Hydrochloric acid or HCl is a very strong acid without any doubt. It is a colorless, pungent-smelling, chlorine-based acid-containing water.
Very often there is confusion regarding hydrogen chloride and hydrochloric acid, as HCl is the chemical formula for both. So it’s very necessary to clear this doubt before moving on to any new concept.
Hydrogen chloride is the gaseous form whereas hydrochloric acid is the aqueous form.
In simple words, hydrochloric acid is the aqueous solution of hydrogen chloride, with the chemical formula HCl. All other things are the same except their physical states.
It’s interesting to know that another name for hydrochloric acid is muriatic acid.
Formation reaction of HCl
Chlorine (Cl2) gas and hydrogen gas (H2) are directly combined rapidly above 250℃ temperature to get HCl
H2 + Cl2 —–> 2HCl
In an aqueous solution, HCl dissociated quickly into hydronium ion (H3O+) and chloride (Cl-). and this makes hydrochloric acid a strong acid.
HCl + H2O ——> H3O+ + Cl-
Along with the above-mentioned method for HCl preparation, there are a few more, like:-
Organic synthesis method-
R-H + Cl2 ——> R-Cl + HCl
R-Cl + HF ——> R-F + HCl
Laboratory method- HCl can be produced by reacting sodium chloride with sulfuric acid or with NaHSO4,
NaCl + H2SO4 —–> NaHSO4 + HCl
NaCl + NaHSO4 ——-> HCl + Na2SO4
Hydrolysis of chloride compounds- Some reactive chlorine compounds like phosphorus chloride, thionyl chlorides, and acyl chlorides are hydrolyzed to give HCl as a product,
PCl5 + H2O —-> POCl3 + HCl
In addition to these, there are some more basic things about HCl that we must know, like the lewis structure, hybridization, etc.
So let’s try to understand these topics in detail and gain some more knowledge about this compound!
HCl Lewis Dot Structure
Before going into the lewis dot structure of HCl, it’s important to know the basics of the lewis structure.
In simple words, lewis structure is the distribution of electrons around the atoms which helps us to find out the number and types of bonds in the compound.
There are certain easy steps we can follow to get the lewis dot structure of any compound.
- First of all, we need to calculate the total number of valence electrons present in the molecule by summing up the valence electrons of all the atoms.
- Secondly, we need to choose a central atom which is generally the least electronegative atom, or the atom with the most available sites.
- The third step will be sketching a skeletal structure of the molecule with the use of only single bonds.
- Next, the electrons remaining after the formation of single bonds in the skeletal structure are to be filled around the atoms for octet fulfillment. It’s advisable to start from the electronegative atoms and move to the electropositive ones.
- We need to recheck the octet fulfillment for all the atoms. If there is any mistake, we have to fulfill the same by giving multiple bonds.
- Lastly, all the atoms should have the lowest possible formal charge. We can check it by using the following formula:-
Now let’s move on to HCl,
HCl has only 2 atoms so making a lewis dot structure for it is pretty easy!
Let’s first find out the total number of valence electrons;
Hydrogen = 1
Chlorine = 7
Total = 8
Chlorine being a halogen needs another one electron to complete its octet.
Likewise, hydrogen also needs one more electron to attain an octet because hydrogen’s outermost shell can hold up to 2 electrons.
Thus a single bond is formed between the two atoms leading to a covalent bond formation.
The image attached below gives a much clearer picture of the above-mentioned concept;
HCl Molecular Geometry
According to VSEPR theory, HCl has linear molecular geometry/shape and tetrahedral electron geometry. The bond angle is 180°.
The above-attached VSEPR theory chart clearly shows that HCl is an AXE3 type molecule where,
A = the central atom = Cl here
X = atom bonded to A = H here
E = lone pair on A = 3 ( Cl have 3 lone pairs)
Now there can be a question regarding the difference between molecular geometry/shape and electron geometry. Let’s find out what these two geometries are!
Electron geometry considers all electron pairs while determining the geometry of the molecule.
Whereas molecular geometry only includes the atoms.
In simple words, lone pairs are taken into consideration in the case of electron geometry, but not in molecular geometry/shape.
Thus we can see, when lone pairs of Cl are considered, the hydrochloric acid structure is tetrahedral. And when the lone pairs are neglected HCl shows a linear shape.
HCl has no hybridization, because HCl being a linear diatomic molecule, has an H atom and a Cl atom bonded covalently. So there is no need for any extra stability.
Being a diatomic molecule, it only has one atom as a surrounding atom. Thus there can be only one possibility of structure and so no extra stability is needed for this molecule.
Hybridization of a molecule can be predicted either from the VSEPR theory chart or by using the formula;
H = ½ [ V+M-C+A]
V= No. of valence electron
M= no. of monovalent atom
C= charge of the cation
A= charge of the anion
Now, if H= 2 = Sp hybridization
H= 3 = Sp2 hybridization
H= 4 = Sp3 hybridization
H= 5 = Sp3d hybridization
H= 6 = Sp3d2 hybridization
With all these, we can find the hybridization of any molecule very easily!
HCl Molecular Orbital Diagram
In simple words, the molecular orbital theory is the formation of molecular orbitals by the combination of atomic orbitals of the atoms in a molecule.
This theory helps us to know about the arrangement of electrons in the orbitals and also bond order.
The molecular orbital diagram can be explained as;
We can clearly see the atomic orbitals of H and Cl and also the MO of HCl from the above image.
The 3s orbital of chlorine is much lower in energy. So interaction with the 1s orbital of hydrogen is not possible.
So the 3p orbital (3px, 3py, 3pz) of chlorine, with comparable energy as that of 1s orbital of hydrogen, mix up with hydrogen’s orbital.
Only sigma overlap is happening over here. This is because, although the 3p orbital of Cl and 1s of H combine together, the symmetry of both is different.
As a result, only the sigma overlap is possible.
Thus we can see that the MO of HCl has 6 pairs of nonbonding electrons and one pair of bonding electrons which is in 𝛔3pz orbital.
Also, the sigma bond electrons will lie closer to Cl because of its higher value of electronegativity.
All these concepts are important to completely understand the molecular orbital diagram of HCl.
Polarity of HCl
The HCl molecule is a polar molecule because of the higher electronegativity of chlorine than hydrogen.
The chlorine pulls the major charge to its side and becomes a negative pole and hydrogen becomes a positive one. In this way, the molecule is considered a polar molecule.
For more detailed information, you must also read an article on the polarity of HCl.
Physical Properties of HCl
Now let’s talk a bit about the physical properties of this compound. The molar mass of HCl is 36.46 g/mol. Also, HCl has a very low boiling point, i.e -85.05℃, and the melting point is -114.2℃.
We can say that the properties can vary with the molarity of HCl.
This compound has immense use in different fields, starting from the formation of polyvinyl chloride for plastic formation in industries to households, where dilute HCl is used as a descaling agent.
HCl is used in the production of gelatin in food industries. It is used in food additives and leather processing as well.
Refining metals, pickling of steel, production of organic and inorganic compounds, ph control are some more uses of HCl.
Although HCl has a different use in different fields, we need to take safety measures while using it. Concentrated HCl has a high corrosive effect on human skin and can cause severe damage. The fumes are harmful as well and can cause coughing, choking, and irritation in the throat.
Sometimes these damages become serious and irreversible. So it’s better to be careful and attentive while using this chemical.
I tried to cover all the important factors of HCl that we must understand before going into the reactions of HCl. I hope it was helpful in some way or the other and you gained a little more knowledge about this compound!