Spontaneity and Entropy - JEE Important Topic

Spontaneity means the continuation of a process without using any external energy. Well, this is a process that, once begun, will keep continuing on its own without using any external energy. On the other hand, speaking about non-spontaneous processes requires a continuous input of energy. 

Let’s understand the meaning of spontaneity here. Speaking about the spontaneity in processes, it occurs without utilizing any external energy. This process, based on the type, can take place slowly or quickly as this is not related to any reaction of kinetic rate. You can consider the example of the transformation process of carbon into graphite. This can be stated in the following reaction. 

C(s,diamond)$\to $ C(s,graphite)

The reaction process is time-consuming and may not be identifiable over a short period. If you can wait for years, you may see carbon in the diamond form, turning into the stable graphite form. On the other hand, this process can be endothermic or exothermic. Based on this, it can be said that the process is not related to any massive changes.

Determination of the Spontaneous Processes

As we have discussed above, spontaneity means the continuation of a process without using any external energy. Even though the process is exothermic. But this is not considered a criterion for spontaneity. As per the experts, the ideal indicator of this reaction is the alteration in Entropy. Based on the Second Law of Thermodynamics, it can be said that for spontaneity in reaction, the entropy will increase.

The criterion for spontaneity of a reaction is a free energy change. What is entropy in Chemistry? Entropy measures the level of energy dispersed during a process. This is generally reflected in the idea that a spontaneous process always leads to the spreading out of energy along with matter.

What is Entropy in Thermodynamics? The Third Law of Thermodynamics can be considered as a definition. The entropy of an element at the temperature of zero will be zero. Absolute zero or 0 Kelvin is a state of temperature reaching which atoms will stop moving. In Chemistry, it is generally considered a point measure for entropy. The unit of Entropy of a system is JK⁻¹mol⁻¹. This means Joules of energy produced per unit of heat per mol.

After getting to the starting point, Entropy is measured by calculating the heat content with the system's temperature. The change should be reversible and can be calculated using this formula:

$dS=\frac{{{q}_{rev}}}{T}$

In Chemistry, this is a process of measuring the thermal energy of a system per unit temperature that is required to complete the process. 

Process in Which Entropy Increases

Entropy increases if a substance breaks into different parts. For example, the dissolution process boosts entropy as all the solute particles separate from each other while forming a solution. With the motion of the system, the Entropy may increase or decrease. Let’s understand this with another example.

The entropy process will increase when the ice melts into water, i.e., the transformation of a solid thing into liquid. However, it will be even more when water evaporates to gas or steam. Based on this, it can be said that:

S_gas >> S_liquid > S_solid

On the other hand, a solid’s Entropy will increase when it dissolves in a solvent. During this process, the number of particles will increase, and they will roam freely in that particular solution. The relationship between entropy of system and surrounding is,

${{S}_{universe}}={{S}_{system}}+{{S}_{surrounding}} \\ \\ \\ {{S}_{universe}}={{S}_{system}}+\frac{{{q}_{surrounding}}}{T}$

However, the change that does not increase Entropy is condensation. This is the process where the intermolecular distance goes down, and the randomness or force of attraction decreases. 

Gibbs Free Energy and Its Relation with Spontaneity

The change in the Gibbs function or Gibbs Free Energy can be utilized to evaluate a process’s spontaneity. Experts say when at a constant level of pressure and temperature, the Gibbs equation will be $\Delta G=\Lambda H-T\Delta S$.

Here, $\Delta H$ means the amount of change in enthalpy.

$\Delta S$ stands for the change in Entropy.

Speaking about $\Delta G$, it means the total amount of available or free energy.

If ΔG is negative, then you can call the process spontaneous. On the other hand, if the ΔG is positive, then that particular process is nonspontaneous. If you get ΔG as zero, then that process is constant and at equilibrium. 

Second Law of Thermodynamics

It may be noted that every spontaneous process generates an increase in Entropy. Both spontaneity and entropy move together. Remember that no process can be done whose primary aim is to transfer the heat to a hotter body from a cooler body.  In an exothermic process (negative change in enthalpy), the heat released by the process increases the randomness of the surroundings, and overall entropy change is positive, which makes the process spontaneous.

Conclusion

The spontaneity of a process can be evaluated from the entropy change. As the randomness increases in the system, entropy also increases; for example, in the state change process, the entropy of a solid is less than that of a liquid state which is less than that of gas. Furthermore, you can also use Gibbs free energy change theory for this. However, experts say the Gibbs free energy change can be more useful here than entropy change to evaluate the spontaneity of a process. This method also uses the change in enthalpy and temperature along with entropy change of the process to predict the spontaneity of a process.