Is Hydrogen Ion a Strong Acid?

HI stands for hydrogen iodide, which belongs to the Hydrogen Halide, HX, family, where X is a halogen, such as F, Cl, Br, and I.

Hydroiodic acid, also known as hydriodic acid, is an aqueous solution of hydrogen iodide (HI). At room temperature, we can’t get 100 percent hydrogen iodide liquid or hydroiodic acid, but we can get 48-57 percent hydrogen iodide by mass, with the rest being water.

Hydrogen iodide has a molar mass of 127.91 g/mol. At typical ambient temperature and atmospheric pressure, hydrogen iodide is a colourless gas, but hydroiodic acid is a colourless aqueous solution.

Hydroiodic acid can be converted to hydrogen halide and vice versa, making them interconvertible.

Let’s look at one of HI’s most crucial characteristics: its acidity.

Is HI a powerful acid, then? Yes, hydrogen iodide, often known as hydroiodic acid, is a powerful acid since its proton can quickly be lost. The strength of the H-I bond is the most important factor in removing a proton from hydroiodic acid. Because the iodine atom is bigger, the H-I bond is weaker, allowing hydrogen to be extracted.

Other elements that contribute to the strong acidity of hydrogen iodide include the electronegativity of the iodine atom and the polarity of hydrogen iodide.

Hydroiodic acid has a pKa of -9.3, making it a strong acid.

The reactions that produce hydrogen iodide are as follows.

2I2 + N2H4 = 4HI + N2H4 is the reaction of iodine with hydrazine.

The reaction of hydrogen sulphide with iodine solution: I2 + H2S → 2HI + S (side product)

I2 + H2 2HI is the reaction of hydrogen with iodine.

The hydrogen iodide is a very useful reagent, and the reactions below produce useful products:

The oxygen reaction is HI + O2 2I2 + 2H2O.

The Reduction of aromatic nitro compounds to aromatic amines: Ar-NO2 + HI → Ar-NH2

Let us elaborate on this explanation for the strong acidity of hydroiodic acid or hydrogen iodide.

Before that, we need to understand the basic concept i.e., what is acid or base? ACID AND BASE.

Various scientists put different theories for defining acids and bases. Three of them have gained widespread acceptance: the Arrhenius Theory, the Bronsted-Lowry Theory, and the Lewis Theory of Acids and Bases.

Let’s begin by looking at the definitions of acid and base from the perspectives of many scientists.

The Acid-Base Theory of Arrhenius

Acids, according to Svante Arrhenius of Sweden, are compounds that may easily give their protons (H+) in water. Hydrochloric acid (HCl), sulphuric acid (H2SO4), and phosphoric acid are examples (H3PO4).

Bases, on the other hand, are compounds that transfer hydroxide ions (OH-) to water. Arrhenius bases include sodium hydroxide (NaOH), barium hydroxide (Ba(OH)2), and magnesium hydroxide (Mg(OH)2).

Because this theory is based on the ionisation of compounds in water, it is only applicable to substances that are soluble in water. As a result, it does not cover all natural substances.

Ammonia, for example, is a base that cannot transfer hydroxide ions to water. Arrhenius’ hypothesis falls short of explaining its fundamentality in this case.

Similarly, there are a number of examples in the literature that show that Arrhenius’ theory is flawed.

Following that, in 1923, two theories, Bronsted-Lowry theory and Lewis Theory, were developed simultaneously to describe the acidity and basicity of substances not covered by Arrhenius theory.

The Acid-Base Theory of Bronsted Lowry

It’s also known as the proton theory of acids and bases, because it explains acidity and basicity in terms of their propensity to lose or receive protons.

As a result, we can see that this theory’s definition of acid corresponds to the Arrhenius Theory. The distinction between the two ideas is based on how Base is defined.

According to the Arrhenius Theory, bases are proton acceptors while bases are hydroxide donors.

Bronsted acid, for example, is HCl, and Bronsted base is NH3.

