15 Facts on HI + HNO3: What, How To Balance & FAQs

HI is a colorless liquified gas, and HNO3 (Nitric acid) is a red- yellow fuming liquid with a suffocating odor. Let us know some facts about how these two react to each other.

HI is the strongest acid among the hydrogen halides and a strong reducing agent. On the other hand, HNO3 is one of the strong acids and is an oxidizing agent.

In this article, we will discuss the products under different reaction conditions, balancing the reaction, reversibility, type, and many more facts about the reaction between HI and HNO3 in more depth.

What is the product of HI and HNO3

Mainly nitric oxide(NO), Iodine (I2), and H2O are the products of the reaction between HI and HNO3 . But in some reaction condition, it produces HIO3 as an intermediate.

  • If the conc. of nitric acid is high (60% solution) then it reacts with hydrogen iodide to produce HIO3, NO2, and H2O. The reaction scheme is :    6HNO3 + HI = HIO3 +6NO2 + 3H2O
  • If the conc. Of HI is higher than the conc. of HNO3, then these react to produce nitric oxide (NO), Iodine (I2), and H2O. The reaction scheme is :    6HI + 2HNO3 = 2NO + 3I2 + 4H2O

What type of reaction is HI + HNO3

The type of reactions between HI and HNO3 are Redox reaction , Irreversible reaction , Endothermic reaction .

How to balance HI + HNO3

  • The unbalanced redox reaction is as follows: HI + HNO3 → I2 +NO +H2O
  • After balancing the I atom and N atom we get the equation as: 2HI + HNO3 → I2 +NO +H2O
  • First, find the oxidation numbers of each atom to find which atoms are oxidized and reduced. From the above reaction scheme, we can make a table about the oxidation states of each atom.
AtomsO.N.  on the reactant side O.N.  on the product side
Oxidation numbers of each atom
  • The oxidation number of Iodine changes from  -1 to 0. the change in oxidation number for one atom is one, for two atoms, the total change in O.N.  is 2 units.
  • The oxidation number of N changes from +5 to + 2 . the total change in O.N.  for one N atom is 3 units.
  • The decrease in the O.N. is balanced with the decrease in the O.N. by cross multiplying.

(2I→ I2) *3

(HNO3 → NO) *2

  • The equation becomes :     6HI + 2HNO3 = 2NO + 3I2 + H2O
  • Finally, we balance the H2O by checking the oxygen atoms on both sides.
  • Therefore the final balanced equation is: 6HI + 2HNO3 = 2NO + 3I2 + 4H2O
  • By the Ion- electron method, we can also do this:-

2N(v) +6e → 2N(II)   (reduction)

6I(-1) – 6e→ 6I(0)  (oxidation)

  • After that, we check the oxygen atom on both sides and balance the equation by adding H2O.

HI + HNO3 titration

The titration between HI and HNO3 cannot be done directly as these two reactants are strong acids.

HI + HNO3 net ionic equation

The net ionic equation of HI + HNO3 is :-

6H+ +6I +2H+ + 2NO3= 2NO + 3I2 + 4H+ +4OH

·       As HI and HNO3 are strong acids so in aqueous solution HI decomposes to H+ and
I ions.

·        HNO3 decomposes to H+ and NO3ions respectively.

·       In the product side as I2 and NO are gases so they do not decompose into

·      H2O ionizes to H+ and OHions.

·      So after decomposition into ions, the net ionic equation becomes:-

                   6H+ +6I+2H+ + 2NO3= 2NO + 3I2 + 4H++4OH

HI + HNO3 conjugate pairs

HI + HNO3  generally does not have conjugate pairs as these two do not conjugate each other to give neutralized compounds.

  • HI is a strong acid so, in an aqueous solution, it donates the proton so that the Iis the conjugate base of HI.
  • HNO3 is a strong acid. It donates its proton rapidly. NO3 is the conjugate base of HNO3.

HI and HNO3 intermolecular forces

In HI + HNO3 reaction, the following intermolecular forces are present in the reactant molecules:

  • HI is a polar covalent strong acid. It possesses  Ionic dipole-dipole interactions, the most important intermolecular forces present between Hand I –  to form the HI.
  • As HNO3  is a polar covalent molecule, It possesses dipole-dipole interactions and London dispersion forces.
  • The product NO has an unpaired electron, so it is paramagnetic in nature. So nitric oxide can participate in dipole-dipole interactions and has weak van-der-walls interactions as well.
  • In Ionly the London dispersion force of attraction is present as it is a nonpolar covalent molecule.
  • In water there is an intermolecular H-bonding is present.

HI + HNO3 reaction enthalpy

The reaction enthalpy between the reaction of HI and HNO3 is +85.06 kJ/mol.

The standard enthalpies of the formation of reactants and products are as follows :

Molecules Enthalpy of formation(KJ/mol)
HI 25.94
HNO3 -173.1
H2O -286
I2 0
NO 90.25
Reaction enthalpy of compounds

Reaction Enthalpy ΔHf = Standard enthalpy of products – Standard enthalpy of reactants

Thus ΔHf = [3*0+2*(90.25) +(-286)] – [6*(25.94) + 2*(-173.1)]

                = (-105.5) – (155.64-346.2) kJ/mol

                 = +85.06 kJ/mol

Is HI + HNO3 a buffer solution

HI + HNO3 is not a buffer solution as HI is a strong acid, and HNO3 is a strong acid also, so it cannot produce any buffer solution .As these two cannot maintain the PH  of the solution.

Is HI + HNO3 a complete reaction

The HI + HNO3  reaction is a complete reaction as no further reaction takes place after that, and no reverse reaction takes place either.

Is HI + HNO3 an exothermic or endothermic reaction

The HI + HNO3 reaction is endothermic in nature, as the positive value (+85.06 KJ/mol) of the overall reaction enthalpy indicates that the reaction requires energy to perform the reaction in the forward direction in the form of heat.

Is HI + HNO3 a redox reaction

The HI + HNO3  reaction is a redox reaction where oxidation of I (-1 to 0) and reduction of N(+5 to +2) occurs simultaneously. Here HI is a reducing agent, and HNO3 is an oxidizing agent.

Screenshot 19

simultaneous oxidation and reduction

Is HI + HNO3 a precipitation reaction

The HI + HNO3  reaction is not a precipitation reaction because nothing precipitated here as a product.

Is HI + HNO3 reversible or irreversible reaction

The HI + HNO3  reaction is an irreversible reaction, as here all the reactants react to each other to form the products, which cannot reverse back to the starting materials in this reaction condition. 

Is HI + HNO3 displacement reaction

The HI + HNO3   reaction is not a displacement reaction as here, no atoms are displaced by each other to form the products.


HNO3 decomposes into the water, NO2, and O2. HI is a strong acid that reacts with HNO3 to give a colorless neutral gas NO. This type of reaction has much interest in the field of organic chemistry as well.