Thermodynamic Definition of Enthalpy (H):
H = E + P x V
E = energy of the system
P = pressure of the system
V = volume of the system
Consider a process carried out at constant pressure.
If work is of the form –Delta V, then:
Delta E = q x p + w
Delta E= q x p – P Delta V
Delta E + P Delta V = q x p
q x p is heat transferred at constant pressure.
Recall: H = E + P x V
Delta H = Delta E + Delta (P x V)
Delta H = Delta E + P Delta V (P is constant)
Delta H = q x p
The change in enthalpy is equal to the heat transferred at constant pressure.
Conversion of a substance from one state to another involves heat input or output
Heat is absorbed when breaking bonds
Solid → liquid → gas
Heat is released when making bonds
Gas → liquid → solid
Transition from solid to gas may lie through liquid phase...
Or transition may be direct – sublimation
Enthalpy change is the same by either route
Delta Hsubl = Delta Hfusion + Delta Hvap
Enthalpy change for a reaction is commonly known as heat of reaction
Exothermic refers to reaction where heat is given out by system
Delta H < 0 or negative
Endothermic refers to reaction where heat is absorbed by system and surroundings get cool:
Delta H > 0 or positive