Differences Between Enthalpy And Entropy

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Enthalpy and Entropy are two concepts in thermodynamics. Enthalpy of a thermodynamic system is the measure of the total amount of heat of the system which is denoted by H. On the other hand, entropy is the degree of disorder in the thermodynamic system which is denoted by S. Thermodynamics is defined as the branch of physics that deals with all forms of energy including heat.

Table of Contents

Enthalpy
Entropy
Relationship Between Enthalpy And Entropy
Differences Between Enthalpy And Entropy
Things to Remember
Sample Questions

Key takeaways: Enthalpy, Entropy, Thermodynamic system, Internal Energy, Endothermic & Exothermic reaction

Enthalpy

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Enthalpy of a thermodynamic system is the measure of the total amount of heat of the system. It is denoted by the symbol 'H'. The change in enthalpy of a system is denoted as '?H'. The unit of enthalpy is kilojoules per mole. It is represented as kJ/mol.

Learn More about Enthalpy

Enthalpy of a system is calculated when the system is at constant pressure. It can be represented by the formula

ΔH = ΔU + PΔV

Where P is the pressure of the system,

ΔU is the change in internal energy of the system,

ΔV is the change in volume of the system.

Thus, the enthalpy of a system is the summation of the internal energy of that system and the non-mechanical work done by the system.

Enthalpy of a System

Enthalpy of a System

Read More:

Enthalpy Formula	Enthalpy and Entropy	Enthalpy change

In a thermodynamic system, if energy is added to the system then the enthalpy of the system increases and if energy is used by the system then the enthalpy of the system decreases. It is thus safe to say that enthalpy is directly proportional to the energy. Thus, for an endothermic reaction, the change in enthalpy of the system is positive. While in an exothermic reaction, the change in enthalpy of the system is negative.

Endothermic and Exothermic Reactions

Endothermic and Exothermic Reactions

Enthalpy(H) itself is immeasurable. One can only measure the change in enthalpy (ΔH) for a system. The measurement of change in enthalpy is very crucial to understanding the changes in a chemical reaction.

Read More:

Lattice Enthalpy	Standard Enthalpy	Bond Enthalpy

Entropy

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The term entropy was coined by Rudolf Clausius. In a thermodynamic system, entropy is the degree of disorder in the system. It is denoted by the symbol 'S'. Change in entropy of a system is denoted by 'ΔS'. The unit of entropy is Joules per Kelvin and it is represented as J/K.

Entropy can be given by the formula

ΔS = ΔQT

Where ΔQ is the change in heat of the system

T is the absolute temperature of the system.

Entropy of Thermodynamic System

Entropy of Thermodynamic System

Enthalpy is associated with the randomness of molecules in a system. As the randomness of a system increases, entropy also increases. If the randomness of a process increases then that process is said to be a spontaneous process.

Read More:

Ionization Enthalpy	Thermochemistry	Bond Dissociation Enthalpy

Relationship Between Enthalpy And Entropy

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The relationship between enthalpy and entropy can be given by a simple expression. In a closed thermodynamic system,

?H = T.?S

Where ΔH is the change in enthalpy

ΔS is the change in entropy

T is the absolute temperature of the system

This signifies that at absolute temperature, a change in enthalpy of a system causes a change in its entropy and vice versa.

Read More:

Third Law of Thermodynamics	Helmholtz Equation	Zeroth Law of Thermodynamics

Differences Between Enthalpy And Entropy

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Differences between enthalpy and entropy are noted in the table below:

Parameter	Enthalpy	Entropy
Definition	Enthalpy is the total heat of the system	Entropy is the randomness in the system
Symbol	Enthalpy is denoted by the symbol 'H'	Entropy is denoted by the symbol 'S'
Unit	Unit of enthalpy is kJ/mol	Unit of entropy is J/K
Nature	Enthalpy is a kind of energy of the system	Entropy is a physical property of the system
System tendency	Any thermodynamic system favours minimum enthalpy	Any thermodynamic system favours maximum entropy
Standardization	Enthalpy is always applicable to related standard conditions of the system	Entropy does not have any specific standard conditions or limitations

Things to Remember

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Enthalpy of a thermodynamic system is the measure of the total amount of heat of the system.
Enthalpy is given by the expression ΔH = ΔU + PΔV
In a thermodynamic system, entropy is the degree of disorder in the system.
Entropy is given by the expression ΔS = ΔQT
The relationship between enthalpy and entropy is given by the expression ΔH = T.ΔS

Read More:

Thermodynamic Process	Second Law of Thermodynamics	Applications of Thermodynamics

Sample Questions

Ques. Clarify the difference between enthalpy and entropy. [2 Marks]

Ans. Enthalpy is the measure of the overall energy of a system whereas entropy is the measure of randomness in the system. Entropy is a property of the system while enthalpy is merely the energy. Both enthalpy and entropy are measured using different methods.

Ques. Is entropy related to chaos? How? [2 Marks]

Ans. Entropy is the randomness in a system. More the randomness in a system the more is the entropy. Randomness is associated with being chaotic. So, entropy is the property that is related to chaos.

Ques. Which of the following statements is true, explain: [2 Marks]
Enthalpy of Hydrogen gas is zero and the enthalpy of water in the liquid state is non-zero.
The entropy of Hydrogen gas is less than the entropy of water in the liquid state.

Ans. The true statement is:

Enthalpy of Hydrogen gas is zero and the enthalpy of water in the liquid state is non-zero.

The reason is only for elemental compounds such as Hydrogen (H₂) and Oxygen (O₂) enthalpy is zero. So, the enthalpy for water (H₂O) in a liquid state cannot be zero.

