Intermolecular Forces

Transcription

1 There are two kinds of forces involving molecules: a) Intramolecular forces b) Intermolecular forces The Intramolecular forces are the bonds between atoms in the molecule: Ionic, Polar covalent, covalent ave you ever wondered: Why does water form on the side of a cold glass? Why does global warming mean there will be bigger and more powerful hurricanes? ow can a tiny change in a gene cause a major disease like sickle cell anemia? The answers to all of these questions can be found in how molecules attract to each other. higher the melting point and the boiling point. lower the vapor pressure at a given temperature. higher the critical temperature.

2 Types of London Dispersion Forces! Non-polar substances Dipole-Dipole Attractions! Polar substances ydrogen Bonding! ydrogen bonded to a small, Types of - What to look for: ydrogen bonding. Types of - What to look for: C C ydrogen bonding. Cl Types of - What to look for: Larger ydrogen bonding. Lo nd o n Dis p e rs io n Fo rc e s! Non-polar substances Dipole -Dipole Attractions! Polar substances y d ro g e n Bo nd ing! ydrogen! bonded to a small,!! very electronegative atom Symmetrical Unsymmetrical

3 Types of - What to look for: Nonpolar ydrogen bonding. Types of - What to look for: ydrogen bonding. Polar Identify the types of in these molecules: a) Methane (C4) Nonpolar b) Methanol (C3O) Polar with O bonds c) Chloroform (CCl3) Polar ydrogen bonding. London D.F. ydrogen bonds dipole-dipole In this pair, select the substance with a higher boiling point. C38 or C4 ydrogen bonding. C38 is virtually nonpolar due to the small electronegativity difference between C and. C4 is symmetrical and nonpolar. C38 has more electrons and a larger size. C38 is therefore more polarizable and has the higher boiling point (-42.1 C vs -164 C).

4 In this pair, select the substance with a higher boiling point. 2S or 2Te ydrogen bonding. 2S or 2Te are both unsymmetrical and polar. 2Te has more electrons and is larger. 2Te is therefore more polarizable and has a larger boiling point (-2 C vs C). In this pair, select the substance with a higher boiling point. I2 or ICl ydrogen bonding. I2 is symmetrical and nonpolar, whereas ICl is unsymmetrical and polar. I2 has more electrons and a larger size. I2 is therefore more polarizable and has the higher boiling point (184.4 C vs 97.4 C). Types of & What to look for London Dispersion Forces! Non-polar substances Dipole-Dipole Attractions! Polar substances ydrogen Bonding! ydrogen bonded to a small, Strength of forces: Strongest Weakest Ionic PC Covalent -Bond Dipole-Dipole London Intramolecular Intermolecular ydrogen bonding.

5 What is ydrogen Bonding? ydrogen bonded to a small, very electronegative element (N, O, F). ydrogen Bonding in water and ice. The effects of ydrogen Bonding.

6 The effects of ydrogen Bonding. The effects of ydrogen Bonding. The effects of ydrogen Bonding. Importance of ydrogen Bonding. Without ydrogen Bonding, water would boil below room temperature. Ice would sink, thus raising the levels of lakes and oceans. Oil and water would mix. DNA and the stuff of life would be much different.

7 higher the melting point and the boiling point. lower the vapor pressure at a given temperature. higher the critical temperature. The importance of IMF. Why does water form on the side of a cold glass? Air contains many separate water molecules, all attracting each other. If they move fast enough they can overcome those attractions and remain a gas mixed in with the air. When water molecules touch the cold glass, they slow down enough that the attractive forces between them cause them to condense on the side of the glass. Why does global warming mean there will be bigger and more powerful hurricanes? Warmer air evaporates more water molecules. During evaporation, heat energy from the air is absorbed by liquid water molecules, which use that energy to overcome their intermolecular attractions. That same energy is released when the water molecules condense back into liquid droplets. The energy released during condensation is what powers hurricanes. More evaporation results in more condensation, resulting in bigger and more powerful hurricanes. The importance of IMF. ow can a tiny change in a gene cause a major disease like sickle cell anemia? Genes are a blueprint for making proteins (a type of large, complex molecule). One of those proteins is hemoglobin, which carries oxygen in our blood cells. There is a gene mutation that results in a small change in the hemoglobin protein, causing hemoglobin molecules to stick to each other, forming fibers that distort blood cells into a sickle shape. Intermolecular attractions are key to understanding these seemingly dissimilar phenomena... and a great deal more. Now it s time to Practice! Effect of IMF on Viscosity Intermolecular Forces-1 & 2