Acid Strength – Order, Chart, Trends & Affecting Factors

You must have read about acids and bases. You have also read the terms like strong or weak acid or base, concentrated or diluted acid or base. But do you know the difference between strong and concentrated acid or base? What makes an acid or base strong? What decides the acid-base strength? Do you know the formula to calculate the strength of an acid or base?

This article is about acid-base strength. If you want to learn about the strength of acid and base or the acid strength formula, read this article till the end.

What Do You Mean by the Strength of Acid and Base?

In general, strength means the ability of an object to hold heavy weights or not to get damaged or break easily. The definition of acid-base strength is similar to this in Chemistry.

Acid-base strength definition:

As per Bronsted-Lowry, the definition of acid and base is all about the tendency to lose and accept H⁺ ions or protons. Acid strength is the estimation of the ability of the acid to lose its protons or H⁺ ions. The easier the removal or loss of H⁺, the stronger the acid is.

Similarly, base strength estimates the base’s ability to accept protons or H⁺ ions. The more easily the acceptance or addition of H⁺, the stronger the base. Suppose the chemical formula HA represents an acid. It is dissociated into H⁺, the proton or cation, and A^–, an anion. It is given by a chemical equation as

HA → H⁺ + A^–

Points to remember:

• The strength of an inorganic acid depends on the atom’s oxidation state to which the proton might attach.
• Acid strength is solvent-dependent. For instance, hydrogen chloride, i.e., HCl, is a strong acid in an aqueous solution. But, when it is dissolved in glacial acetic acid, it is a weak acid.

What Are Strong and Weak Acids?

Based on the dissociation of acids into ions, they are classified into two types, i.e., strong and weak acids.

1. Strong acids:

A strong acid is a kind of acid that dissociates as per the given below reaction:

HA + S ⇌ SH⁺ + A⁻

For example, HCl + H₂O → H₃O⁺ + Cl⁻ Here, S represents a solvent molecule like a water molecule. The seven most common strong acid strength examples are nitric acid (HNO₃), hydrochloric acid (HCl), hydrobromic acid (HBr), sulfuric acid (H₂SO₄), perchloric acid (HClO₄), hydroiodic acid (HI), and chloric acid (HClO₃).

2. Weak acids:

A weak acid is a kind of acid that does not completely dissociate when solvated in a solvent. The following reaction gives it:

HA ⇌ H⁺ + A⁻

The solvent, like water, omits this assertion when its concentration is either low or finely stable by acid dissociation. Some most common weak acid strength examples are oxalic acid (HO₂C₂O₂H), phosphoric acid (H₃PO₄), hydrofluoric acid (HF), benzoic acid (C₆H₅COOH), methanoic acid (HCOOH), and acetic acid (CH₃COOH).

Point to remember:

When reacted with water, a strong acid gets 100% ionised in the solution. A weak acid hardly ionises in the solvents and leaves some undissociated molecules in the solution.

Dissociation of Acids

The strength of a weak acid is calculated by the dissociation constant, K_a. As the following reaction gives the weak acid dissociation, HA ⇌ H⁺ + A⁻  The dissociation constant will be K_a = [H⁺][A⁻] / [HA] It is also used as the acid strength formula. The larger the value of K_a, the stronger the acid strength and the more acid ionises in the water. Whereas the lower the value of K_a, the weaker the acid. Hence, acid strength becomes low and fewer acid ions are formed in the water.

The below table is an acid-base strength chart according to their Ka value. Acid strength examples are also mentioned in this chart.

