Q. Define Enthalpy and Calculate the Enthalpy of a system?

Ans.

Enthalpy is a measure of the total energy content of a system.  It is represented by a big H. Change in enthalpy is represented by a delta H.

ΔH

Change in enthalpy

As long as a reaction occurs at a constant pressure the enthalpy change of a reaction is the same as the heat change.  This is a safe thing to assume in most common lab reactions because they all occur under the pressure of the atmosphere. 

There is not need to feel lost or confused when dealing with enthalpy.  The ΔH  for a chemical reaction depends on the enthalpy of the reactants and the enthalpy of the products.  Chemists have shown repeatedly that the path from the reactants to the products affect the change in enthalpy.

Whichever path you take results in the same change in entropy.