3.3 Polarity and intermolecular forces

Polarity and intermolecular forces shape how molecules interact. These concepts explain why water beads up on a leaf or why oil and water don't mix. Understanding them is key to grasping how chemicals behave in biological systems.

From electronegativity differences to hydrogen bonding, these forces influence physical properties. They determine melting points, boiling points, and solubility. Recognizing these patterns helps predict how substances will behave in various environments.

Polarity and Electronegativity Differences

Polarity and Electron Density Distribution

• Polarity uneven distribution of electron density within a molecule
• Results in regions of partial positive (δ+) and negative (δ-) charge
• Polar molecules have an asymmetrical arrangement of polar bonds (H2O)
• Nonpolar molecules have a symmetrical arrangement of polar bonds (CO2) or no polar bonds (O2)

Electronegativity and Bond Polarity

• Electronegativity measures an atom's ability to attract electrons in a chemical bond
• Greater electronegativity difference between atoms in a bond leads to a more polar bond
• In a polar covalent bond, the more electronegative atom has a partial negative charge (δ-) and the less electronegative atom has a partial positive charge (δ+)
• Examples of polar bonds: H-O, C-N, C-Cl

Dipole Moment and Molecular Polarity

• Dipole moment measures the polarity of a molecule
• Determined by the magnitude and direction of individual bond dipoles
• Higher dipole moment indicates a more polar molecule
• Molecules with a net dipole moment are polar (NH3), while those with a zero net dipole moment are nonpolar (CH4)

Intermolecular Forces: Types and Examples

Dipole-Dipole Interactions

• Occur between polar molecules
• Partial positive end of one molecule attracts the partial negative end of another molecule
• Examples: HCl molecules, acetone molecules

Hydrogen Bonding

• Special type of dipole-dipole interaction
• Occurs when a hydrogen atom bonded to a highly electronegative atom (N, O, or F) interacts with a lone pair of electrons on another N, O, or F atom
• Strongest type of dipole-dipole interaction due to high electronegativity and small size of hydrogen atom
• Examples: water molecules, DNA base pairs, protein secondary structure

London Dispersion Forces

• Also known as induced dipole-induced dipole interactions
• Occur between nonpolar molecules due to temporary fluctuations in electron density
• Create instantaneous dipoles that attract each other
• Strength increases with molecule size and surface area
• Examples: noble gas atoms, hydrocarbon molecules

Other Intermolecular Forces

• Ion-dipole interactions: attraction between ions and polar molecules (Na+ and H2O)
• Ion-induced dipole interactions: ion's electric field induces a dipole in a nonpolar molecule (Na+ and Ar)

Predicting Intermolecular Force Strength

Relative Strength of Intermolecular Forces

• Strength generally increases in the following order: London dispersion forces < dipole-dipole interactions < hydrogen bonding < ion-dipole interactions
• London dispersion forces are the weakest and present in all molecules
• Dipole-dipole interactions are stronger than London dispersion forces due to attraction between permanent dipoles
• Hydrogen bonding is the strongest type of dipole-dipole interaction

Factors Affecting Intermolecular Force Strength

• Molecule size and surface area: larger molecules have stronger London dispersion forces
• Magnitude of dipole moments: stronger dipole-dipole interactions with higher dipole moments
• Presence of hydrogen bonding sites: molecules with multiple hydrogen bonding sites (DNA, proteins) form extensive networks of hydrogen bonds
• Electronegativity differences: greater differences lead to stronger dipole-dipole interactions and hydrogen bonding

Intermolecular Forces and Physical Properties

Melting and Boiling Points

• Melting point: temperature at which a substance transitions from solid to liquid
• Boiling point: temperature at which vapor pressure equals atmospheric pressure, causing liquid to vaporize
• Substances with stronger intermolecular forces have higher melting and boiling points
• More energy required to overcome attractions between molecules
• Examples: H2O (strong hydrogen bonding) has higher melting and boiling points than CH4 (weak London dispersion forces)

Solubility and the "Like Dissolves Like" Principle

• Solubility: ability of a substance to dissolve in a solvent
• Substances with similar intermolecular forces tend to be more soluble in each other
• Solvent molecules effectively interact with and surround solute molecules
• Examples: polar substances dissolve in polar solvents (ethanol in water), nonpolar substances dissolve in nonpolar solvents (oil in hexane)

Surface Tension and Viscosity

• Surface tension: property arising from cohesive forces between liquid molecules at the surface
• Liquids with stronger intermolecular forces have higher surface tension (water)
• Viscosity: measure of a fluid's resistance to flow
• Liquids with stronger intermolecular forces tend to have higher viscosity
• Attractive forces between molecules hinder their ability to move past one another
• Examples: honey (high viscosity) vs. water (low viscosity)