The key difference between spontaneous and nonspontaneous reactions is that the spontaneous reactions have a negative Gibbs free energy whereas the non-spontaneous reactions have a positive Gibbs free energy.
Reactions can be either chemical reactions or biological reactions. We can divide these reactions into two categories as spontaneous reactions and non-spontaneous reactions. A spontaneous reaction occurs without any external influence. But non-spontaneous reactions cannot progress without an external influence. Let’s discuss more details on these reactions and tabulate the difference between spontaneous and nonspontaneous reactions.
CONTENTS
1. Overview and Key Difference
2. What are Spontaneous Reactions
3. What are Nonspontaneous Reactions
4. Side by Side Comparison – Spontaneous vs Nonspontaneous Reactions in Tabular Form
5. Summary
What are Spontaneous Reactions?
Spontaneous reactions are chemical or biological reactions that take place without the influence of any external factor. Moreover, these reactions favour increasing the entropy while decreasing the enthalpy of a thermodynamic system. Since these reactions need no external factor, they occur naturally. Therefore these reactions favour the formation of the products under the conditions at which the reaction occurs. The Gibbs free energy of a spontaneous reaction is a negative value.
Most of the spontaneous reactions occur quickly because it favours forming products rather than keeping the reactant as it is. Ex: combustion of hydrogen. But some reactions occur extremely slowly. Ex: Conversion of graphite into diamonds. Moreover, in some reversible reactions, one reaction direction is favoured over the other direction. For example, in the formation of carbon dioxide and water from carbonic acid, the forward reaction is favoured; formation of carbon dioxide and water is spontaneous.
H2CO3 ↔ CO2 + H2O
What are Nonspontaneous Reactions?
Non-spontaneous reactions are chemical or biological reactions that cannot take place without the influence of any external factor. Therefore, these reactions do not undergo at the natural conditions.
Figure 01: Comparison Of Spontaneous and Non-spontaneous Reactions
Thus, we have to provide some external factor for these reactions to progress. Ex: we can provide heat, give some pressure, add a catalyst, etc. Moreover, the Gibbs free energy is positive for these reactions.
Almost all the nonspontaneous reactions are endothermic; they release energy to outside. These reactions accompany a decrease in entropy. Ex: formation of nitrogen monoxide (NO gas) from the reaction between oxygen and nitrogen in our atmosphere is nonspontaneous at normal temperature and pressure conditions. However, this reaction occurs at very high temperatures.
What is the Difference Between Spontaneous and Nonspontaneous Reactions?
Spontaneous reactions are chemical or biological reactions that take place without the influence of any external factor. They favour increasing the entropy while decreasing the enthalpy of a thermodynamic system. Moreover, the Gibbs free energy of a spontaneous reaction is a negative value. Whereas, the non-spontaneous reactions are chemical or biological reactions that cannot take place without the influence of any external factor. They do not favour increasing entropy or decreasing enthalpy at normal conditions. Furthermore, the Gibbs free energy of a non-spontaneous reaction is a positive value.
Summary – Spontaneous vs Nonspontaneous Reactions
All the reactions belong to two types of reactions such as spontaneous reactions and non-spontaneous reactions. The difference between spontaneous and nonspontaneous reactions is that spontaneous reactions have a negative Gibbs free energy whereas non-spontaneous reactions have a positive Gibbs free energy.
Reference:
1. Libretexts. “11.5: Spontaneous Reactions and Free Energy.” Chemistry LibreTexts, Libretexts, 13 July 2018. Available here
2. Science, Ck12. “Spontaneous and Nonspontaneous Reactions.” CK-12 Foundation, CK-12 Foundation, 28 Mar. 2017. Available here
Image Courtesy:
1.’Gibbs free energy’By Stephjc – Own work, (Public Domain) via Commons Wikimedia