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
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).
FAQs on Acid Strength
1. What does 'acid strength' actually mean in Chemistry?
Acid strength refers to the ability of an acid to lose a proton (H⁺ ion) when dissolved in a solution, typically water. It is not about how corrosive an acid is, but rather its degree of ionisation. A strong acid almost completely ionises, releasing all its H⁺ ions, while a weak acid only partially ionises, meaning most of its molecules remain intact.
2. What are the main factors that determine how strong an acid is?
Several factors influence acid strength, primarily related to the stability of the molecule after losing a proton:
- Bond Strength: The weaker the bond between hydrogen and the rest of the acid molecule (H-A), the more easily the proton can be released, making the acid stronger.
- Bond Polarity: A more polar H-A bond makes it easier for the H⁺ ion to separate, increasing acidity.
- Stability of the Conjugate Base: After an acid donates a proton, it forms a conjugate base (A⁻). If this conjugate base is very stable (due to factors like resonance or electronegativity), the acid is more likely to donate its proton and is therefore stronger.
3. How are Ka and pKa values used to measure and compare acid strength?
The acid dissociation constant (Ka) is a quantitative measure of acid strength. It represents the equilibrium constant for the ionisation of an acid.
- A higher Ka value indicates a greater extent of dissociation, meaning the acid is stronger.
- The pKa value is the negative logarithm of Ka (pKa = -log(Ka)). Because of the negative sign, the relationship is inverse. A lower pKa value signifies a stronger acid.
For example, HCl has a very low pKa, while acetic acid has a pKa of about 4.76, making HCl the much stronger acid.
4. How can the periodic table help predict trends in acid strength?
The periodic table provides clear trends for the strength of binary acids (H-A):
- Across a Period (left to right): Acid strength increases. This is because the electronegativity of the atom 'A' increases, making the H-A bond more polar and the conjugate base more stable. For example, HF is a stronger acid than H₂O.
- Down a Group: Acid strength increases. In this case, atomic size is the dominant factor. As the atom 'A' gets larger, the H-A bond becomes longer and weaker, making it easier to break and release the proton. For example, HI is a stronger acid than HCl.
5. Why is an acid like HCl considered strong, while acetic acid (CH₃COOH) is weak?
The difference lies in their degree of ionisation. Hydrochloric acid (HCl) is a strong acid because it dissociates almost 100% in water to form H⁺ and Cl⁻ ions. Its conjugate base, Cl⁻, is very stable. In contrast, acetic acid (CH₃COOH) is a weak acid because it only partially dissociates. An equilibrium is established where most of the acid remains as undissociated CH₃COOH molecules, with only a small fraction forming H⁺ and acetate (CH₃COO⁻) ions.
6. Why is phenol more acidic than alcohol, but less acidic than a carboxylic acid?
This is explained by the stability of their conjugate bases due to resonance.
- When an alcohol loses a proton, the resulting alkoxide ion has no resonance stabilisation.
- When phenol loses a proton, the negative charge on the resulting phenoxide ion is delocalised (spread out) over the benzene ring through resonance, making it relatively stable. This makes phenol acidic.
- When a carboxylic acid loses a proton, the negative charge on the carboxylate ion is delocalised over two highly electronegative oxygen atoms. This resonance is much more effective, making the carboxylate ion extremely stable and the acid much stronger than phenol.
7. What are some common examples of strong acids?
Strong acids are those that completely ionize in solution. The most common examples taught in school are:
- Hydrochloric acid (HCl)
- Sulphuric acid (H₂SO₄)
- Nitric acid (HNO₃)
- Perchloric acid (HClO₄)
- Hydrobromic acid (HBr)
- Hydroiodic acid (HI)
8. Is there a 'strongest' acid in existence?
Yes, acids stronger than 100% pure sulphuric acid are called superacids. The most famous and often cited 'strongest' acid is fluoroantimonic acid (H₂SbF₆). It is incredibly powerful because it creates an extremely stable conjugate base, which is very effective at holding the electron pair after the proton has been donated, making it over ten quadrillion (10¹⁶) times stronger than pure sulphuric acid.
