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 = q_p

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.