Ques. Why is the entropy of the universe continuously increasing? [2 Marks]

Ans. The universe is always expanding. It has no known borders. Due to the continuous expansion and addition of new stars and planets to the universe, the total energy given out is always increasing. Since energy has a direct relationship with entropy, it is true that the entropy of the universe is continuously increasing.

Ques. Give applications of enthalpy in real life. [2 Marks]

Ans. Real-life applications of enthalpy are:

Refrigerants in a refrigerator evaporate on heating.
Our food gets cold when left out in the open.
Running outside increases the temperature of our bodies.

Ques. Does converting a substance from its solid state to a liquid state increase its entropy? [2 Marks]

Ans. When a substance is in a solid-state, the molecules are closely packed together. There is no randomness in the molecules. But when that solid gets converted into liquid either by melting or by dissolving in a solvent, the kinetic energy of the molecules increases. Due to this the randomness in the substance increases. Since entropy is an increase in randomness, one can say that on converting a substance from its solid-state to liquid state its entropy increases.

Ques. Define enthalpy and entropy. [2 Marks]

Ans. Enthalpy: Enthalpy of a thermodynamic system is the measure of the total amount of heat of the system.

Entropy: In a thermodynamic system, entropy is the degree of disorder in the system.

Ques. Give the formula for enthalpy, entropy, and the expression that gives the relationship between enthalpy and entropy. [2 Marks]

Ans. The formula for enthalpy is

ΔH = ΔU + PΔV

The formula for entropy is

ΔS = ΔQT

The expression that gives the relationship between enthalpy and entropy is

ΔH = T.ΔS

Read More:

Vaporization	Huckel’s rule of Aromaticity	Hydrogen spectrum

Differences Between Enthalpy And Entropy

Enthalpy

Entropy

Relationship Between Enthalpy And Entropy

Differences Between Enthalpy And Entropy

Things to Remember

Sample Questions

CBSE CLASS XII Related Questions

1.
Draw the number of scattered particles versus the scattering angle graph for scattering of alpha particles by a thin foil. Write two important conclusions that can be drawn from this plot.

2.
Draw a circuit diagram of a full-wave rectifier using p-n junction diodes. Explain its working and show the input-output waveforms.

3.
Write the expression for the magnetic field due to a current element in vector form. Consider a 1 cm segment of a wire, centered at the origin, carrying a current of 10 A in positive x-direction. Calculate the magnetic field \( \mathbf{B} \) at a point \( (1 \, \text{m}, 1 \, \text{m}, 0) \).

4.
Two thin lenses of focal length \( f_1 \) and \( f_2 \) are placed in contact with each other coaxially. Prove that the focal length \( f \) of the combination is given by \[ f = \frac{f_1 f_2}{f_1 + f_2}. \]

5.
If Bohr’s quantization postulate (angular momentum \( = \frac{nh}{2\pi} \)) is a basic law of nature, it should be equally valid for the case of planetary motion also. Why, then, do we never speak of quantization of orbits of planets around the Sun? Explain.

Similar Physics Concepts

Comments

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Differences Between Enthalpy And Entropy

Enthalpy

Entropy

Relationship Between Enthalpy And Entropy

Differences Between Enthalpy And Entropy

Things to Remember

Sample Questions

CBSE CLASS XII Related Questions

1. Draw the number of scattered particles versus the scattering angle graph for scattering of alpha particles by a thin foil. Write two important conclusions that can be drawn from this plot.

2.Draw a circuit diagram of a full-wave rectifier using p-n junction diodes. Explain its working and show the input-output waveforms.

3.Write the expression for the magnetic field due to a current element in vector form. Consider a 1 cm segment of a wire, centered at the origin, carrying a current of 10 A in positive x-direction. Calculate the magnetic field \( \mathbf{B} \) at a point \( (1 \, \text{m}, 1 \, \text{m}, 0) \).

4.Two thin lenses of focal length \( f_1 \) and \( f_2 \) are placed in contact with each other coaxially. Prove that the focal length \( f \) of the combination is given by \[ f = \frac{f_1 f_2}{f_1 + f_2}. \]

5. If Bohr’s quantization postulate (angular momentum \( = \frac{nh}{2\pi} \)) is a basic law of nature, it should be equally valid for the case of planetary motion also. Why, then, do we never speak of quantization of orbits of planets around the Sun? Explain.

Similar Physics Concepts

Comments

SUBSCRIBE TO OUR NEWS LETTER

1.
Draw the number of scattered particles versus the scattering angle graph for scattering of alpha particles by a thin foil. Write two important conclusions that can be drawn from this plot.

2.
Draw a circuit diagram of a full-wave rectifier using p-n junction diodes. Explain its working and show the input-output waveforms.

3.
Write the expression for the magnetic field due to a current element in vector form. Consider a 1 cm segment of a wire, centered at the origin, carrying a current of 10 A in positive x-direction. Calculate the magnetic field \( \mathbf{B} \) at a point \( (1 \, \text{m}, 1 \, \text{m}, 0) \).

4.
Two thin lenses of focal length \( f_1 \) and \( f_2 \) are placed in contact with each other coaxially. Prove that the focal length \( f \) of the combination is given by \[ f = \frac{f_1 f_2}{f_1 + f_2}. \]

5.
If Bohr’s quantization postulate (angular momentum \( = \frac{nh}{2\pi} \)) is a basic law of nature, it should be equally valid for the case of planetary motion also. Why, then, do we never speak of quantization of orbits of planets around the Sun? Explain.