Enthalpy and Entropy

Every day, from melting ice cubes to burning fuel, we encounter countless chemical processes like these. So, have you ever wondered what makes these transformations possible? The answer lies within two striking concepts of enthalpy and entropy in thermodynamics. These two forces determine the pattern of energy transfer within a system. Here, we will be exploring these two important concepts of thermodynamics, along with the difference between them, equations, and their relationship. 

1.0Definition of Enthalpy and Entropy 

Enthalpy (H) 

Enthalpy is a term used for the total energy of a thermodynamic system. It comprises internal energy (U) and the product of the pressure (P) and volume (V) of the system. Enthalpy is utilised in thermodynamics to account for the exchange of heat during chemical reactions that take place under constant pressure.

Entropy (S)

Entropy is a measure of disorder or randomness. In thermodynamics, entropy indicates the amount of energy in a system that cannot be utilised to perform work. The more disordered a system, the greater the entropy. In simple words, entropy measures the extent of disorder or the number of microscopic states available to a system.

2.0Enthalpy and Entropy Equation 

Enthalpy and entropy possess their own separate formula to numerically quantify energy changes, particularly in reactions or phase transitions. These formulas or equations of enthalpy and entropy are: 

Enthalpy Equation 

The enthalpy equation is defined as the sum of the total internal energy of the system and the energy related to the pressure and volume of the system. Mathematically, the equation of enthalpy can be written as: 

H=U+p V

Here, 

• H is the Enthalpy (total heat content)
• U is the Internal energy of the system
• p is the Pressure of the system
• V is the Volume of the system

The above equation, when the applied pressure remains constant, can be rewritten as: 

$\Delta H=\Delta U+p\Delta V$

Here, 

$\Delta H=\text{ change in enthalpy }=H_{\text{Final Enthalpy }}-H_{\text{Initial Enthalpy }}$

Entropy Equation 

The entropy is the measure of disorder or randomness of a thermodynamic system. Hence, the equation for change of entropy can be written as: 

$\Delta S=\frac{q_{rev}}{T}$

Here:

• ΔS = Change in entropy
• q_rev = Reversible heat transfer
• T = Temperature in Kelvin

Units of Enthalpy and Entropy 

Both enthalpy (H) and entropy (S) are thermodynamic measures of energy, but they have different units because of how they're used and defined. For instance: 

• The SI unit of Enthalpy or change of enthalpy is Joule (J). 
• Whereas, the SI unit of Entropy is Joule/Kelvin (J/K)

3.0Relation Between Enthalpy and Entropy

In thermodynamics, the relation between enthalpy and entropy is given by the Gibbs Free Energy Equation. This relation is a resourceful factor in determining whether a process will occur spontaneously or not. The Gibbs relation can be given as: 

$\Delta G=\Delta H-T\Delta S$

Here: 

• ΔG = Change in Gibbs Free Energy
• ΔH = Change in Enthalpy (heat content)
• ΔS = Change in Entropy (disorder)
• T = Temperature in Kelvin

This formula explains the reason why certain reactions occur spontaneously and others require energy input. It demonstrates how the energy (enthalpy) and disorder (entropy) balance controls the direction of chemical processes.

Interpretation of the Equation 

• If $\Delta G<0$, then the process will occur naturally or spontaneously, that is, no additional energy will be required once it starts. 
• If $\Delta G>0$, then the process will be non-spontaneous, meaning the system requires additional energy to continue the process. 
• If $\Delta G>0$, then the process will occur in equilibrium, which does not mean the process stops; however, it means the rate of forward and reverse processes is the same. 

4.0Difference Between Enthalpy and Entropy 

Feature	Enthalpy (H)	Entropy (S)
Definition	It is the measurement of the total heat involved in a system.	It is the measure of disorder or randomness in a given system.
What it tells us	Enthalpy tells us the amount of heat either absorbed or released in a system.	Entropy tells us the extent of molecular disorder or energy distribution within a system.
Equation and Units	H=U+p V SI Unit: Joules or kilojoules, commonly written as KJ/mol	\Delta S=\frac{q_{r e v}}{T} SI Unit: Joules per kelvin, commonly J/Kmol.
Process type involved	The reaction involved can be both endothermic and exothermic.	Reaction helps determine if the system is becoming more or less ordered.
Spontaneity role	When combined with the Gibbs free energy equation, it affects the spontaneity of a system.	Entropy can directly influence the spontaneity of a system; the higher the entropy, the higher the spontaneity.
Example	Heating or combustion reaction.	Melting ice, evaporation, or mixing substances.
Enthalpy and Entropy diagram

5.0Practical Examples of Enthalpy and Entropy 

Examples of Enthalpy 

• Exothermic Reaction: 
• Burning wood or gasoline releases a huge amount of heat, causing an enthalpy decrease. 
• Mixing of acid and base, also known as a neutralisation reaction, releases heat in the system. 
• Endothermic Reaction: 
• Water absorbs heat from the source to change the form of water from liquid to vapour. 
• A cup of tea absorbs heat from the surroundings. 

Examples of Entropy 

• Increases: 
• When ice melts, solid water becomes liquid, and particles become freer to move randomly. 
• Decreases: 
• During condensation, gas molecules become more ordered as they turn into a liquid.