Hydroiodic acid-formula-properties-uses with pH calculation

What is hydroiodic acid in halogen chemistry?

Hydroiodic acid is an important chemical in chemistry, especially in inorganic chemistry. It is slightly polar [ionic character 5%] covalent molecule with chemical formula, HI.  

The others name of hydroiodic acid is iodane, hydrogen iodide, hydriodic acid, hydronium iodide etc. 

The pKa value of hydroiodic acid is –11. This means that hydroiodic acid is very strong hydracids of iodine. It is a mono basic acid. 

Hydroiodic acid molar mass

The molecular formula of hydroiodic acid is HI. Hence, the molar mass of hydroiodic acid is 127.91[1.01 + 126.90 = 127.91].

Hydroiodic acid formula and Lewis structure

Hydroiodic acid is a diatomic molecule which consists of hydrogen and iodine molecules.The chemical formula and Lewis structure of hydroiodic acid are shown below.

Hydroiodic acid physical and chemical properties

Hydroiodic acid physical properties

 

Hydroiodic acid is a colorless with pungent smelling gas. It is slightly heavier than air and creates smoke with wet air.

Hydrogen iodide is highly soluble inwater. When it dissolved in water, it forms a constant boiling mixture with boiling point 400K.

At 127ᵒC temperature, an aqueous solution of hydroiodic acid contains 58% of hydrogen iodide gas in stoichimetric ratio. The density of hydroiodic acid is approximately 2.46 g/ml at 298 K.

 

What is the pH of hydroiodic acid?

The density of hydroiodic acid is 2.46 g/ml. That is, 1 ml aqueous solution of hydroiodic acid contains 2.46 g of hydrogen iodide gas. 

Therefore, 1000 ml or 1 liter contains 2460 g of HI gas. Now molar mass of HI molecule is 127.91.

Hence the strength of hydroiodic acid solution is [2460/ 127.91] = 19.4(M). Since hydroiodic acid is a strong acid, hence it is completely ionized in aqueous solution [HI = H⁺ + I ^–].

So, the concentration [H⁺] = [I^–] = 19.4(M) or 19.4(N), as HI is a mono basic acid.

Therefore, the pH of HI acid= –log [H⁺] = –log 19.4 = –1.29. Since the pH value of hydroiodic acid is negative, hence it is a very strong acid.

 

Physical state

Hydrogen iodide is a gas at room temperature. But it is easily compressed to liquid and finally to solid. HI gas is slightly heavier than air and creates smoke with aquatic air.

Melting and boiling point of hydroiodic acid

Due to less polarity of H–I bond, hydrogen iodide can’t form inter molecular hydrogen bonding. 

That is, HI molecules remain as an individual molecule. Hence, HI is a gas at room temperature.

The melting point of HI acid is 222 K and the boiling point is 238 K.

Bond dissociation energy and stability of hydroiodic acid

The bond length of H–I bond is 160.9 pm due large size of iodine atom. Now, bond length is inversely proportional to the bond energy.

Hence, H–I bond dissociation energy is 65-71 Kcal/mole, which is less than other halogen acid.

Consequently, on heating or in presence of sunlight, H–I bond is dissociated into hydrogenand iodine easily. That is, hydroiodic acid is thermally less stable than the other halogen acids.

 

Chemical properties of hydrobromic acid

Hydroiodic acid reducing properties

Since the bond dissociation energy of H–I bond is less than H–F, H–Cl and H–Br bond, hence HI acid act as a strong reducing agent.

Hydroiodic acid is used as a good reducing agent. Because, all types of oxidizing agents [strong, medium and weak].are reduced by hydroiodic acid easily.

But other halogen acids can’t reduce the weak oxidizing agent like FeCl₃, HNO₂, H₂O₂, CuSO₄ etc.

Consequently, hydroiodic acid is a stronger reducing agent than other halogen acids.

Is hydroiodic acid a strong or weak acid?

The aqueous solution of hydrogen iodide, that is,hydroiodic acid is a very strong in nature. It is stronger than HF, HCl and HBr acids. Because hydroiodic acid be completely ionized in aqueous solution.

The calculated pH value of hydroiodic acid is –1.29, which indicates its strong acidic character.

The aqueous solution of hydroiodic acid changes the litmus paper color from blue to red. 

It reacts with metal oxide, metal carbonate to form carbon dioxide, salt and water. Hydroiodic acid is also reacts with metal, like Zn and produce hydrogen gas.

Laboratory preparation of hydroiodic acid

Hydrogen iodide is prepared in laboratory by the reaction of red phosphorous, iodine and water.

This is two step process reactions. In the first step, iodine reacts with red ‘P’ to form phosphorous tri iodide.

In the second step, this phosphorous tri iodide forms hydrogen iodide when it under goes hydrolysis.

 

This hydrogen iodide gas produces hydroiodic acid when dissolved in water solvent.

How to synthesis hydroiodic acid ?

When a mixture of hydrogen gas and iodine vapor is passed over hot ‘Pt’ catalyst at a specific temperature, then hydrogen iodide gas is obtained.

 

The reaction is reversible. Hence, in this method production of hydrogen iodide is incomplete.

This method for preparation of hydrogen iodide is known as Synthesis method. On dissolving of this hydrogen iodide in water, hydroiodic acid is obtained.

Common uses of hydroiodic acid in chemistry  

Hydroiodic acid plays an important role in inorganic,organic and also in industrial chemistry. Some application of HI acid is discussed below.

 

The potassium salt of HI acid is used to prepare medicine in medicine industry. The silver salt of HI acid is used in photography.

 

In organic chemistry, hydroiodic acid is used to prepare iodo compound, Such as, iodoalkane, alkyl magnesium iodide, iodobenzene and other aromatic benzene derivative compound.

It is used as a good reducing agent both in organic and inorganic chemistry. It is also used as a laboratory reagent.

Is hydroiodic acid a poisonous substance?

Hydroiodic acid is corrosive in nature. When it comes in contact with skin, it causes of burns. If for any reason, hydroiodic acid comes in contact in eyes, it may cause severe irritation.

 

How does Identify the hydroiodic acid

When HI acid or metal iodide is heated with concentrated sulfuric acid, a violet vapor of iodine is evolved.

If a few drops of AgNO₃ solution is added to a portion of aqueous iodide solution, a yellow color of precipitate is appeared, which is insoluble both in nitric acid and ammonium hydroxide.

What is hydroiodic acid in halogen chemistry?
Hydroiodic acid formula and Lewis structure
Hydroiodic acid physical properties
What is the pH of hydroiodic acid?
Is hydroiodic acid a strong or weak acid?
Melting and boiling point of hydroiodic acid
Bond dissociation energy and stability of hydroiodic acid
Chemical properties of hydroiodic acid
Laboratory preparation of hydroiodic acid
Common uses of hydroiodic acid in chemistry  
How does Identify the hydroiodic acid
Molar mass of hydroiodic acid.

Read more: HF acid strength and pH calculation.