12.6 Tro's Introductory Chemistry, Chapter 12 1

2 Effect of the Strength of Intermolecular Attractions on Properties The stronger the intermolecular attractions are, the more energy it takes to separate the molecules. Substances with strong intermolecular attractions have higher boiling points, melting points, and heat of vaporization; they also have lower vapor pressures.

Tro's Introductory Chemistry, Chapter 12 3 Why Are Molecules Attracted to Each Other? Intermolecular attractions are a result of attractive forces between opposite charges. + ion to – ion. + end of one polar molecule to − end of another polar molecule. H-bonding is especially strong. Even nonpolar molecules will have temporary induced dipoles. Larger charge = stronger attraction.

Tro's Introductory Chemistry, Chapter 12 4 IM Force #1 Dispersion Forces Also known as London forces or instantaneous dipoles. Caused by distortions in the electron cloud of one molecule inducing distortion in the electron cloud on another. Distortions in the electron cloud lead to a temporary dipole. The temporary dipoles lead to attractions between molecules—dispersion forces. All molecules have attractions caused by dispersion forces Weak compared to the other IM forces

Tro's Introductory Chemistry, Chapter 12 5 Instantaneous Dipoles

Tro's Introductory Chemistry, Chapter 12 6 Strength of the Dispersion Force Depends on how easily the electrons can move, or be polarized. The more electrons and the farther they are from the nuclei, the larger the dipole that can be induced. Strength of the dispersion force gets larger with larger molecules.

Tro's Introductory Chemistry, Chapter 12 7 Dispersion Force and Molar Mass

Tro's Introductory Chemistry, Chapter 12 8 IM Force #2 Permanent Dipoles Because of the kinds of atoms that are bonded together and their relative positions in the molecule, some molecules have a permanent dipole. Polar molecules.

Tro's Introductory Chemistry, Chapter 12 9 Dipole-to-Dipole Attraction Polar molecules have a permanent dipole. A + end and a – end. The + end of one molecule will be attracted to the – end of another.

Tro's Introductory Chemistry, Chapter Polarity and Dipole-to-Dipole Attraction

Tro's Introductory Chemistry, Chapter Attractive Forces _ _ _ _ Dispersion forces—All molecules. Dipole-to-dipole forces—Polar molecules.

Tro's Introductory Chemistry, Chapter Intermolecular Attraction and Properties All molecules are attracted by dispersion forces. Polar molecules are also attracted by dipole- dipole attractions. Therefore, the strength of attraction is stronger between polar molecules than between nonpolar molecules of the same size.

Tro's Introductory Chemistry, Chapter CS 2 Nonpolar bonds = nonpolar molecule. CH 2 F 2 Polar bonds and asymmetrical = polar molecule. CF 4 Polar bonds and symmetrical shape = nonpolar molecule. Practice—Determine Which of the Following Has Dipole–Dipole Attractive Forces. (EN C= 2.5, F = 4, H = 2.1, S = 2.5) CS 2 CH 2 F 2 CF 4 

Tro's Introductory Chemistry, Chapter Attractive Forces and Properties Like dissolves like. Miscible = Liquids that do not separate, no matter what the proportions. Polar molecules dissolve in polar solvents. Water, alcohol, CH 2 Cl 2. Molecules with O or N higher solubility in H 2 O due to H-bonding with H 2 O. Nonpolar molecules dissolve in nonpolar solvents. Ligroin (hexane), toluene, CCl 4. If molecule has both polar and nonpolar parts, then hydrophilic-hydrophobic competition.

Tro's Introductory Chemistry, Chapter IM Force #3 Hydrogen Bonding HF, or molecules that have OH or NH groups have particularly strong intermolecular attractions. Unusually high melting and boiling points. Unusually high solubility in water. This kind of attraction is called a hydrogen bond.

Tro's Introductory Chemistry, Chapter Properties and H-Bonding NameFormula Molar mass (g/mol) Structure Boiling point, °C Melting point, °C Solubility in water EthaneC2H6C2H Immiscible EthanolCH 4 O Miscible

Tro's Introductory Chemistry, Chapter Intermolecular H-Bonding

Tro's Introductory Chemistry, Chapter Hydrogen Bonding When a very electronegative atom is bonded to hydrogen, it strongly pulls the bonding electrons toward it. Since hydrogen has no other electrons, when it loses the electrons, the nucleus becomes deshielded. Exposing the proton. The exposed proton acts as a very strong center of positive charge, attracting all the electron clouds from neighboring molecules.

Tro's Introductory Chemistry, Chapter H-Bonds vs. Chemical Bonds Hydrogen bonds are not chemical bonds. Hydrogen bonds are attractive forces between molecules. Chemical bonds are attractive forces that make molecules.

Tro's Introductory Chemistry, Chapter Practice–Pick the Compound in Each Pair Expected to Have the Higher Solubility in H 2 O. CH 3 CH 2 OCH 2 CH 3 or CH 3 CH 2 CH 2 CH 2 CH 3. CH 3 CH 2 NHCH 3 or CH 3 CH 2 CH 2 CH 3. CH 3 CH 2 OH or CH 3 CH 2 CH 2 CH 2 CH 2 OH.

Types of Intermolecular Forces Type of force Relative strength Present inExample Dispersion force Weak, but increases with molar mass All atoms and molecules H2H2 Dipole– Dipole force Moderate Only polar molecules HCl Hydrogen Bond Strong Molecules having H bonded to F, O, or N HF