Dipole hydrogen halides

The unequal distribution of charge produced when elements of different electronegativities combine causes a polarity of the covalent bond joining them and, unless this polarity is balanced by an equal and opposite polarity, the molecule will be a dipole and have a dipole moment (for example, a hydrogen halide). Carbon tetrachloride is one of a relatively few examples in which a strong polarity does not result in a molecular dipole. It has a tetrahedral configuration... 

The dipole moments of the hydrogen halides decrease with increasing atomic number of the hydrogen, the largest difference occurring between HF and HCl, and association of molecules is not an important factor in the properties of FICl, HBr and HI. This change in dipole moment is reflected in the diminishing permittivity (dielectric constant) values from HF to HI. 

Table 1 3 lists the dipole moments of various bond types For H—F H—Cl H—Br and H—I these bond dipoles are really molecular dipole moments A polar molecule has a dipole moment a nonpolar one does not Thus all of the hydrogen halides are polar molecules To be polar a molecule must have polar bonds but can t have a shape that causes all the individual bond dipoles to cancel We will have more to say about this m Section 1 11 after we have developed a feeling for the three dimensional shapes of molecules... 

Hydrocarbons normally have very small dipole momen Why (Hint Consider the relationship betwe electronegativity differences and dipole momer established above for hydrogen halides.) Does sing methylene possess a small dipole moment Explain. W1 direction do you expect singlet methylene s dipole point Explain. In what direction does it point ... 

This equation, with a = 0.25, was based on the observed electric dipole moments of HC1, HBr, and HI. Since then the value of the dipole moment of HF has been determined it is 1.98 D, which corresponds to 47% ionic character, whereas Equation 5 with a = 0.25 gives 59%. It seems justified to formulate an empirical function, based on the values 5, 11, 17, and 47% for the hydrogen halides HI, HBr, HC1, and HF, as calculated from their... 

Dipole moments depend on bond polarities. For example, the trend in dipole moments for the hydrogen halides follows the trend in electronegativity differences the more polar the bond (indicated by Ax), the larger the molecular polarity (indicated by the dipole moment, fi ... 

The dipole moments of hydrogen halides decrease with the period of the halogen element the increase in the bond length / is overpowered by the decrease... 

This curve corresponds to the amounts h U. J14, and t>0 percent of ionic character for HI, HBr, HCJ, and HF. respectively. The values for the first three of these hydrogen halides a re cloeehr equal to those indicated by the electric dipole moments of ihe molecules, as given in Table 3-1. At the time when the equation was formulated the value of the dipole moment of HF was not known, and an estimate of 60 percent was made for the partial ionic character in this molecule. As shown in Table 3-1, the dipole moment of HF corresponds to only 45 percent partial ionic character. 

Dipo/e Moment The evidence comes from an examination of the dielectric constant of the hydrogen halides. In an electric field, say between the plates of a condenser, molecules that have a charge separation within them will tend to orient themselves with the electric field. Such molecules behave like electric dipoles (Fig. 4.3) and are called polar molecules. The extent of orientation is reflected by a change in the dielectric... 

Table 4.2 gives the dipole moments for the hydrogen halides. 

The other molecules in Table 2 have permanent dipoles and exhibit all three forces. Note that in the series of hydrogen halides, the dipole moment decreases as the size and polarizability increase hence orientation and induction forces decrease as dispersion forces increase in the series. 

Dipole moments were discussed briefly in Chapter 9 in connection with the ionic character of hydrogen halides. It will be recalled that molecules in which the centers of positive and negative charge do not coincide are said to have a permanent dipole. The electric dipole moment, p, is defined ... 

Spectral studies of rotational energy levels have proved most profitable for linear molecules having dipole moments, particularly diatomic molecules (for example, CO, NO, and the hydrogen halides). The moment of inertia of a linear molecule may be readily obtained from its rotation spectrum and for diatomic molecules, interatomic distances may he calculated directly from moments of inertia (Exercise 14d). For a mole-... 

In addition to R, electric dipole moment a-components are given in Table 7. Assuming that charge transfer can be neglected, this dipole moment is simply the projection of the hydrogen halide moment along the a-axis plus induced moment terms [Eqn. (8)],... 

Table 2.12 Experimental values of dipole moment, p, and dipole polarizabilities, a, of hydrogen halide HX molecules. ...

The structure of the complex between a pair of hydrogen halide molecules is depicted in Fig. 2.7 where three lone electron pairs are placed on the proton-accepting molecule. In the classical case of sp hybridization, one might expect an angle p of some 109°. a. measures the nonlinearity of the H-bond as in the above cases, A nonzero value of a might be expected based on the direction of the dipole moment of the acceptor molecule. 

In a very large number of compounds the chemical bond cannot be considered as purely ionic or purely covalent the dipole moments of such bonds are greater than zero but considerably less than the values given in Table XXV. Typical examples of such bonds are HF, HCl, HBr, HI, NH, OH, GCl etc. If the experimental values for the bond energies of the hydrogen halides are compared with those calculated by the Pauling s method it is found Table XXVI) that for HI the calculated value is greater than the experimental. Such a result is clearly impossible since, because of resonance which was not taken into account... 

The angular orientation sometimes found between the dipoles of A—H and B—in crystals involving H bonds A—H B—[for example, the molecules in the hydrogen halide crystals take a zigzag arrangement in preference to the parallel orientation preferred by a dipolar array (957, 82)]. 

Compare the boiling points and electronegativity differences of the hydrogen halides, shown in Table 3. As the electronegativity difference increases, the boiling point increases. The boiling points increase somewhat from HCl to HBr to HI but increase a lot more for HE What accounts for this jump The answer has to do with a special form of dipole-dipole forces, called a hydrogen bond. 

Generally, as electronegativity differences increase in diatomic molecules, the measured dipole moments increase. This can be seen clearly from the data for the hydrogen halides (see Table 7-3). 

Table LXXVIII gives the dipole moments and the calculated weights of structures for the hydrogen halides. The data show, in agreement with the chemical behaviour, the increasing homopolar nature of the bond in the series, HF, HC1, HBr, HI, the transitional structure in each step making a greater contribution than the ionic structure.

The dipole moments of the hydrogen halides decrease from HF to HI (see Table 10.3). Explain this trend. 

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