8-3 Intermolecular forces Intermolecular forces: are the forces of attractions that exist between molecules in a compound. 10~100KJ/mol These affect the melting and boiling points of compounds as well as the solubilities of one substance in another, and cause the compound to exist in a certain state of matter: solid, liquid, or gas. 2019/1/17

8-3.1 The Polarity of Molecules covalent bond 2019/1/17

Polarity of molecule A non-polar molecule has no net dipole. This can be achieved by the molecule having : (1) only non-polar bonds O2, Cl2 (2) polar bonds arranged symmetrically so that the dipoles cancel out CH4, O=C=O A polar molecule has a net dipole (has a partial positive charge at one site and a partial negative charge at another site). NH3, H2O, HCl This is achieved in the molecule by the molecule being made up of polar bonds arranged unsymmetrically so that the dipoles do not cancel out. 2019/1/17

For diatomic molecule: H-H, Cl-Cl; Molecular polarity is dependent on bond polarity and the molecular geometry For diatomic molecule: H-H, Cl-Cl; HCl, CO, HF For multiatomic molecule: symmetrically: O=C=O, CH4, BF3 nonpolar molecules unsymmetrically: NH3 , H2O polar molecules 2019/1/17

Dipole moment (μ): which is the product of the charge (Q) and the distance d between charges: μ= Q × d Dipole moments are usually expressed in debye units (D) (after Peter Debye, 1884-l966). 1 D = 3.33 × 10 –30 C·m where C is coulomb and m is meter. 2019/1/17

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possesses a dipole moment nonpolar molecule: a molecule that does not possess a dipole moment 2019/1/17

Polarizability of molecule The charge distribution in a molecule can be distorted by an external electric field is called its polarizability. Figure: Instantaneous and induced dipoles 2019/1/17

8-3.2 Van Der Waals Forces Induced dipole Forces Van Der Waals Forces Dipole-Dipole Forces Induced dipole Forces Van Der Waals Forces ion- induced dipole interaction dipole-induced dipole interaction Dispersion Forces 2019/1/17

Dipole-dipole force Dipole-Dipole Forces orientation force Dipole-dipole forces are forces that act between polar molecules, that is, between molecules that possess dipole moments. Dipole-dipole force 2019/1/17

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Induced dipole forces Induced dipole: because the separation of positive and negative charges in the neutral atom (or a nonpolar molecule). The attractive interaction between an ion and the induced dipole is called ion-induced dipole interaction. The attractive interaction between a polar molecule and the induced dipole is called dipole-induced dipole interaction. polar molecule-nonpolar molecule polar molecule-polar molecule 2019/1/17

This forces exists between the ion and the induced dipole, or between the permanent dipole and the induced diople 2019/1/17

dipole-induced dipole interaction ion-induced dipole interaction dipole-induced dipole interaction 2019/1/17

Dispersion Forces A kind of attractive interaction caused by instantaneous dipole exists among any two molecules. 2019/1/17

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Van der Waals Forces polar-polar √ √ √ molecule polar-nonpolar √ √ Orientation Dipole-induced Dispersion polar-polar √ √ √ molecule polar-nonpolar √ √ nonpolar-nonpolar √ 2019/1/17

8-3.3 The Hydrogen Bond Decreasing molar mass Decreasing boiling point 2019/1/17

The Hydrogen Bond H bond is formed when an H atom bonded to one highly electronegative atom (F, O, or N) is simultaneously attracted to a highly electronegative atom (F, O, or N) of a neighboring molecule. 2019/1/17

This interaction is written A­H······B or A­H······A The hydrogen bond is a special type of dipole-dipole interaction between the hydrogen atom in a polar bond such as OH or NH, and the electronegative atoms O, N, or F. This interaction is written A­H······B or A­H······A A and B represent O, N, or F; A­H is one molecule and B is a part of another molecule, and the dashed line represents the hydrogen bond. 2019/1/17

A­H······B or A­H······A Large electronegativity Small radius 2019/1/17

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In solid HF, for example, the molecules HF molecule In solid HF, for example, the molecules do not exist as individual units; instead, they form long zig-zag chains: 2019/1/17

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Intermolecular hydrogen bonds 2019/1/17

Intramolecular hydrogen bonds 2019/1/17