) for the process is equal to the change in enthalpy defined by the equation:
DH = q
By looking at whether q is exothermic or endothermic we can determine a relationship between ΔH 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 ΔH 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 ΔH will also be negative
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. 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 (ΔH ) for a process between initial and final states.
DH =DU + DPV.
ΔH=ΔU+ΔPV 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:
DH = DU +P DV
Also at constant pressure the heat flow (q) for the process is equal to the change in enthalpy defined by the equation:
DH = q
By looking at whether q is exothermic or endothermic we can determine a relationship between ΔH 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 ΔH 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 ΔH will also be negative
For a beeter understanding of the topic, watch this video.
References.
CrashCourse (2013). Enthalpy: Crash Course Chemestry #18. Retrieved from: https://www.youtube.com/watch?v=SV7U4yAXL5I
UCDAVIS CHEMWIKI. (n. d.). Enthalpy. Retrieved from: http://chemwiki.ucdavis.edu/Physical_Chemistry/Thermodynamics/State_Functions/Enthalpy
CrashCourse (2013). Enthalpy: Crash Course Chemestry #18. Retrieved from: https://www.youtube.com/watch?v=SV7U4yAXL5I
UCDAVIS CHEMWIKI. (n. d.). Enthalpy. Retrieved from: http://chemwiki.ucdavis.edu/Physical_Chemistry/Thermodynamics/State_Functions/Enthalpy