INTERMOLECULAR FORCES MRS. LOZANO CHEMISTRY

ATTRACTIVE FORCES electrostatic in nature Intramolecular forces bonding forces These forces exist within each molecule. They influence the chemical properties of the substance. Intermolecular forces nonbonding forces These forces exist between molecules. They influence the physical properties of the substance.

BONDING IS AN INTRAMOLECULAR FORCE

stronger cohesive forces Which is stronger? Intermolecular or Intramolecular? capillarity stronger cohesive forces adhesive forces H2O Hg

DISPERSION/ VANDERWAALS forces 3 INTERMOLECULAR FORCES 1. DIPOLE-DIPOLE forces 2. ION-DIPOLE forces DISPERSION/ VANDERWAALS forces Cl2 molecules

3 INTERMOLECULAR FORCES 1. DIPOLE-DIPOLE forces When elements in a molecule have very different electronegativities each side has an opposite charge. We call this polar. This results in two poles, and we call it a di-pole!

INTERMOLECULAR FORCE: DIPOLE-DIPOLE This occurs when two polar substances come together. They are attracted because of the partial charges on each molecule! As suggested through (a) Lewis structures and (b) ball-and-stick models, each water molecule is linked to four others through hydrogen bonds. Each H atom lies along a line that joins two O atoms. The shorter distances (100 pm) correspond to covalent bonds, and the longer distances (180 pm) to the hydrogen bonds.

The H-bonding ability of the water molecule. WATER: dipole-dipole Hydrogen bonding: a special type of dipole-dipole bond. The H-bonding ability of the water molecule.

THE HYDROGEN BOND a dipole-dipole intermolecular force A hydrogen bond may occur when an H atom in a molecule, bound to small highly electronegative atom. The elements which are so electronegative are F, O, and N. .. N H .. H O F H ..

HYDROGEN BONDING IN ACETIC ACID: VINEGAR

Which one will H bond? (c) (a) (b) SOLUTION: (a) C2H6 has no H bonding sites. (b) (c)

The molecular basis of surface tension. Figure 12.19 The molecular basis of surface tension. the net vector for attractive forces is downward hydrogen bonding occurs across the surface and below the surface hydrogen bonding occurs in three dimensions

HYDROGEN BONDING IN DNA Hydrogen bonding connects A to T and G to C, holding the two strands together! Each NH group is hydrogen bonded to a CO group one helical turn (3.6 amino acid units) away in the same chain, giving a fairly rigid cylindrical structure with side chains on the outside.

INTERMOLECULAR FORCES IN PROTEIN STRUCTURE The 4 types of interactions that determine tertiary structure of proteins are shown. Though varying in strength, each set of interactions is significant in the formation of intrachain bonds that fixes the structure of the overal polymer.

The forces that maintain protein structure. Figure 13.7 The forces that maintain protein structure.

Intermolecular forces and membrane structure. Figure 13.10 Intermolecular forces and membrane structure.

Figure 12.14 Hydrogen bonding and boiling point: Stronger intermolecular forces result in a higher boiling point

3 INTERMOLECULAR FORCES 2. ION-DIPOLE forces

INTERMOLECULAR FORCE: ION-DIPOLE When ionic substances dissolve in water you have an ion interacting with the partial charges on water. These are Ion-Dipole forces

3. DISPERSION/ VANDERWAALS forces 3 INTERMOLECULAR FORCES 3. DISPERSION/ VANDERWAALS forces Cl2 molecules

Liquid Helium boils at 4.22 K or -269 °C. WHY SO LOW? VANDERWAALS FORCE OR DISPERSION FORCE: This occurs in a non polar substance which can have an instantaneous dipole causing weak attractions (a) In an unpolarized molecule the electron charge distribution is symmetrical. (b) A displacement of electron charge density (to the left) produces an instantaneous dipole. (c) The instantaneous dipole on the left induces charge separation in the molecule on the right, making it also a dipole, called an induced dipole. The attraction between the two dipoles constitutes an intermolecular force.

Gecko toes: setae and van der Waals forces (seta is a stiff hair or bristle) Recent studies of the spatula tipped setae on gecko footpads demonstrate that the attractive forces that hold geckos to surfaces are van der Waals interactions between the finely divided setae and the surfaces themselves. Every square millimeter of a gecko's footpad contains about 14,000 hair-like setae.

Dispersion forces among nonpolar molecules. Figure 12.15 Dispersion forces among nonpolar molecules. separated Cl2 molecules instantaneous dipoles

When molecules move VERY SLOWLY they can cause the electrons to temporarily move to one side. This causes a temporary change in charge on each side and can induce (cause) a dipole in the neighboring molecule Non-polar substances are only able to do this at a very low temperature.

Types of bond What force(s) would a diatomic molecule such as N2 have? London Dispersion because they have the same electronegativity What force(s) would a molecule such as NH3 (ammonia) have? All forces… they have hydrogen bonding which makes them a dipole, but at low temps london dispersion can also occur