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Enthalpy

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Enthalpy is nothing but the total energy of a thermodynamic system. In other words, it is a thermodynamic potential and an extensive quantity. The SI unit of entropy is Joule. And it is denoted by H. Enthalpy of a system cannot evaluate directly.

One can measure the change in enthalpy rather than its absolute value very easily. In the case of endothermic reaction, the change in enthalpy is positive and in the case of exothermic reaction it is found to be negative. Detailed description about the enthalpy is given in the following section.

Define Enthalpy

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Enthalpy can be defined as the amount of total energy of a thermodynamic system. It consists of internal energy (the energy required to create a system), pressure and volume of the system.

Enthalpy Equation

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Mathematical representation of enthalpy is given as,
ΔH=ΔU+PV
where,
H
is the enthalpy of the system
U is the internal energy of the system
P is the pressure
V is the volume

Enthalpy of Reaction

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Enthalpy of reaction is defined as the total enthalpy change that takes place in a system while one mole of substance is transformed during a chemical reaction under normal conditions.
$\Delta H_{reaction}$ = $\sum H_{products}$ - $\sum H_{reactants}$

Enthalpy Change

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Enthalpy change denotes the heat absorbed or evolved in a reaction if it is taken in a constant pressure. The notation for the enthalpy change is ?H. This change can be calculated using the Hess,s law. It states that, if a process is taking place in different steps, the total enthalpy change of this process is equal to the sum of enthalpy change of the each steps.

Specific Enthalpy:
The equation of specific enthalpy is same as that of we discussed above.
The equation is
$\Delta $H = $\Delta $U+PV
where,
H is the specific enthalpy
U is the specific internal energy
P is the pressure
V is the specific volume

Standard Enthalpy Table

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Standard enthalpy table is illustrated below, which consists of enthalpies of some important substances.

Substances
Phase
ΔH (kJ/mol)
Aluminium solid 0
Barium chloride solid -858.6
Beryllium oxide solid -609.4
Bromine gas 30.91
Calcium chloride aqueous -877.3
Benzene liquid 48.95
Graphite solid 0
Diamond solid 1.8
Cesium gas 76.50
Water liquid -285.8

Enthalpy Examples

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The examples of enthalpy in daily life is given below:
  • Compressor in refrigerator
  • Chemical reaction involved in production of water
  • Combustion of substances

Calculating Enthalpy

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Some solved problems of enthalpy is given in this section.

Solved Examples

Question 1: Calculate the change in enthalpy if the internal energy change is 50J and pressure is 1atm and volume is 20g/mol?

Solution:
 
Given,
$\Delta $U = 50 J, P = 1 atm, V = 20 g/mol

The enthalpy equation is,
$\Delta $H = $\Delta $U + PV
$\Delta $H = 50+1×20 = 70 J/mol

 

Question 2: Estimate the change in enthalpy if the internal energy change is 420 J and pressure and volume is 1atm and 60 g/mol respectively?

Solution:
 
Given,
$\Delta $U = 420 J, P = 1 atm, V = 60 g/mol

The enthalpy equation is,
$\Delta $H = $\Delta $U + PV
$\Delta $H = 420+1×60 = 480 J/mol