Acid strength
The strength of a weak organic acid may depend on substituent effects.
The strength of an inorganic acid is dependent on the oxidation state for the atom to which the proton may be attached.
Acid strengths also depend on the stability of the conjugate base.
value measures the tendency of an acidic solute to transfer a proton to a standard solvent (most commonly water or DMSO), the tendency of an acidic solvent to transfer a proton to a reference solute (most commonly a weak aniline base) is measured by its Hammett acidity function, the
Although these two concepts of acid strength often amount to the same general tendency of a substance to donate a proton, the
values are measures of distinct properties and may occasionally diverge.
values indicating that it undergoes incomplete dissociation in these solvents, making it a weak acid.
However, as the rigorously dried, neat acidic medium, hydrogen fluoride has an
value of –15,[1] making it a more strongly protonating medium than 100% sulfuric acid and thus, by definition, a superacid.
This usage is consistent with the common parlance of most practicing chemists.)
When the acidic medium in question is a dilute aqueous solution, the
is approximately equal to the pH value, which is a negative logarithm of the concentration of aqueous
For weak acid solutions, it depends on the degree of dissociation, which may be determined by an equilibrium calculation.
A strong acid is an acid that dissociates according to the reaction where S represents a solvent molecule, such as a molecule of water or dimethyl sulfoxide (DMSO), to such an extent that the concentration of the undissociated species
For practical purposes a strong acid can be said to be completely dissociated.
This results from the very high buffer capacity of solutions with a pH value of 1 or less and is known as the leveling effect.
[3] The following are strong acids in aqueous and dimethyl sulfoxide solution.
A weak acid is a substance that partially dissociates or partly ionizes when it is dissolved in a solvent.
The solvent (e.g. water) is omitted from this expression when its concentration is effectively unchanged by the process of acid dissociation.
The strength of a weak acid can be quantified in terms of a dissociation constant,
is known it can be used to determine the extent of dissociation in a solution with a given concentration of the acid,
This equation shows that the pH of a solution of a weak acid depends on both its
) is said to be dibasic because it can lose two protons and react with two molecules of a simple base.
It is sometimes stated that "the conjugate of a weak acid is a strong base".
An acid which is strong in water may be weak in a less basic solvent, and an acid which is weak in water may be strong in a more basic solvent.
217) An important example of a solvent which is more basic than water is dimethyl sulfoxide, DMSO,
Superacids are strong acids even in solvents of low dielectric constant.
[11] Lewis acids reacting with Lewis bases in gas phase and non-aqueous solvents have been classified in the ECW model, and it has been shown that there is no one order of acid strengths.
[13][14] It has been shown that to define the order of Lewis acid strength at least two properties must be considered.
The effect decreases, the further the electronegative element is from the carboxylate group, as illustrated by the following series of halogenated butanoic acids.
