Hydrogen chloride electronegativity difference

We see from this diagram that for the reaction of hydrogen and chlorine to form hydrogen chloride the two spans of zero length (for the two bonds H—H and Cl—Cl) are converted into two longer spans (for the two bonds II—Cl). (The word span is used to represent the difference in electronegativity of the two atoms.) Application of Equation 3-12 shows that such a reaction is predicted to be exothermic. 

Finally, consider hydrogen chloride, or hydrochloric acid. Hydrogen has an electronegativity of 2.20, and chlorine has an electronegativity of 3.16. Therefore, the electronegativity difference for the chemical bond in hydrochloric acid, HC1, is 0.96. Hydrogen chloride is a gas at room temperature, but its water solution conducts electricity. Is hydrogen chloride a covalent compound or an ionic compound Its AEN can help you decide, as you will see below. 

As you can see, both carbon dioxide (COz) and methane (CH4) are symmetrical and, therefore, must be non-polar molecules. Water (HzO) and hydrogen chloride (HC1) are asymmetrical and therefore might be polar molecules. In order to be sure that water and hydrochloric acid are polar molecules, you must check their electronegativities to be sure that they have polar covalent bonds, which they do. Water, with its asymmetrical shape and polar covalent bonds, is the classic of a polar molecule. All tetrahedral molecules, because of their symmetrical shape, must be non-po-lar. All of the diatomic molecules, such as Oz and H2, must be non-polar because the electronegativity difference between the elements involved will be zero. 

Polar molecules have negative and positive polarity. This is because of the difference in electronegativities of the bonded atoms in such molecules. This polarity results in the formation of attractive forces among molecules. Such interactions are called dipole-dipole interactions. Consequently, the positive end of one molecule is attracted to the negative end of another. Figure 7-2 illustrates the dipole-dipole interactions between hydrogen chloride molecules. 

Polar molecules can attract one another through dipole-dipole forces. The dipole-dipole force is an attractive intermolecular force resulting from the tendency of polar molecules to align themselves such that the positive end of one molecule is near the negative end of another. Recall that a polar molecule has a dipole moment as a result of the electronic structure of the molecule. For example, hydrogen chloride, HCl, is a polar molecule because of the difference in electronegativities of the H and Cl atoms. 

In contrast, the bonding electrons in hydrogen chloride, water, and ammonia are more attracted to one atom than to another because the atoms that share the electrons in these molecules are different and have different electronegativities. Electronegativity is a measure of the ability of an atom to pull the bonding electrons toward itself. 

C—H bonds are relatively nonpolar, because carbon and hydrogen have similar electronegativities (electronegativity difference = 0.4 see Table 1.3) N—H bonds are more polar (electronegativity difference = 0.9), but not as polar as O—H bonds (electronegativity difference = 1.4). Even closer to the ionic end of the continuum is the bond between sodium and chloride ions (electronegativity difference = 2.1), but sodium chloride is not as ionic as potassium fluoride (electronegativity difference = 3.2). 

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