Ka Value	Acid Name	Base Name
Large	Perchloric acid	Perchlorate ion
3.2 x 109	Hydroiodic acid	Iodide
1.3 x 106	Hydrochloric acid	Chloride
1.0 x 103	Sulfuric acid	Hydrogen sulphate ion
2.4 x 101	Nitric acid	Nitrate ion
5.4 x 10-2	Oxalic acid	Hydrogen oxalate ion
7.1 x 10-3	Phosphoric acid	Dihydrogen phosphate ion
1.8 x 10-4	Methanoic acid	Methanoate ion
6.3 x 10-5	Benzoic acid	Benzoate ion

Factors Affecting Acid Strength

Various factors affect the acid strength of an acid in the solvent solution. Some of them are

1. Bond Strength

Acid strength is calculated in terms of dissociation constants and pK_a values. In general, the dissociation of acids is given by the following chemical equation, HA ⇌ H⁺ + A⁻ So, its acid strength depends on the H+ and A-bond strength. Therefore, the stronger the bond between H⁺ and A^–, the higher the energy needed to break it. As a result, the acid becomes weak. The weaker the bond, the lesser the bond-breaking energy required. Hence, the acid becomes stronger and the acid strength increases.

2. Bond Polarity

The electronegativity of the atoms normally determines bond polarity. The smaller the size of an atom or its ion, the larger its tendency to attract electron pairs. Suppose the bond between H⁺ and A^– is highly polar in an acid. In that case, the proton leaves the molecule easily by making it a strong acid. If the electronegativity difference is low, the proton leaves the bond with difficulty, making the acid weak.

3. Inductive Effect

The inductive effect also plays a critical role in affecting the acid strength of an acid. The higher the electronegativity of an atom, the more it tends to attract the electron pair itself. As a result, polarization occurs among the ions of the molecules. Therefore, it becomes easy for H⁺ ions or protons to get released from the grip of the bond. Therefore, the acid becomes strong, and hence, the acid strength increases.

Acid Strength Orders and Trends

As mentioned earlier, the acid strength depends upon the polarity and strength of the H-A bond and the inductive effect of the ions or atoms present in the acid. So, different acids have different acid strengths depending upon these factors. Order of acidic strength helps you compare the strength of the H-A bond formed by the elements in the periodic table in the same group.

Generally, in an acid HA, ‘A’ plays the key role in deciding the acid strength of the acid. When you move down in a group, the size of ‘A’ increases. Thus, the bond strength of H-A decreases. As a result, the acid strength increases, making the acid strong. For instance, the acid strength of the halogen family is HF < HCl < HBr < HI, In contrast, the atoms’ sizes decrease when you move left to the right in the periodic table, i.e., according to a period. As a result, electronegativity increases. Thus, the polarisation and inductive effect play their roles in deciding the acid strength of an acid.

Conclusion

You are well-informed about the strength of acid and base from the above information. It is concluded that the inductive effects and charge delocalisation significantly impact the basicity or acidity of a compound. A molecule’s acid-base strength depends strongly on the types of bonds its ions form. The weaker the bonding between protons and anions, the more likely it will dissociate H+ ions. In addition, any factor that stabilises the lone pair on the conjugate base favours the dissociation of H+.

Frequently Asked Questions

1. What makes an acid strong or weak?

1. A strong acid, HA, dissociates its ions when dissolved in the solvent solution. Therefore, the bonds holding H⁺ and A^– together must be weak. While in a weak acid, there are some undissociated molecules with ions when dissolved in the solvent solution. It means the bonds that bind H⁺ and A^– together are strong. Hence, the bonds between protons and anions of the acid decide whether it is a strong or weak acid.

2. What is the most powerful acid?

A. A stronger acid with an acid strength higher than pure sulfuric acid is termed a super-acid. Fluoroantimonic acid is the most powerful super-acid, which is many times more acidic than gastric acid, with a pH of -31.3. But carborane superacids are the strongest acid because fluoroantimonic acid is a mixture of acids. The pH of carborane is -18.

3. How does bond strength affect acid strength?

A. When an acid is solvated in water or another solvent solution, it dissociates into protons and anions. For instance, HA ⇌ H⁺ + A⁻ The acid becomes weak if the H⁺ and A^– strength is strong for a general acid. If the strength is less, the acid will be strong. The bond strength of an acid normally depends on the size of the ‘A’ atom: the smaller the ‘A’ atom, the stronger the H-A bond.

4. Which are all said to be Seven strong acids? 

The seven-strong acids are nitric acid, hydrochloric acid, hydroiodic acid, hydrobromic acid, and sulphuric acid. However, being included on a list of strong acids does not indicate how damaging or deadly an acid is.