Question

Define enthalpy in terms of internal energy, heat, and P-V work. What measurable quantity is equal to enthalpy change?

Answer 1

The enthalpy of system is similar to the internal energy of system which is the total energy of system but it is equal to amount of absorbed or released hat of system at constant pressure.  It is denoted with the symbol of H. H = U + pV   where,

H is the enthalpy of the system
U is the internal energy of the system
p is the pressure at the boundary of the system and its surrounding
V is the volume of the system.

In a chemical reaction, the heat energy that is absorbed or released during the formation of the products by the reaction, given the fact that the reaction takes place under constant pressure, is called enthalpy change. The energy change during a reaction is called the enthalpy of the reaction.

Consider any chemical reaction, we can see the change in enthalpies in the reactants and the end products. This difference in the enthalpies of the products and the reactants can be called as Enthalpy Change.

Δ H = H₂ -H₁.Where H₁ = enthalpy of Initial state

H₂ = enthalpy of Final state.
Basically the enthalpy of reaction are of two types:
1) Exothermic reaction
If the energy is released during a reaction. Such a reaction is called as exothermic and the delta H is negative.
2) Endothermic reaction
If the energy could be consumed in a reaction. Such reactions are called endothermic and the delta H is positive here. The formula is as follows: Δ H = H₂ -H₁ ;   where, H₁ = enthalpy of Initial state before reaction and H₂ = enthalpy of Final state after the reaction