Difference Between Enthalpy and Entropy

Entropy and Enthalpy

In physics, thermodynamics is the study of the effects of heat, energy and work on the system.

Entropy is the measure of thermal energy of a system per unit temperature.

It is the measure of unavailable energy in a closed thermodynamic system and is concerned with measuring the molecular disorder, or randomness, of the molecules inside the system.

In simple terms, entropy is the degree of disorder or uncertainty in the system.

Enthalpy is a central factor in thermodynamics. It is the total heat contained in the system.

Which means if the energy is added, the enthalpy increases. 

If the energy is given off then the enthalpy of the system decreases.

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Enthalpy And Entropy In Thermodynamics

Enthalpy

 In thermodynamics, 

                H = U + PV

Where U is the internal energy 

P is the external pressure

V = Volume

H = Enthalpy

For example, 

Consider a system that has molecules moving in random motion. These molecules have some attraction between them, and so they have potential energy.

Whole together these energies are made up by an internal energy.

Here, the system is in a free-state.

To establish the pressure and volume inside the system, some work is done. The energy used is actually, the PV.

So,

Heat content = Internal energy = It comes from atoms and molecules or electrons at a quantum level as they are very small in range.

Here,  PV = is the energy required to establish a system at pressure P, volume V from empty space.

Hence, the total energy which is required in the whole process is called the enthalpy. 

The unit of H is KJ/mole.

Here, per mole means an energy required in per mole of a reaction.

The major drawback of H is that it is immeasurable, so we consider ΔH (enthalpy change).


Entropy 

The term entropy took birth from spontaneous process (a process that happens itself or by a little push).

When the randomness of the system increases, the process is said to be spontaneous.

For example, you are sitting in the classroom according to your wish.

Your teacher makes you sit in your respective seats. In this way, randomness is created in the classroom because of external energy or a little push (by your teacher) applied to you. Such a process is spontaneous by nature.

The measure of such a disorganized motion of molecules is called the entropy (denoted by S).

When we consider the nature, entropy keeps on increasing. Therefore, nature is spontaneous.


Relation Between Entropy And Enthalpy

Enthalpy is the sum total of all the energies, whereas entropy is the measure of the change in enthalpy/temperature.

Let’s understand by an example,

Suppose you have Rs. 100 with you. If someone gives you a Rs. 50 note, it will make a difference. 

If you have Rs. 1,00,00,000 with you and someone gives, you Rs. 50, though the change is the same.

So, we need another term to measure the difference made by this 50 rupee note.

We can divide the change with the amount you initially had.

Like in the first case,

It is  50/100=0.5

While in the second case, it is

50/1,00,00,000=0.000005

In thermodynamics, 

This ratio is called entropy i.e., the heat supplied (roughly the change in enthalpy, ΔH) divided by temperature (directly related to enthalpy).

Difference Between Enthalpy And Entropy

              Enthalpy

            Entropy

It is an energy

It is an attribute.

Directly related to the internal energy of the system.

Entropy is the measurement of molecular randomness 

It is the sum total of all the energies inside the system.

It increases with the increase in temperature.

Symbolized as H.

Symbolized as S.

Unit = KJ /mol

Unit = J/ K

Termed by a scientist named Heike Kamerlingh Onnes.

Termed by a scientist named Rudolf Clausius.


Spontaneity 

As we discussed above, there is a randomness in the molecules under the free gas expansion. 

 ∆H = -ve (heat is given off to the surroundings or exothermic), the stable system and hence spontaneous.

∆H = +ve  (heat added from surroundings), the entropy increases. 

∆S = +ve, randomness increases.

If  ∆S = -ve when water changes to ice, entropy decreases.

Here, due to variation we couldn't obtain perfect spontaneous reactions. 

So,

ΔG came as the perfect step to determine the spontaneity of reaction with 100% guarantee.

                            G =  H - T S

Where G is the Gibbs free energy.

If the reaction is carried out at ΔT=0 (T in kelvin), then

                         ΔG = ΔH - TΔS

This equation is called the Gibbs Helmholtz equation.

Where ∆G is the change in free energy.

For spontaneous reactions, ∆G is always <0.

Hence, Gibbs free energy is a thermodynamic potential that can be used to calculate the useful work done by a thermodynamic system at a constant temperature and pressure.  

FAQ (Frequently Asked Questions)

1. Is Entropy A Chaos?

Yes, because entropy is the chaotic or a property of random motion of particles inside the system.

2. Why Is Entropy Of The Universe Increasing?

Energy in nature keeps on increasing, and this causes an increase of entropy.  Entropy is the expansion of energy, and  energy tends to spread out as much as possible. Just like the sun and every other star is continuously radiating energy into the universe. Therefore, nature is spontaneous.

3. Does Dissolving Increase Entropy?

Crystals of salt are in solid state so there’s no randomness as they’re closely bound. When these crystals are mixed in the hot water, the kinetic energy of molecules increases. Since, kinetic energy is directly related to entropy. Therefore, the entropy of the molecules on dissolving the salt crystals increases.

4. How Is Enthalpy Used In Real Life?

Change in enthalpy can be applied in real life such as:

  • In refrigerators and hand warmers. In a fridge, refrigerants such as Freon evaporate. 

  • The enthalpy of vaporization i.e., change from liquid to gaseous state  is equivalent to the coldness of your food. 

  • While working, or playing outdoors on a cold day, people use a hand warmer to warm their hands.