Characteristics of enthalpy:
1. Enthalpy is a thermodynamic property of a system. Enthalpy H is defined as sum of the internal energy (U) of a system and the product of pressure and volume of the system.
That is, H = U + PV
2. Enthalpy is a state function which depends entirely on the state functions T, P and U.
3. Enthalpy is usually expressed as the change in enthalpy (∆H) for a process between initial and final states.
∆H = ∆U + P∆V
4. At constant pressure, the heat flow (q) for the process is equal to the change in enthalpy which is defined by the equation.
∆H = qp
5. In an endothermic reaction heat is absorbed by the system from the surroundings that is q > 0 (positive).
Therefore, at constant Tand P, by the equation above, if q is positive then ∆H is also positive.
6. In an exothermic reaction heat is evolved by the system to the surroundings that is, q < 0 (negative). If q is negative, then ∆H will also be negative.
7. Unit of enthalpy is KJ mol-1.
