[Physics Class Notes] on Difference Between Enthalpy and Entropy Pdf for Exam

In physics, thermodynamics is the study of the effects of heat, energy and work on the system. Entropy is the measure of the 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. This means if the energy is added, the enthalpy increases. 

 

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

 

 

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.

 

Together these energies are made up of 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.

 

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

 

Entropy 

The term entropy took birth from a 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 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.eThe heat supplied (roughly the change in enthalpy, ΔH) divided by temperature (directly related to enthalpy).

 

Difference Between Enthalpy And Entropy

 

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 a 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.