Entropy

Thermodynamics is the study of the energy changes that occur as a result of temperature and heat variations. Entropy is the measurement of  system’s unpredictability or disorder in general. In the year 1850, a German physicist named Rudolf Clausius proposed this hypothesis. We don’t consider the tiny characteristics of a system while looking at entropy from a thermodynamics standpoint. Entropy, on the other hand, is used to characterise a system’s behaviour in terms of thermodynamic parameters like temperature, pressure, entropy, and heat capacity. The condition of equilibrium of the systems was taken into account in this thermodynamic description.

Because the value of entropy or Entropy Change is dependent on the substance present in a thermodynamic system, entropy is indicated by the letter ‘S.’Entropy is a fascinating concept since it calls into question the notion of full heat transfer. It aids in the reinterpretation of the second law of thermodynamics.

Entropy is related to spontaneity, i.e., the higher the degree of disorder in a thermodynamic process, the higher the entropy. Entropy, in simple terms, tells us how much energy does not convert to labour and instead adds to the system’s disorder. It is essentially impossible to spend all of the energy in accomplishing work because energy provides the ability to undertake labour. Entropy is a metric that measures this.

 Because the rule of thermodynamics states that energy cannot be generated or destroyed but can be changed from one form to another, entropy cannot be expressed at a single point and must be measured as a change. That is why the Entropy Change is calculated .

Entropy change

The change in the state of disorder of a thermodynamic system associated with the conversion of heat or enthalpy into work is known as entropy change. Entropy is higher in a system with a high degree of disorderliness. Entropy is a state function factor, which means that its value is independent of the thermodynamic process’s pathway and is solely a determinant of the system’s beginning and final states. The changes in entropy in chemical reactions are caused by the rearranging of atoms and molecules, which alters the system’s initial order. This can result in an increase or decrease in the system’s randomness, and hence in an increase or decrease in entropy.

Change in Entropy Formula Thermodynamic

A thermodynamic system’s Entropy Change is denoted by ∆S. Using the change in entropy formula, we can compute the Entropy Change of a chemical reaction or a system:

ΔS = (Q/T)rev

Temperature

Q the heat transfer to or from the thermodynamic system

T is the absolute temperature.

The SI unit Entropy Change is J/Kmol.

Example: The entropy of water vaporisation can be estimated by dividing the heat of vaporisation by the boiling point, which is 1000 degrees Celsius or 373 degrees Fahrenheit.

Entropy Changes During Phase Transition

Entropy of Fusion

When a solid starts melting into a liquid, the entropy increases. With phase shift, the entropy increases as the freedom of movement of molecules increases.

The entropy of fusion is calculated by dividing the enthalpy of fusion by the melting point (fusion temperature).

∆fusS=∆fusH / Tf

When the related change in the Gibbs free energy is negative, a natural event such as phase transition (e.g. fusion) will occur.

∆fusS is almost always positive.

Exception: At temperatures below 0.3 K, helium-3 has a negative entropy of fusion. Below 0.8 K, helium-4 has a slightly negative entropy of fusion.

Entropy of Vaporization

The entropy of vaporisation is a situation in which the entropy of a liquid increases as it transforms into a vapour. This is caused by an increase in molecular mobility, which causes motion to become random.

The enthalpy of vaporisation divided by the boiling point gives the entropy of vaporisation. It can be expressed as follows:

∆vapS=∆vapH / Tb

Standard Entropy of Formation of a Compound

When one mole of a compound in the standard state is created from the elements in the standard state, there is an entropy change.

Spontaneity

Exothermic reactions occur naturally because the environment is positive, making the total positive.

Because ∆Ssystem is positive and ∆Ssurroundings is negative, but  ∆S total positive, endothermic reactions are spontaneous.

Free energy change criteria are preferable to entropy change criteria for predicting spontaneity because the former only requires free energy change in the system, whereas the latter requires entropy change in both of the surroundings and the system.

Negentropy

It’s the polar opposite of entropy. It implies that things are becoming more organised. The term ‘order’ refers to the organisation, structure, and function of something. It is the polar opposite of chaos or randomness.

A star system, such as the solar system, is an example of negentropy.

Enthalpy

The measurement of energy in a thermodynamic system/environment is enthalpy. The total content of heat in a system is the enthalpy, which is actually equal to the internal energy of the system plus the product of volume and pressure.

Enthalpy is a technical term that denotes the total amount of internal energy required to create a system as well as the amount of energy required to make room for it by establishing its pressure and volume and displacing its surroundings.

The heat developed(either absorbed or released)  is equal to the change in enthalpy when a process starts at constant pressure.

H=U+PV

Enthalpy can also be thought of as a state function composed entirely of the state functions P, T, and U. It is usually demonstrated by the difference in enthalpy (H) of a process between its initial and final states.

ΔH=ΔU+ΔPV

The change in enthalpy is given by, if the pressure and temperature do not change throughout the operation and the task is confined to pressure and volume.

ΔH=ΔU+PΔV

According to the following equation, the flow of heat (q) at constant pressure in a process equals the change in enthalpy.

ΔH=q

Knowing whether q is endothermic or exothermic can help define a relationship between q and H.

Conclusion

In nature, systems have a proclivity to progress toward increasing chaos or unpredictability. The tendency of the cosmos to move towards disorder or unpredictability is known as entropy. The adiabatic process has constant entropy because the change in entropy is zero. The sum of a thermodynamic system’s internal energy and the product of its pressure and volume is called enthalpy. Enthalpy is an energy-like attribute or state function that has the dimensions of energy (and so is measured in joules or ergs), and its value is entirely determined by the temperature, pressure, and composition of the system, not by its history.