Enthalpy and Entropy

A Guide to Enthalpy and Entropy 

Enthalpy and Entropy are two significant terms related to thermodynamics. Both of them are partly related to each other in a reaction because the fundamental rule of any reaction is releasing or absorbing heat or energy. Relying on these two factors, a new product is formed through a standard reaction of several compounds.  

What is Enthalpy?

Enthalpy is defined as a change in internal energy and volume at constant pressure. It deals with the heat contained in any system. Thereby it changes when heat enters or leaves a system. For example, it increases when heat is added and decreases when heat is withdrawn from that system. 

There are some molecules which take part in this change are called “internal enthalpy” and the molecules that do not are referred to as “external enthalpy”. The enthalpy is represented through the following equation. 


Where E is enthalpy, U is internal energy of any system, P is pressure, and V is volume. 

Change in Enthalpy

The change of enthalpy in a reaction is almost equivalent to the energy gained or lost during a reaction. Also, it is concluded that if the enthalpy decreases, a reaction is successful. The reason behind it is if a system participates in a reaction, it releases energy. Hence, its own energy content gets low, according to the fundamental concept of energetics. 

It happens because during a chemical reaction, some bonds of reactions need to be broken to produce the product. Therefore, it requires some energy to break the bonds, and in return, some energy is released as well after the product is formed. 

This change in enthalpy is represented by delta H. Also, the equation looks like 


Standard Enthalpy Change

It refers to a change in enthalpy that occurs in a reaction taking place under standard conditions and where the reactants are in a standard state. These standard states are also denoted as “reference state”. 

The symbol of standard enthalpy change is Delta H nought or H. In case of this change in a reaction; the symbol will become ∆H⁰r

For example, record the standard enthalpy change in the reaction between H and O₂ to form water or H₂O. 

2H₂(g) + O₂(g)  →  2H₂O(I)         ΔH⁰ᵣ = -572kJmol⁻¹ 

If you observe the reaction, you will see that the energy is not indicating at any specific substance. Instead, it is denoting that, if 2 M of hydrogen gas reacts with 1 M of oxygen gas, 2 M of liquid water is made, and 572 kJ heat creates. 

Standard Condition

  • 298 K (25⁰ C).

  • Pressure- 1 bar or 100kPa. 

  • The concentration of a solution has to be 1 mol dm-3.

Standard States 

In case of a reaction, all the physical and chemical states have to be in standard condition. It means that standard state of water is in liquid form and not in ice or water vapour. Similarly, the standard state of oxygen is the gas form of it. 

However, in case of allotropic elements, we have to consider the one which is the most energetically stable. For example, oxygen under standard condition exists as both 02 and ozone (O3), but O2 is more stable energetically; hence it is oxygen’s standard state. 

Standard Enthalpy Change of Formation

It is expressed as ∆H⁰f. It happens when only 1 M of product is formed in a reaction. For example, 

               2H₂(g) + ½ O₂(g)        →       H₂O(I)         ΔH⁰f = -286kJmol⁻¹ 

                              ↑                               ↑   

Don’t worry about fractions           1 mole of water formed. That’s why the fraction 

in equations                                       has to be there on the left-hand side.

This reaction shows that to form 1 M of liquid water, 286 kJ heat evolves. 

Standard Enthalpy Change of Combustion 

It happens when in the presence of oxygen, 1 M of any compound is completely burned. As burning always produces heat, the value of this change will be negative in all circumstances. Following is the example of such a reaction. 

 CH₄(g) + 2O₂(g)  →  CO₂(g) + 2H₂O(I)         ΔH⁰c = -890kJmol⁻¹ 

From the above equation, it is proved that, whatever compound is burned, has to take 1M of its heat energy only. 

Also, it is to be noted that, the standard enthalpy change of combustion for hydrogen is same as a change of formation of water.  

What is Entropy?

The Entropy is the measure of the disorder of the energy of a collection of particles. This idea is derived from Thermodynamics, which explains the heat transfer mechanism in a system. This term comes from Greek and means “a turning” point. It was first coined by Rudolf Clausius, a German physicist.   

