HNO2 or Nitrous Acid comes under the category of monoprotic acids (acids that donate one proton during dissociation). It is a weak acid and exists only in solution form in the form of nitrite salts (NO2-).
Nitrous acid has a relatively lower percentage of oxygen than nitric acid (HNO3). Quite unstable in nature, it was discovered by Scheele. It has a pungent odor and appears as a pale blue solution.
Nitrous Acid is prepared by acidifying sodium nitrite and mineral acid. This is usually done at ice temperatures and the product HNO2 is produced in the reaction mixture itself.
It can also be prepared by dissolving dinitrogen trioxide in water. The reaction is as follows:
N2O3 + H2O ——> 2HNO2
HNO2 Lewis Structure
Making a Lewis structure is the first and most crucial step for identifying various properties related to the bonding of the molecule.
Hence, whenever bonding is mentioned besides a molecule or a compound your mind should immediately jump to the Lewis structure of the given compound.
Now, before we proceed to make the Lewis structure for HNO2, there are a few things you need to note.
The number of valence electrons of an atom is denoted by the Lewis structure. Valence electrons of a particular group can be determined by looking at the number written on top of their column in a periodic table. The valence electrons are shown in the form of dots around the atom.
These electrons are arranged in such a way that each atom completes its octet. This basically means each atom should have 8 electrons around it so that it can achieve its stability.
Only hydrogen and helium are exceptions as they follow the duplet rule i.e 2 electrons in its outermost shell.
Now let us take a look at the steps of drawing a Lewis diagram:-
Step 1. We begin by counting the total number of valence electrons of the molecule.
Looking at HNO2, we see that H has 1 valence electron, N has 5 valence electrons and O has 6 valence electrons and there are two atoms of O, so 6×2 = 12 valence electrons.
Thus, when we add it all up we get the total number of valence electrons which is 1+5+12 = 18 valence electrons.
Step 2. Now we come to the second step which is to determine the central atom of the compound (one which has the highest number of bonding sites).
In the case of HNO2, we need to note that whenever H is attached to a polyatomic molecule (in this case NO2) the H will always be attached to one of the oxygen atoms.
Hence we see that N is the central atom as it has the highest bonding sites and is less electronegative than O.
Step 3. Now we put 2 valence electrons between each atom which resembles a chemical bond.
Step 4. Now we start arranging the rest of the valence electrons so that each atom attains its octet or duplet (H).
Step 5. After arranging these electrons if the atoms do not achieve their octet form then we turn the valence electrons into a double or a triple bond so that each atom has its complete octet.
You can also check out the formal charge of each atom as a last step. It should be the lowest possible and can be calculated by the formula given below.
Now let us take a look at HNO2
Total number of valence electrons= 18.
Central atom = N
After arranging all the 18 valence electrons around the molecule, we notice that N is short of 2 valence electrons for it to complete its octet.
Thus we turn one pair of valence electrons from O to form a double bond with N so that each atom achieves its octet. Now the Lewis structure of HNO2 is complete and if we check the formal charge of each atom it turns out to be zero.
You can use the above-mentioned steps to find the Lewis structure of any molecule.
After understanding the Lewis structure we next come to the Hybridization of a molecule. Hybridization is the formation of new hybrid orbitals which help determine a molecule’s shape and properties.
The Hybridization of HNO2 is Sp2.
There are two ways to understand Hybridisation:-
1. We can find hybridization by understanding the theory behind it. Hybridization is determined by adding the number of bonds and the lone pair of the central atom.
The value of Hybridization (H) goes like this,
If H=2 then it’s sp hybridized.
If H=3, then its sp2 hybridized.
H=4 means it sp3 hybridized.
H=5 means it sp3d hybridized.
And H=6 means it is sp3d2 hybridized.
In HNO2, we know N is the central atom. It’s bonded to two oxygen atoms and has a lone pair. Thus the total (H) is 2+1=3 making it sp2 hybridized.
2. We also have a formula that is useful to determine the Hybridization of a molecule.
The formula to find the Hybridization is as follows:-
H= Hybridization, V= Number of Valence electrons, C= Charge on cation or more electropositive atom, and A= Charge on anion or more electropositive atom.
When we take HNO2 we see that,
V = 5 (valence electrons of the central atom N)
M= 1. Oxygen (O) is a divalent atom. So it is not counted. The only monovalent atom is H of which there is only one atom.
Since HNO2 is a neural molecule (overall charge is 0), the charge of cation or anion will also be zero.
H=3, indicating that HNO2 is Sp2 hybridized. Hence, the hybridization of HNO2 can be found using these two methods.
HNO2 Molecular Geometry
The next important step is to determine the molecular geometry of HNO2. It gives us the shape and the bond angles between each atom.
The molecular geometry of HNO2 is Bent (a bent-shaped molecule). The bond angles formed are close to 120°.
This can be determined by using the AXN notation where A represents the central atom which is N in this case. So A=1. Next, we look at X which represents the number of atoms bonded to the central atom. So X=2 (2 Oxygen atoms bonded to Nitrogen).
Now N here represents the non-bonding electrons i.e lone pairs on the central atom. Here, N=1 as there is a pair of lone pair on N atom. Thus we get the notation as AX2N.
If we look up this notation in the VSEPR chart given below we find that HNO2 has a bent shape.
HNO2 is considered to be a polar molecule.
When there is an electronegativity difference between different atoms within a molecule then polarity occurs. Polar molecules have an asymmetrical structure their net dipole moment equals zero.
To find the polarity of HNO2, we first have to draw its Lewis structure. From the Lewis structure, it is clear that HNO2 is not a symmetrical molecule.
One side of N is bonded to OH and the other side is a double bond with O. From here we can already infer that HNO2 is a polar molecule.
Now to make sure of this fact, we take a look at its molecular geometry. The bent shape due to the lone pair makes it asymmetrical.
Also, we notice that one end of the molecule is highly electronegative due to O and the other end is highly electropositive due to H.
Thus we have oppositely charged poles at either end of the molecule and this coupled with the fact that HNO2 has an asymmetrical shape makes it a polar molecule.
Properties of HNO2
- It is an extremely volatile compound and gives off thick fumes when it rises in the air.
- The molar mass of HNO2 is 47.013 g/mol.
- Density is approximately equal to 1 g/ml.
- The boiling point of HNO2 is 82 degrees Celsius.
- When HNO2 is reacted with water, nitrous gas is evolved. HNO2 in a fuming state allows the combustion of phosphorus and charcoal.
1. It is used to prepare diazonium salts in amines as well as for the preparation of azo dyes (sandmeyer reaction).
2. Acts as a potent oxidizer in liquid fuel rockets. It can also be used to remove the toxicity of sodium azide.
To summarize this article, we discussed the Lewis structure, hybridization, molecular geometry, and polarity of HNO2. Now, you should be able to master the basics of the molecule HNO2.
We hope you got all the answers you were searching for and if you have doubts regarding any topic please feel free to reach out to me anytime.