Predicting if a Reaction will Occur Spontaneously

The second law of thermodynamics deals with the physical property of entropy. Entropy change describes a reaction to be spontaneous or nonspontaneous. If the entropy change of a reaction is zero or above zero, a reaction is said to be spontaneous. If entropy change is negative, the reaction is said to be non-spontaneous. Later on, it was found that in some cases, even though the reaction was spontaneous, the change in entropy was negative. Though the change in entropy of the system was negative, it was found that the overall entropy change ΔS = ΔSsurroundings + ΔSsystem was positive because of the large change in ΔS value of surroundings. To prevent such confusion, a new and more relevant term was introduced. Gibbs free energy is now used to determine the spontaneity of the reaction. Gibbs free energy is the free available energy that can be utilised to conduct a chemical reaction or chemical transformation. It is also the maximum work that can be done in a closed system.

The formula for the formula of Gibbs free energy is: ΔG =  ΔH – TΔS

Spontaneous Reactions

A reaction is said to be spontaneous if the change in Gibbs free energy is negative. That is, the reaction does not require any starting boost or external source of energy. Based on the formula, it was then said that if ΔG is negative, TΔS must be positive, which implies ΔS is positive. For example, the rusting of iron is spontaneous because the moment iron comes in contact with water and oxygen, it starts to corrode.

Non-Spontaneous Reactions

A reaction is said to be non-spontaneous if the change free energy is positive. This means that the reaction requires external ignition or external energy to proceed. From the formula of Gibbs free energy, if ΔG is positive, then TΔS must be negative, which implies ΔS is negative. For example, boiling water is non-spontaneous as it requires an external source of energy.

Exergonic and Endergonic Reactions

An exergonic reaction is one in which the reactant possesses more energy than the product such that during the course of the reaction, energy is released to the surroundings. Generally, spontaneous reactions are exergonic in nature. A reaction that requires energy to proceed and in which the energy of the product is more than the energy of the reactant is known as an endergonic reaction. In these reactions, energy from the surroundings is absorbed. Generally, non-spontaneous reactions are endergonic in nature.

Examples of Spontaneous and Non-spontaneous Reactions

• Carbonic acid is present in some beverages: It decomposes into water and carbon dioxide as the following reaction: 

H2CO3(aq)⇌CO2(g)+H2O(l)

Carbonic acid decomposing into water and carbon dioxide is a spontaneous reaction, as in beverages, as no external energy is provided. In fact, carbonic acid itself decomposes on its own. The reverse of the reaction is non-spontaneous as it requires external energy to convert water and carbon dioxide back into carbonic acid.

• The decomposition of calcium carbonate:

CaCO3  →  CaO + CO2

Heating calcium carbonate yields calcium oxide (“quick lime”) and carbon dioxide. From this reaction, it was found that the ΔG value of this reaction is positive as energy is to be supplied for this reaction. Hence, the reaction is non-spontaneous.

• The melting of ice:

H2O (s)  →  H2O (l)

Although this reaction seems to be non-spontaneous and the ΔG value for it is zero, the entropy of this reaction is in positive value. Hence, even though the reaction appears to be non-spontaneous, it is actually spontaneous.

• Dissolution of salts in water:

The dissolution of salts such as NaCl and KCl in water is an exergonic process. The process releases energy on its course. Moreover, the ΔG value of this reaction is negative, and hence, the reaction is spontaneous.

• Dissolution of sand in water:

Sand is insoluble in water, and dissociating it in water requires a lot of heat energy. Hence, the ΔG value for this reaction is positive. This implies that the reaction is non-spontaneous.

• The burning of candles:

The burning of candles appears to be non-spontaneous, but it is spontaneous in nature. This is because once the candle is ignited, it continues to burn without any external source required to proceed with the process.

Conclusion

For a reaction to be spontaneous, it must satisfy the condition that the change in Gibbs free energy must be negative in magnitude. Furthermore, the spontaneity of a reaction is not related to the speed of the reaction. The kinetics of a reaction is independent of its spontaneity. Moreover, some reactions, such as the melting of ice, appear to be non-spontaneous but are spontaneous because of the positive value of change in entropy.