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Acid Strength and Molecular Structure

Acid Strength and Molecular Structure

Binary Acids and Bases
In the absence of any leveling effect, the acid strength of binary compounds of hydrogen with nonmetals (A) increases as the H-A bond strength decreases down a group in the periodic table. For group 17, the order of increasing acidity is HF < HCl < HBr < HI. Likewise, for group 16, the order of increasing acid strength is H2O < H2S < H2Se < H2Te. Across a row in the periodic table, the acid strength of binary hydrogen compounds increases with increasing...

Bronsted-Lowry Acids and Bases

Bronsted-Lowry Acids and Bases

The acid-base reaction class has been studied for quite some time. In 1680, Robert Boyle reported traits of acid solutions that included their ability to dissolve many substances, to change the colors of certain natural dyes, and to lose these traits after coming in contact with alkali (base) solutions. In the eighteenth century, it was recognized that acids have a sour taste, react with limestone to liberate a gaseous substance (now known to be CO2), and interact with alkalis to form neutral...

Brønsted-Lowry Acids and Bases

Brønsted-Lowry Acids and Bases

In 1923, the Brønsted–Lowry definition of acids and bases was proposed by Johannes Brønsted and Thomas Lowry. According to this theory, a Brønsted acid is defined as a species that donates a proton in a chemical reaction and gets converted to its conjugate base. A Brønsted base is defined as a species that accepts a proton in a chemical reaction and gets converted into its conjugate acid. These transfers of protons are caused by the displacement of electrons in these reactions, which is...

Acids, Bases and Neutralization Reactions

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An acid-base reaction is one in which a hydrogen ion, H+, is transferred from one chemical species to another. Such reactions are of central importance to numerous natural and technological processes, ranging from the chemical transformations within cells or lakes and oceans to the industrial-scale production of fertilizers, pharmaceuticals, and other substances essential to the society.

Acids, Bases and Neutralization Reactions

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Acids and bases play several important roles in biology. The pH of a biological system can significantly impact the function of biological molecules, including enzymes, proteins, and nucleic acids. For example, enzymes have optimal pH ranges for their activity, and changes in pH can denature or alter their structure, affecting their function. Acids and bases also play a crucial role in cellular signaling and communication. The pH of the extracellular fluid around cells can influence the...

Molecular Structure and Acidity

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An acid can be deprotonated to form a conjugate base or an anion. If the produced anion is more stable, then the acid is stronger. On the contrary, if the anion is unstable, then the acid is weaker. Hence, to determine the acidity of the compound, the stability of its conjugate base is studied using various factors.
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Related Experiment Video

Updated: May 21, 2026

Determination of the Gas-phase Acidities of Oligopeptides

Determination of the Gas-phase Acidities of Oligopeptides

Published on: June 24, 2013

Hard and soft acids and bases: structure and process.

¹Department of Chemistry, Center for Functional Nanoscale Materials, Clark Atlanta University, 223 Brawley Dr. SW, Atlanta, Georgia 30314, United States. jreed@cau.edu

The Journal of Physical Chemistry. A

|June 7, 2012

Summary

This summary is machine-generated.

This study reveals that responding electrons in simple anionic bases are the key structure and relaxation during charge transfer is the process driving chemical hardness. This challenges traditional views attributing hard-soft behavior to electrostatic and covalent interactions.

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Area of Science:

Chemical theory
Quantum chemistry
Materials science

Background:

Hard-soft acid-base (HSAB) theory is a cornerstone of chemical interaction.
Traditional explanations for HSAB behavior focus on electrostatic and covalent contributions.
The extrinsic nature of chemical hardness in simple atomic bases requires further elucidation.

Purpose of the Study:

To identify the specific structure and process responsible for hard-soft behavior in simple anionic atomic bases.
To challenge and refine the prevailing understanding of chemical hardness.
To establish a thermochemically based operational scale for chemical hardness.

Main Methods:

Utilized a thermochemically based operational scale to quantify chemical hardness.
Investigated the correlation between electron relaxation energies and operational chemical hardness.
Analyzed the role of "responding electrons" within anionic bases.

Main Results:

Identified "responding electrons" as the structural origin of chemical hardness in anionic bases.
Determined that electron relaxation during charge transfer is the process governing hard-soft behavior.
Demonstrated that responding electrons directly cause hard-soft behavior, contrasting with electrostatic/covalent models.

Conclusions:

The study substantiates that chemical hardness is an extrinsic property in simple atomic bases.
Electron relaxation in responding electrons is the fundamental mechanism behind hard-soft behavior.
This finding offers a new perspective on acid-base interaction mechanisms.