Difference Between Spontaneous and Nonspontaneous Reactions

Main Difference – Spontaneous vs Nonspontaneous Reactions

Spontaneous and nonspontaneous reactions are the two types of chemical reactions that can occur in the environment. Spontaneous reactions take place on their own under a given set of conditions. However, energy should be provided for the nonspontaneous reactions to proceed. The change in the Gibbs free energy is negative for spontaneous reactions. Therefore, these reactions release energy to the surroundings in the form of heat. In nonspontaneous reactions, the change in the Gibbs free energy is positive. They absorb energy from the environment. On that account, spontaneous reactions are exergonic while nonspontaneous reactions are endergonic. The main difference between spontaneous and nonspontaneous reactions is that spontaneous reactions release free energy from the system, making it more stable whereas nonspontaneous reactions increase the total energy of the system.

Key Areas Covered

1. What are Spontaneous Reactions
     – Definition, Thermodynamics, Examples
2. What are Nonspontaneous Reactions
     – Definition, Thermodynamics, Examples
3. What are the Similarities Between Spontaneous and Nonspontaneous Reactions
     – Outline of Common Features
4. What is the Difference Between Spontaneous and Nonspontaneous Reactions
     – Comparison of Key Differences

Key Terms: Endergonic Reactions, Entropy, Exergonic Reactions, Gibbs Free Energy, Nonspontaneous Reactions, Spontaneous Reactions

What are Spontaneous Reactions

Spontaneous reactions refer to the chemical reactions that occur without being driven by an outside force. The two driving forces of a chemical reaction are enthalpy and entropy. Enthalpy is a thermodynamic property of a system that is the sum of the internal energy added to the product of the pressure and the volume of the system. Entropy is the other thermodynamic property that accounts for the system’s thermal energy per unit temperature. It describes the randomness and disorder of molecules. When the occurrence of a chemical reaction decreases the enthalpy and increases the entropy of the system, it is considered as a favourable reaction. As spontaneous reactions fulfil the above two conditions, they occur without inside intervention.

Figure 1: Wood Combustion

Combustion is an example of spontaneous reactions. The products of the fire partly consist of the two gases: carbon dioxide and water vapour. Combustion generates heat. Thus, it is an exergonic reaction. Heat increases the entropy of the system. But, the entropy of the products of the combustion has a reduced entropy.

What are Nonspontaneous Reactions

Nonspontaneous reactions refer to the chemical reactions that require an energy input to proceed. In nonspontaneous reactions, both enthalpy and entropy prefer the reactants. Thus, reactants are more stable than products. On that account, the chemical reaction is endergonic, absorbing heat. It decreases the entropy as well. The change of the Gibbs free energy over time in nonspontaneous reactions is shown in figure 2.

Figure 2: The Change of the Gibbs Free Energy/Time

The reaction between atmospheric nitrogen and oxygen is an example of a nonspontaneous reaction. It forms nitrogen monoxide. At the normal atmospheric pressure and temperature, this reaction is unfavourable. This means the reactants of the chemical reaction, i.e., nitrogen and oxygen gases, are more stable than the product: nitrogen monoxide. But, at very high temperatures such as when lightning, this reaction is favourable.

Similarities Between Spontaneous and Nonspontaneous Reactions

• Both spontaneous and nonspontaneous reactions occur in the environment.
• Both spontaneous and nonspontaneous reactions occur in a system with defined boundaries.
• Both spontaneous and nonspontaneous reactions obey the three laws of thermodynamics.

Difference Between Spontaneous and Nonspontaneous Reactions

Definition

Spontaneous Reactions: Spontaneous reactions refer to the chemical reactions that occur without being driven by an outside force.

Nonspontaneous Reactions: Nonspontaneous reactions refer to the chemical reactions that require an energy input to proceed.

Favorable/Unfavorable

Spontaneous Reactions: Spontaneous reactions are favourable.

Nonspontaneous Reactions: Nonspontaneous reactions are unfavorable.

Energy Input

Spontaneous Reactions: Spontaneous reactions do not require energy input to proceed under a set of conditions.

Nonspontaneous Reactions: Nonspontaneous reactions require an energy input to proceed.

Change in the Gibbs Free Energy

Spontaneous Reactions: The change in the Gibbs free energy in a spontaneous reaction is negative (ΔG° < 0).

Nonspontaneous Reactions: The change in the Gibbs free energy in a nonspontaneous reaction is positive (ΔG° > 0).

Total Energy of the System

Spontaneous Reactions: Spontaneous reactions release the free energy from the system, making it more stable.

Nonspontaneous Reactions: Nonspontaneous reactions increase the total energy of the system.

Total Energy of the Products

Spontaneous Reactions: The total energy of the products of a spontaneous reaction is less than that of the reactants.

Nonspontaneous Reactions: The total energy of the products of a nonspontaneous reaction is higher than that of the reactants.

Enthalpy

Spontaneous Reactions: Spontaneous reactions have a negative enthalpy.

Nonspontaneous Reactions: Nonspontaneous reactions have a positive enthalpy.

Exergonic/Endergonic Reactions

Spontaneous Reactions: Spontaneous reactions are exergonic reactions.

Nonspontaneous Reactions: Nonspontaneous rections are endergonic reactions.

Entropy

Spontaneous Reactions: Spontaneous reactions increase the entropy.

Nonspontaneous Reactions: Nonspontaneous reactions decrease the entropy.

Speed of the Reaction

Spontaneous Reactions: Spontaneous reactions occur rapidly.

Nonspontaneous Reactions: Nonspontaneous reactions occur with less speed.

Examples

Spontaneous Reactions: Combustion reactions are spontaneous.

Nonspontaneous Reactions: The reaction between atmospheric nitrogen and oxygen is an example of a nonspontaneous reaction.

Conclusion

Spontaneous and nonspontaneous reactions are the two types of chemical reactions that occur in the environment. Spontaneous reactions are exergonic. So, they decrease the enthalpy and increase the entropy of the system. However, nonspontaneous reactions are endergonic. They increase the enthalpy of the system as they require external energy input to proceed. They also decrease the entropy. On that account, spontaneous reactions are favourable under a given set of chemical reactions. But, nonspontaneous reactions are unfavourable. The main difference between spontaneous and nonspontaneous reactions is the thermodynamic properties of each type of reaction.

Reference:

1. “The Laws of Thermodynamics.” The Laws of Thermodynamics | Boundless Chemistry, Available here.
2. “11.5: Spontaneous Reactions and Free Energy.” Chemistry LibreTexts, Libretexts, 1 Nov. 2017, Available here.
3. “Endergonic reaction.” Wikipedia, Wikimedia Foundation, 16 Dec. 2017, Available here.

Image Courtesy:

1. “Large bonfire” By Fir0002 – Originally uploaded to the English Wikipedia here by the author (CC BY-SA 3.0) via Commons Wikimedia
2. “Endergonic Reaction” By Provenzano15 – Own work (CC BY-SA 3.0) via Commons Wikimedia