Acid Strength

Acid Strength Definition 

Acid strength can be defined as the tendency of an acid, to dissociate into a proton, H+, and an anion, A−, and symbolized by the formula HA. The dissociation of a robust acid in solution is effectively complete, except in its most concentrated solutions

HA → H+ + A

Strong acid examples are hydrochloric acid (HCl), perchloric acid  (HClO4),  nitric acid (HNO3), and sulfuric acid (H2SO4).

A weak acid is partially dissociated, with both the dissociated acid and its undissociated product being present, 

In equilibrium with each other.

HA ⇌ H+ + A.

The best example of a weak acid is Acetic acid  (CH3COOH).

The strength of a weak acid is quantified by its acid equilibrium constant, pKa value.

What is Acid Strength?

The ability of the acid is to lose its H+ ion is the measure of Acid strength

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It depends on several factors which we will discuss in the subsequent sections.

Strong Acids

Strong acid is an acid that dissociates according to the chemical reaction

HA + S ⇌ SH+ + A

S represents the solvents molecule, like the molecule of water Or DMSO, to an extent that the concentration of the undissociated species HA is too low to be measured.

A strong acid can be completely dissociated for practical purposes,. An example of a robust acid is acid

HCl → H+ + Cl (in aqueous solution)

Any acid with a pKa value which is a smaller amount than about -2 is classed as a robust acid. This results from the very high buffer capacity of solutions with a pH value of 1 or less and is understood because the leveling effect

Weak Acids

A weak acid is a substance that partially dissociates when it is dissolved in a solvent. In solution, there's an equilibrium between the acid, HA, and therefore the products of dissociation.

HA ⇌ H+ +A+

The solvent (e.g. water) is omitted from this expression when its concentration is effectively unchanged by the method of acid dissociation. The strength of a weak acid is often quantified in terms of an equilibrium constant, Ka, defined as follows, where [X] signifies the concentration of a chemical moiety, X. 

[H]2/ka + [H]- TH=0

This equation shows that the pH of a solution of a weak acid depends on both its Ka value and its concentration. Typical samples of weak acids include ethanoic acid and hypophosphorous acid. An acid like ethanedioic acid (HOOC–COOH) is claimed to be dibasic because it can lose two protons and react with two molecules of an easy base. Phosphoric acid(H3PO4) is tribasic.

Factors Determining Acid Strength

Different acids have different acid strengths. As already discussed earlier 

If An acid has a greater degree of dissociation it behaves as a stronger acid.

Now allow us to understand the factors on which the strength of an acid depends. The degree of dissociation of an acid depends on the following two factors. 

Strength of H-A bond 

Polarity of H-A bond

In general weaker the strength of H-A bond, stronger is that the acid. And also, greater the polarity of the H-A bond is, stronger is the acid. Both these factors make the dissociation of acid molecules into H+ and A- easier thereby increasing the acidity.

While comparing elements in the same group of the periodic table the strength of the A-H bond is a more important factor in deciding the acidity than its polarity. As the size of A increases on descending a gaggle, H-A bond strength decreases, and thus the acid strength increases. For example, the acid strengths of hydrides of group-17 elements increase in the order. 

HF < HCl < HBr < HI

Fun Facts 

The following is strong acids in aqueous and dimethyl sulfoxide solution. The values of pKa, cannot be measured experimentally. The values within the following table are average values from as many as 8 different theoretical calculations




in Water


Hydrochloric acid


-5.9 ± 0.4

−2.0 ± 0.6

Hydrobromic acid


-8.8 ± 0.8

−6.8 ± 0.8

Hydroiodic acid


-9.5 ± 1

−10.9 ± 1

Triflic acid


−14 ± 2

−14 ± 2

Perchloric acid


−15 ± 2

−15 ± 2


Also, in water

  • Nitric acid (HNO3), pKa = -1.6 

  • Sulfuric Acid (H2SO4), pKa1 ≈ −3 (Only first dissociation)

The following can be used as protonation in organic chemistry

  • Fluoroantimonic acid H[SbF6]

  • Magic acid H[FSO3SbF5]

  • Carborane superacid H[CHB11Cl11]

  • Fluorosulfuric acid H[FSO3] (pKa = −6.4)

A class of strong organic oxyacids is Sulfonic acids, such as p-toluenesulfonic acid (tosylic acid).

FAQ (Frequently Asked Questions)

1. Is Acetic Acid a Strong aAcid or Weak Acid? Why?

Acetic acid is a WEAK acid. 

Acetic acid, CH3COOH, is a weak acid because it is present in a solution primarily as whole CH3COOH molecules and very little as H+ and CH3COOions. CH3COOH(aq)⇌H+(aq)+CH3COO(aq)The acid dissociation constant (Ka) of acetic acid is 1.76 ×10−5. This basically means that the ratio of CH3COOH concentration to the concentration of H+ and CH3COO ions is 1:1.76×10−5


Which furthermore indicates that ethanoic acid is weak because strong ions ionize almost completely. 

2. What Determines the Strength of an Acid?

Strength of acid is as follows

  • The strength of an acid depends on the H-A bond. The stronger the bond, the max energy is required to break it. Hence, the acid is weak.

  • The acid strength is affected by the polarity of the H-A bond. If the bond is very polar, the proton tends to go away the molecule more easily, making it a robust acid. 

  • However, bond strength is more important once we consider and compare acid strengths of elements within the same group of the table, using the above two factors.

  • Nevertheless, once we compare the acid strengths of elements within the same row, priority is given to the polarity of the H-A bond.

  • The atomic size of acid also affects the acid strength. As the atom becomes bigger, the bond becomes weaker. Consequently, acid strength increases.