CaCl2, referred to as calcium chloride, is a member of alkaline earth metal halides i.e., MX2, where, M is an alkaline earth metal. It is an inorganic compound of white color without any odor.
Calcium chloride is crystalline in nature and can be easily dissolved in water. It is hygroscopic in nature and hence, used in the desiccators for drying chemicals.
The melting and boiling point of anhydrous calcium chloride is very high. However, the melting point of hexahydrate calcium chloride is only 29.9 °C. The molecular weight of its anhydrous form is 111 g/mol.
Generally, calcium chloride is found in the hydrated form i.e., CaCl2 (H2O)n, where n = 0, 1, 2, 4, and 6. Calcium chloride is prepared by reaction of calcium hydroxide with hydrogen chloride.
Ca(OH)2 + 2HCl —-> CaCl2 + 2H2O
Calcium chloride can also be produced from limestone by the Solvay process i.e. CaCO3 + 2NaCl → CaCl2 + Na2CO3
Calcium chloride produces free calcium ions i.e., the aqua complex of calcium ion and chloride ion by dissolving themselves in water as follows
CaCl2 + 6H2O —–> [Ca(H2O)6]2+ + 2Cl
This reaction results in an appreciable increase in temperature i.e., exothermic reaction, and hence, dissolution of calcium chloride in water has a high enthalpy of solvation.
So, is CaCl2 Ionic or Covalent? CaCl2 is an ionic compound owing to the large electronegativity difference between the calcium atom and chlorine atom, which is greater than 2.0.
In calcium chloride, the calcium atom donates its two electrons and become cation whereas each chlorine atom gain one electron, donated by Calcium, and get a negative charge.
Now, the calcium cation and the chlorine anions are attracted by the electrostatic force of attraction and hence, there is the formation of an ionic bond between these two atoms.
Let us start the discussion of the ionic nature of calcium chloride in depth.
Let us initiate this discussion with the basics of chemical bonds i.e., Ionic and Covalent Bond.
The chemical bond is the force of attraction between the atoms of the molecule, which brings atoms together in their chemical structure.
When an atom forms a chemical bond, it tends to achieve the nearest noble gas configuration.
A chemical bond is formed either by transferring electrons or by sharing electrons between atoms of the molecule.
The chemical bond is formed by the participation of only valence electrons, electrons of the outermost shell, of the atom.
There are two types of chemical bonds based on their formation.
- Covalent Bond
- Ionic Bond
The covalent bond results when there is a sharing of electron pair between atoms of the molecule. However, this sharing may be equal or unequal depending on the electronegativity of the constituent atoms of the molecules.
Hence, a covalent bond may be polar or nonpolar.
In covalent bonding, the ionization energy of the atom is very high that it cannot donate its electron or electron pair.
The ionic bond is the chemical bond between metal and nonmetal.
As metals have low ionization energy and hence, they can easily transfer their electron to another atom. Nonmetals easily accept these electrons owing to their high electron gain enthalpy or electron affinity.
Hence, ionic bond results when there is transferring of electron (s) either from one atom to another atom or from metal to nonmetal.
As there is the formation of cations and anion, there will be the electrostatic force of attraction between them.
How Ionic Bond formation takes place in Calcium Chloride
The formation of calcium chloride can be explained by the Born Haber cycle.
Let us understand the formation of calcium chloride in detail.
The calcium chloride compound consists of one calcium atom and two chlorine atoms. As we know that the calcium atom belongs to group 2 of the modern periodic table and hence, it is a metal.
Whereas the chlorine atom belongs to group 17 of the modern periodic table and therefore, it is a nonmetal.
Metals have very low ionization energy (I.E.) and hence, the calcium atom will lose its electron easily. It is a two-step process i.e., loss of one electron per step.
As calcium is a white crystalline solid and hence, the first step would be the conversion of solid calcium to gaseous calcium atom by providing sublimation energy (ΔHsub) because metals can donate their electrons only in gaseous form.
Ca (s) + ΔHsub → Ca (g)
Ca (g) + (I.E.)1 → Ca+(g) + e-
Ca+(g) + (I.E.)2 → Ca2+ (g) + e
Usually, the second ionization energy is greater than the first ionization energy. However, it is not true in the case of the calcium atom i.e., alkaline earth metals.
Let us explain this controversy.
The ground state electronic configuration of the calcium atom is [Ar] 4s2. Here, we need to provide a large amount of energy for the extraction of an electron from the 4s subshell as it is a filled subshell.
After the removal of an electron, the electronic configuration of the unipositive calcium ion becomes [Ar] 4s1.
Now, it is easy to remove an electron by providing a small amount of energy because it will achieve a stable noble has configuration i.e., the Argon atom.
Hence, the second ionization energy of calcium is smaller than the first ionization energy
The chlorine atom is a nonmetal and hence, it has a high tendency to gain electrons owing to its high electron affinity (E.A.) as compared to the calcium atom. Hence, the chlorine atom will accept the electrons donated by the calcium atom.
The chlorine atom exists in diatomic form i.e., Cl2. Therefore, the first step would be the dissociation of chlorine elements into chlorine atoms and the next step would be acceptance of electrons.
½ Cl2 (g) + ΔHdiss → Cl (g)
Cl (g) + e- → Cl-(g) + E.A.
Now, one calcium ion will combine with two chlorine ions, and hence, ionic bond formation takes place between them by releasing energy. This energy is known as Lattice Energy (U) and its value depends upon the strength of the Ionic bond.
Ca2+ (g) + 2Cl-(g) → CaCl2 (s) + U
The lattice energy (U) of the calcium chloride is -2195 kJ/mol.
How do we get to know that calcium chloride is an Ionic compound?
Differentiation between Ionic and Covalent bond
The ionic and covalent bonds can be differentiated on the basis of the electronegativity difference of the bond between atoms.
A chemical bond is an Ionic bond if the electronegativity difference of the chemical bond is greater than 2.0 and it will be a covalent bond if the electronegativity difference is less than 2.0 on the Pauling scale.
How Calcium Chloride is an Ionic compound?
In the case of the calcium chloride compound (CaCl2),
On Pauling Scale,
The electronegativity value of the calcium atom = 1.0
The electronegativity value of the chlorine atom = 3.16
The electronegativity difference of the Ca-Cl bond = 2.16
The electronegativity difference of the Ca-Cl bond in the calcium chloride compound is 2.16 on the Pauling scale, which is greater than 2.0 and confirms the ionic nature of the Ca-Cl bond.
Hence, calcium chloride is an ionic compound.
Calcium chloride is highly soluble in water owing to its ionic nature.
Anhydrous calcium chloride crystallizes in the orthorhombic and tetragonal structure whereas hexahydrate calcium chloride crystallizes itself in trigonal structure.
Uses of Calcium Chloride
- Calcium chloride in water decreases the freezing point of water and hence, it prevents the formation of ice and is used for de-icing.
- A highly concentrated solution of calcium chloride on the road prevents the formation of the dust owing to its hygroscopic nature and forming a liquid layer on the surface of the road.
Here, we have discussed the ionic nature of calcium chloride.
In summary, calcium chloride is an ionic compound owing to the large electronegativity difference of the Ca-Cl bond in calcium chloride, which is greater than 2.0. The calcium atom forms a positive calcium ion by losing two electrons and the chlorine atom forms a negative chlorine ion by accepting one electron.
The calcium ion and chlorine ion attract each other via electrostatic force of attraction and an ionic bond formation takes place between them by releasing energy in the form of Lattice energy.
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Thank you for reading.