Enthalpy of a Thermodynamic System

"Energy is the quantitative property that must be transferred to a physical system in order to do work on that system or heat it". While for the purpose of thermodynamics analysis, energy is classified into two broad categories namely "MACROSCOPIC ENERGIES" and "MICROSCOPIC ENERGIES". 
.......The energy which can be described by taking reference point outside the system or possessed by a body as a whole are called Macroscopic Energy, examples are potential energy, kinetic energy of whole system. While the energy which can be described by taking reference point inside the system are called Microscopic Energy........
➛ Microscopic is related to molecular or atomic level whereas macroscopic is not related to molecular or atomic level. Thus, Kinetic energy and Potential energy are the external forms of energies (or macroscopic forms of energies) while Internal energy is an internal form of energy (or Microscopic forms of energies).

INTRODUCTION TO ENTHALPY
HISTORY??
The term "Enthalpy" was introduced by a Dutch Scientist, Heike Kamerlingh Onnes in 1909. The word enthalpy means ‘Total Heat Content’. Enthalpy tells us how much heat is added or removed from the system at constant pressure condition. It is an important quantity because most of the reactions occur at constant pressure and it is used to measure the heat of a reaction.

INTRODUCTION??
In thermodynamics, there are two types of processes: "Flow And Non-Flow Processes". 
  1. In non-flow process, there is no change in mass of system i.e no mass flows in or out of system. In this case, the microscopic energy of that system is simply internal energy.
  2. In flow process, mass can flow in and out of system. So, in addition to internal energy fluids possesses flow energy (i.e., the energy by virtue of pressure difference between various points in system due to which fluids flow)
FOR NON-FLOW PROCESS
Total Microscopic Energy =Internal Energy 
FOR FLOW PROCESS
Total Microscopic Energy =Internal Energy + Flow Energy
☛ Flow energy at a point in open system=P.V
          P= pressure at that point.
          V=volume at point in system.

WHAT IS FLOW ENERGY??
The energy of flowing fluid is called flow energy. The flowing fluid can do work on the piston placed in its path.
DEFINITION??
Thus, "For flow process, total microscopic energy consists of internal energy and flow energy which is called Enthalpy". 
It is defined as a measure of the total microscopic forms of energy (kinetic energy+potential energy+internal energy) of a thermodynamic system.

Mathematical Definition Of Enthalpy
Enthalpy is the amount of energy either lost or gained by the system at constant pressure. It can also be explained as the total energy contained inside a system. It is denoted by a symbol 'H'. It is given by a formula;
             ➩ { Enthalpy, H ＝ U＋PV }
Thus, the mathematical definition of enthalpy is;
"Enthalpy of a thermodynamic system is equal to the summation of internal energy of the system and the amount of energy required to make room for it (means system) by displacing its environment and establishing it's volume 'V' and pressure 'P' ". 
According to the first law of thermodynamics, energy can neither be created nor be destroyed, it remains constant. So, we can say that the energy of the universe is constant. Because if the energy of a system decreases, the surrounding energy increases, but the total energy remains constant and vice versa.
The absolute value of enthalpy cannot be determined but we can measure the change in enthalpy. These enthalpy changes are of several types.

SIGNIFICANCE OF ENTHALPY
There is a lot of significance of enthalpy in a thermodynamic system.
  1. Enthalpy tells us whether the reaction is endothermic or exothermic.
  2. From enthalpy, we can calculate the heat of the reaction at constant pressure.
  3. Enthalpy tells us how much heat is absorbed or is released by the system during a chemical reaction.
To know about the nature of the reaction whether it is exothermic or endothermic the term enthalpy is used.
• When enthalpy (H) is positive, the reaction is endothermic.
• When enthalpy (H) is negative, the reaction is exothermic.

PHYSICAL INTERPRETATION
1. It is defined as amount of energy associated with the system plus the amount of work required to make space for the system to be exist in universe and the amount of work can be calculated from the pressure and volume associated with the system at that particular state.
2. If we supply energy to the system which is enclosed in nature or we can say if we supply energy to the closed system at constant pressure then we can say that the enthalpy change of the system is equal to the amount of energy transfer at the constant pressure.
EXAMPLE:-
(a) If we supply 50 KJ of energy through heat transfer to the closed system then we can say that the enthalpy change of the system is equal to 50 KJ.
(b) If we supply 50 KJ of energy by mean of electrical work to the closed system then we can say that enthalpy change of the system is equal to 50 KJ.