After giving its proton, bronsted acid forms a conjugate base. The Bronsted base, on the other hand, produces conjugate acid when it accepts protons.

Apart from protons, the tendency of acids and bases to donate or take electrons, as determined by Lewis, can be used to identify them.

The theory of acid and base by Lewis

Acids, according to Lewis, are electron-deficient entities that readily acquire electrons from other species.

Lewis bases, on the other hand, are electron-rich or electron-donor species.

Why is HI so acidic?

It has a proton for donation in the case of Hydrogen Iodide (HI). As a result, it is acid according to both the Arrhenius and Bronsted theories.

The tendency of HI to lose proton, or how easily it donates a proton, determines whether it is a strong or weak acid.

Let’s start with the idea of strong and weak acids.

There are two types of acids: strong acid and weak acid.

Strong acids are those acids that dissociate entirely or totally in the solution. Weak acids are acids that do not entirely dissociate in a solution.

Acetic acid (CH3COOH), for example, is a weak acid because it maintains equilibrium with its ions.

It does not entirely dissociate in water and exhibits a reversible interaction with it.

CH3COOH   +   H2O   ↔   CH3COO-   +   H3O+

Hydrogen iodide is a strong acid because it dissociates entirely in solution.

HI   +   H2O   →   I-   +   H3O+

What factors have an impact on the acidic strength of acids?

Factors that influence the acid’s acidic strength (HX)

The strength of an acid is determined by a number of factors, including

The atom’s electronegativity (X)

Atomic Dimensions (X)

The molecule’s polarity and the strength of the H-X bond

Let’s take a look at each of these factors individually in the context of hydrogen iodide (HI).

The atom’s electronegativity (X)

It is more acidic if the hydrogen atom is linked to the more electronegative atom.

On the Pauling scale, iodine and hydrogen have electronegativity values of 2.66 and 2.20, respectively. As a result, hydrogen iodide is a powerful acid.

Atomic Dimensions (X)

The acid’s potency is also determined by the size of the X atom. The H-X bond grows larger and weaker as the X atom gets bigger. As a result, HX’s acidic strength increases.

Because the iodine atom is so huge in comparison to the hydrogen atom, the H-I bond is both larger and weaker.

As a result, hydrogen iodide readily donates its proton, making it a powerful acid.

The molecule’s polarity and the strength of the H-X bond

The electron density will move towards a more electronegative atom if the molecule is more polar.

It makes the H-X bond weaker, which means it takes less energy to break it. The acidic strength of the molecule increases as a result.

The electronegativity difference between the hydrogen and iodine atoms in HI is 0.46. As a result, the H-I bond is polar, and a dipole will point towards the iodine atom.

This dipole will give the hydrogen atom a partial positive charge and the iodine atom a partial negative charge, causing electron density to shift towards the iodine atom.

The H-I bond is now weaker, and it will be disrupted by a lack of energy. As a result, in water, the H I bond will easily dissociate.

HI   +   H2O   —–>   I-   +   H3O+

Hydrogen iodide is a powerful acid due to its complete or 100 percent dissociation. Because of the aforementioned factors, hydrogen iodide is a powerful acid.


The acidity of hydrogen iodide, which is a gas at room temperature and atmospheric pressure, has been discussed in this article. At typical conditions, however, the hydroiodic acid, which is an aqueous solution of hydrogen iodide, is liquid.

The electronegativity of X in comparison to the hydrogen atom, the size of the X atom, the polarity of the HX molecule, and the strength of the H-X bond all influence the acidity of the acid (HX).

All of these things lead to hydrogen iodide’s or hydroiodic acid’s severe acidity. Thank you for taking the time to read this article.

Suggestions are much appreciated.

Good luck with your studies.

Read more: Is it true that milk is a homogeneous mixture?

Misha Khatri
Misha Khatri is an emeritus professor in the University of Notre Dame's Department of Chemistry and Biochemistry. He graduated from Northern Illinois University with a BSc in Chemistry and Mathematics and a PhD in Physical Analytical Chemistry from the University of Utah.


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