The key difference between autoionization and autoprotolysis is that autoionization is the conversion of a neutral state of a chemical species into an ionized state whereas autoprotolysis is the transfer of a proton between two identical chemical species to form ionized forms.
Both terms autoionization and autoprotolysis describe the two methods of formation of ionized species, i.e. cations and anions. These are spontaneous reactions where ionization occurs without the effect of an external factor.
CONTENTS
1. Overview and Key Difference
2. What is Autoionization
3. What is Autoprotolysis
4. Side by Side Comparison – Autoionization vs Autoprotolysis in Tabular Form
5. Summary
What is Autoionization?
Autoionization is the process of converting a neutral state of a chemical species into an ionized state. The term usually describes the ionization of water molecules. Therefore, we can call it either self-ionization of water or auto-dissociation of water as well. Here, a water molecule deprotonates to form a hydroxide ion, OH– and a hydrogen ion, H+ (a proton). In here, the deprotonation immediately protonates another water molecule and leads to the formation of a hydronium ion (H3O+). So, this process is a good example of the amphoteric nature of water.
Figure 01: Self-ionization of a Water Molecule
Further, this process describes the amphoteric nature of water. Amphoteric nature means that water can act as both acid and base because the autoionization forms both protons and hydroxide ions, which gives the water the ability to neutralize both acids and bases to a little extent; for example, hydronium ion or H3O+ ion can neutralize mild bases, and hydroxide ions can neutralize mild acids.
What is Autoprotolysis?
Autoprotolysis is the process of transfer of a proton between identical chemical species to form ionized species. Here, one of the two identical molecules acts as the Brønsted acid, and it releases a proton. The other molecule can accept this proton. Therefore, this other molecule acts as the Brønsted base. Self-ionization of water is an example for autoprotolysis. Furthermore, this term is different from autoprotonolysis because autoprotonolysis describes the cleavage of a chemical bond by acids.
Some other examples of chemical compounds that undergo autoprotolysis include ammonia and acetic acid;
Autoprotolysis of Ammonia:
2NH3 ⇌ NH2– + NH4+
Autoprotolysis of Acetic Acid:
2CH3COOH ⇌ CH3COO– + CH3COOH2+
What is the Difference Between Autoionization and Autoprotolysis?
Both autoionization and autoprotolysis are spontaneous reactions. The key difference between autoionization and autoprotolysis is that autoionization is the conversion of a neutral state of a chemical species into an ionized state whereas autoprotolysis is the transfer of a proton between two identical chemical species to form ionized forms. An example of autoionization is water while water, ammonia, acetic acid are some examples for autoprotolysis.
Moreover, in the process of autoionization (also known as self-ionization of water or auto-dissociation), a water molecule deprotonates to form a hydroxide ion, OH- and a hydrogen ion, H+ (a proton), while in the process of autoprotolysis, one of the two identical molecules involved acts as the Brønsted acid and it releases a proton which is accepted by the other molecule that acts as the Brønsted base. Furthermore, the autoionization process of water describes the amphoteric nature (it can neutralize both mild acids and mild bases) of water. On the other hand, autoprotolysis describes the amphoteric nature of chemical compounds such as water, acetic acid and ammonia.
The below infographic summarizes the difference between autoionization and autoprotolysis.
Summary – Autoionization vs Autoprotolysis
Both autoionization and autoprotolysis are spontaneous reactions. The key difference between autoionization and autoprotolysis is that autoionization is the conversion of a neutral state of a chemical species into an ionized state whereas autoprotolysis is the transfer of a proton between two identical chemical species to form ionized forms.
Reference:
1. “Self-Ionization of Water.” Wikipedia, Wikimedia Foundation, 3 June 2019, Available here.
2. “Autoprotolysis.” Wikipedia, Wikimedia Foundation, 24 Sept. 2019, Available here.
3. “Water Autoionization.” Chemistry LibreTexts, Libretexts, 5 June 2019, Available here.
Image Courtesy:
1. “Autoprotolyse eau” By Cdang – Own work (Public Domain) via Commons Wikimedia