NOTE
➛ Enthalpy is measured in term of 'JOULE' according to International system of unit, In FPS it is measured in 'Btu', In CGS, it is measured in 'CALORIE' 
➛ "A thermodynamic system always favors minimum enthalpy". Enthalpy of a system signifies its capacity to absorb or release heat from surrounding.
➛ Enthalpy of the system is a "State Function"(it means that this property is depend upon the mass associated with the system, more the mass of system more the value of enthalpy and less the mass less is the enthalpy can be observed of the system).
➛ Enthalpy of the system is an "Extensive Property". It is point function.
➛ Enthalpy is an energy change between two states of a particular system. It is a function of temperature and pressure.
➛ Differential form of enthalpy; 
           ➩ dH ＝ dU＋d(PV)
Since, enthalpy is defined at constant pressure condition therefore;
           ➩ dH ＝ dU＋P dV＋ V dP 
                               ( ∵ dP＝0 )
           ➩ { dH ＝ dU＋ P (dV) }
If we integrate it for finite changes then,
                    ➩ { ∆H ＝ ∆U＋ P∆V }
➛ For a case of chemical reaction; change in enthalpy is equal to the difference between the enthalpy of products and the enthalpy of reactants. Thus;
          ➩ ∆H ＝ H(products) ＋H(reactants)

Hess's Law Of Thermodynamic
"The Hess's law of thermodynamics deals with the enthalpy of a reaction". Since we know that enthalpy is a state function and whenever a reaction occurs in one or more steps, the enthalpy associated with this chemical reaction depends only on the identity of the products and reactants. This means that it is completely independent whether the reaction occurs in one step or in more than one step. 
       "Hess's law is an important consequence of the first law of thermodynamics. This allows us to calculate the overall change in enthalpy by adding up the individual enthalpy change of each step of a chemical reaction". Mathematically;
       ➩ {∆Hr＝ ∆H₁＋∆H₂＋∆H₃＋∆H₄＋..........}

FAQs
What is the difference between heat and enthalpy??
Heat is a transfer of energy due to temperature differences whereas enthalpy is total heat content in a system. It is a change in heat at constant pressure.
If in case, the work done is zero then heat and enthalpy are interchangeable.
What is the effect of pressure and temperature on enthalpy??
When the temperature increases the kinetic energy of particles increases which increases its internal energy so enthalpy increases too.
Enthalpy occurs at constant pressure only.
Is enthalpy H or Delta H??
H is enthalpy which is equal to the sum of internal energy and product of pressure-volume work.
➩ H = U+ PV
Whereas ∆H is a change in enthalpy which is equal to the difference of enthalpy of product and enthalpy of reactants.
➩ ∆H = Hf – Hi
Or
➩ ∆H = ∆U + P∆V
What is enthalpy in layman’s terms??
The word enthalpy comes from the Greek word ‘Enthalpos’ meaning ‘To Put Heat Into’.
When changes in a system occur at constant pressure, it tells us how much heat and work were added or removed from the system.
Is enthalpy negative or positive??
Enthalpy change is positive for endothermic processes while negative for exothermic processes.
Is Enthalpy Just Energy??
Enthalpy is the sum of internal energy and product of pressure-volume work. Energy is a state of matter where enthalpy is an energy change between two states. So, enthalpy is not just energy.
What Does It Mean When Enthalpy Is Zero??
Zero enthalpy means reactants and products are identical. As enthalpy is a state function, if initial and final enthalpies are equal then the change in enthalpy is zero. It also means that there exists either no reaction or complete equilibrium.
What Is The Enthalpy Of Products Or Reactants??
Enthalpy of product is the total heat content per mole of product and enthalpy of reactant is the total heat content per mole of reactant.
Enthalpy of reaction is the difference between the enthalpy of products and the enthalpy of reactants.
What Is The Difference Between Heat And Enthalpy??
Heat is a transfer of energy due to temperature differences. Enthalpy is the change in the amount of heat at constant pressure.


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