Spontaneous Process

Introduction

A process is spontaneous if it occurs naturally without any external force. In general, Spontaneous means something that happens continuously. But in thermodynamics, there is a different explanation for the spontaneity of process. Thermodynamics is the branch of science that deals with work, temperature, energy, and heat. The laws of thermodynamics are used to define the direction of the apontaneous process. Some may get confused that the spontaneity of process is related to the process’s speed. But that’s not true. It may be a slow process of a speed process. A spontaneity of process is explained using Gibbs free energy and a system’s entropy. Let us read further.

Second Law Of Thermodynamics

The second law of thermodynamics clearly states that all spontaneous processes increase entropy over time. It means that the entropy of an isolated system tends to increase and would never decrease. It also means that if entropy increases in a process, it is more likely to be a spontaneous process. As the second law of thermodynamics says more about the entropy of a system, it is also known as the law of entropy.

Gibbs Free Energy

Gibb’s free energy is used to explain the spontaneous process. Gibbs free energy is the maximum energy used by a thermodynamic system while keeping pressure and temperature constant. The change of Gibb’s free energy is used to determine the spontaneity of a process. The Gibbs free energy also denotes the maximum work extracted from a system.

ΔG = ΔH – TΔS

The above is the expression for Gibbs free energy.

G = Gibbs free energy

T = temperature

S = entropy

Based on the value of Gibbs free energy, we could determine the nature of the process.

ΔG > 0; the reaction is nonspontaneous and endergonic

ΔG < 0; the reaction is spontaneous and exergonic

ΔG = 0; the reaction is at equilibrium

Spontaneous Process

A spontaneous process is an irreversible process that occurs continuously without any external force. We cannot reverse a spontaneous process without the application of some energy. Example melting of ice is a spontaneous process. The system’s energy remains high at the beginning of the process, lost. In the end, the energy of the system is minimal.

Non Spontaneous Process

The process which require an external agency to progress is known as a non-spontaneous process. For a non-spontaneous process to occur, an external force is needed. For example, melting ice is a spontaneous process, but the reverse of it, the crystallization of water, is a non-spontaneous process. The energy at the initial state is the lowest. During the process, energy is absorbed by the system, and the final state has the highest amount of energy.

Difference Between Spontaneous and Non-Spontaneous Process

The critical difference lies in the value of Gibb’s free energy. If the Gibbs free energy is less than zero or negative for a reaction, then the reaction is said to be spontaneous. But for a non-spontaneous process, the Gibbs free energy is always greater than zero. Any chemical or biological reaction is spontaneous if it doesn’t require external energy or force. Instead, a non-spontaneous process would always require an external force or energy. A spontaneous reaction would also increase the entropy of a system, but a non-spontaneous processwould decrease the entropy of a system. Also, a spontaneous reaction would decrease the enthalpy of a system. Contrarily, a non-spontaneous processwould increase the enthalpy of a system. These are the differences between spontaneous and nonspontaneous reactions.

Conditions for Spontaneous Process

• ΔG < 0, The Gibbs free energy decreases and Gibbs free energy change is negative
• ΔH < 0, The enthalpy of a system decreases, and the change in enthalpy is negative
• ΔS > 0, The entropy of a system increases, and the change of entropy are positive

Conditions for Non-Spontaneous Process

• ΔG > 0, The Gibbs free energy increases, and the change of Gibbs free energy are positive
• ΔH > 0, The enthalpy of a system increases, and the change in enthalpy are positive
• ΔS < 0, The entropy of a system decreases, and the change of entropy is negative
• These are the conditions for non-spontaneous process to occur

Endergonic and Exergonic Process

• An exergonic reaction is when free energy is released from the system to the surroundings. The energy flows from the system to the surroundings. So the Gibbs free energy for exergonic reaction decreases
• The energy flows from the surroundings to the system for an endergonic reaction. In an endergonic reaction, the energy is absorbed by the system. Therefore the Gibbs free energy for an exergonic reaction increases

Differences Between Endergonic and Exergonic Process

• In an endergonic process, energy is absorbed, but energy is released to the surrounding in an exergonic reaction
• To initiate an endergonic reaction, some energy needs to be supplied, but an exergonic reaction does not require a supply of external energy or force
• All exothermic reactions are exergonic, and all endothermic reactions are endergonic
• As Gibb’s free energy increases in an endergonic reaction, Gibb’s free energy change is negative. Instead, in an exergonic reaction, the Gibbs free energy decreases; therefore, the change of Gibbs free energy is positive
• All endergonic reactions are called severe reactions, and all exergonic reactions are called downhill reactions
• For an exergonic reaction, the energy level at the initial state is the highest, released during the process. After the compilation of reactions, the system’s energy remains the lowest. But it is the opposite in an ergonomic reaction. During the initial phase of the reaction, the system’s energy is deficient. The system absorbs energy during the reaction, and the final product has more energy than the initial state
• The endergonic and exergonic processare called non-spontaneous and spontaneous reactions. Therefore all the conditions of spontaneous and non-spontaneous process applied to endergonic and exergonic process, respectively

Conclusion

The key takeaways from the earlier points are conditions for spontaneous and non-spontaneous processes. It is important to remember at least one condition to apply the rest. A spontaneous process or exergonic reaction does not require external energy, and the reaction can occur on its own when the required conditions are met. But for a non-spontaneous process or endergonic reaction, there is a need for external energy or force to be supplied to initiate the process.