He documented a precise form of the Second law of thermodynamics through entropy. It states that any spontaneous change in an isolated system for irreversible reaction always leads towards the increasing entropy. For example, when we put a block of ice on a stove, these two make an integral part of a single isolated system. Thereby the ice melts and entropy increases. 

Since all the spontaneous processes are irreversible, we can say that the entropy of the universe is increasing. Moreover, it can be concluded that more energy will be unavailable for work. Due to this, it is said that the universe is “running down”. 

The S.I unit of entropy is Joules per Kelvin. Also, it is expressed as Delta S, and following is the equation. 

ΔSsystem = Qrev / T

Where S denotes the change in entropy, Q denotes reverse of heat and temperature is represented by T in Kelvin scale. 

Absolute Entropy

It is a related term and is expressed by S. It is derived from the third law of thermodynamics. The entropy is zero at absolute zero, and it is made so by adding a constant. 

Entropy Characteristics

  • It increases with mass.

  • In the course of vaporisation, melting and sublimation, entropy increases. 

  • When liquid or hard substances dissolve in water, Entropy increases. 

  • In contrast, when gas is dissolved in the water, it decreases. 

  • In malleable solids such as metals, entropy is higher.

  • On the other hand, entropy is lesser in brittle and hard substances. 

  • As chemical complexity increases, the entropy increases as well.  

Calculation of Entropy

 In an isothermal reaction, the entropy change is defined as Delta S= the change in heat (Q) divided by absolute temperature or T. The equation as follows

Delta S= Q/T. 

For a reversible thermodynamic process, Entropy can be expressed in calculus as an integral from the initial state of a process to its final state that is dQ/T. More specifically, entropy is a measure for probability and molecular randomness of a macroscopic entity. In a system which can be presented by variables, they can predict a certain number of changes. If each configuration is probable equally, then the entropy is the natural logarithm of the total number of changes, multiplied by Boltzmann's constant. 

S = kB ln W                                          

Where kB is Boltzmann's constant, S is entropy, ln is natural logarithm, and W denotes the number of possible states. 

Note: Boltzmann's constant= 1.38065 × 10-23 J/K.

Enthalpy and Entropy Relation

Also, enthalpy entropy and free energy are closely related to each other as both entropy and enthalpy are combined into a single value by Gibbs free energy. This free energy is dependent on chemical reaction for doing useful work. This relation is first stated in 1070’s by Josiah Willard Gibbs. 

It is expressed by G. The equation is as follows - 


Where H is enthalpy, T is temperature and S is entropy. If we subtract the product of T and S from Enthalpy, we get Gibbs free energy. 

In constant temperature,


The direction of a chemical reaction is determined by Delta G. For a spontaneous process, G is negative, and for a non-spontaneous process, G is positive. 

DIY: Find out the value of T from the enthalpy and entropy change for the reaction below.

 Br₂(l) + Cl₂(g) → 2BrCl(g) 

Where Delta H is 30 kJ mol-1 and Delta S is 105 J K mol-1  

If you want more information about enthalpy and entropy in Thermodynamics, check out our website or download Vedantu’s app on your Smartphone. We have a great collection of notes and model question on every chapter of chemistry be it physical, organic or inorganic. 

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FAQ (Frequently Asked Questions)

1. What is the Difference Between Enthalpy and Entropy?

Ans. The primary difference between Enthalpy and Entropy is that Enthalpy refers to the overall energy of a system, whereas entropy refers to the randomness and chaos within a particular system. 

2. How is Enthalpy Used in Real Life?

Ans. The change in the enthalpy can be applied to hand warmers and refrigerators. For example, Freon is a refrigerant that evaporates in a fridge. In this process, the enthalpy of vaporisation equals to coolness of the food.   

3. Which Substances have Highest and Lowest Entropy?

Ans. Typically, solids have the lowest entropy as they contain very few microstates. On the other hand, gases contains the highest number of microstates, therefore, has highest entropy. Nonetheless, liquid has lesser entropy than gas and higher entropy than solids.  

4. What are the Entropy Units?

Ans. Joule per Kelvin is entropy S.I unit whereas the S.I unit of enthalpy is Joule or Joule per Kilogram.