Intermolecular Forces Unit 9 Lesson 1

Forces that occur BETWEEN molecules (covalent compounds) Intramolecular (ionic and covalent) bonds are inside a compound. They are stronger than intermolecular forces.

* temporary - forms when electrons momentarily shift to one When there is a positive (δ+) and a negative (δ-) side to an atom or molecule Dipole - Can be…… * temporary - forms when electrons momentarily shift to one side of an atom at random * induced - occurs when a neighboring molecule moves past one with a temporary dipole * permanent - the result of a polar bond in the molecular structure

1. London dispersion AKA van der Waals There are three types of Intermolecular Forces (IMFs) 1. London dispersion AKA van der Waals Weakest of the IMFs The only IMF found between nonpolar molecules

While the electrons in the 1s orbital of helium normally repel each other (and, therefore, tend to stay far away from each other), they do occasionally wind up on the same side of the atom. At that instant, then, the helium atom has a temporary dipole (polar), with an excess of electrons on the left side and a shortage on the right side.

Another helium nearby, then, would have a dipole induced in it, as the electrons on the left side of helium atom 2 repel the electrons in the cloud on helium atom 1. London dispersion forces are attractions between an induced dipole and an induced dipole.

2. Dipole – Dipole Interactions Stronger than dispersion forces Occur between molecules with permanent dipoles (polar molecules)

3. Hydrogen Bonds Strongest of the IMFs Very strong dipole-dipole interactions that occur when hydrogen attached to N, O, or F in one molecule is attracted to a N, O, or F in another molecule This is because of the very high electronegativities of N, O & F * Although Cl also has a high electronegativity, it’s larger size keeps it from forming H bonds.

Label the molecules and the location of the H bond in between the molecules. Hydrogen Bonds

Hydrogen bonds are what make water have so many special properties

Intermolecular Forces Affect Many Physical Properties Viscosity – resistance of liquid to flow Increases with stronger intermolecular forces and decreases with higher temperature. Surface Tension - results from the inward force experienced by the molecules on the surface of a liquid Melting & Boiling Points – stronger IMFs equal higher melting & boiling points

States of Matter

The fundamental difference between states of matter is the distance between particles.

Solid Strong cohesive forces Particles in fixed lattice positions Constant shape Constant volume Minimal compressibility

Liquid Particles are close together Not held in fixed positions Take the shape of container Have fixed volume Little compressibility

Gas Particles are far apart Completely fill container Easily compressed

The state a substance is in at a particular temperature and pressure depends on two things: The kinetic energy (KE) of the particles 2. The strength of the attractions between the particles (IMFs) The intermolecular forces between particles become stronger as particles are packed closer together and move less rapidly (less KE)

Phase Changes Melting Vaporization (boiling) Sublimation Freezing Require Energy Release Energy Melting Vaporization (boiling) Sublimation Freezing Condensing Deposition Remember: All phase changes are physical changes!

Phase Diagrams display the state of a substance at various pressures and temperatures and the places where equilibria exist between phases

The AB line is the liquid-vapor interface. It starts at the triple point (A), the point at which all three states are in equilibrium. It ends at the critical point (B); above this critical temperature and critical pressure the liquid and vapor are indistinguishable from each other. Each point along this line is the boiling point of the substance at that pressure.

The AD line is the interface between liquid and solid. The melting point at each pressure can be found along this line.

Below A the substance cannot exist in the liquid state. Along the AC line the solid and gas phases are in equilibrium; the sublimation point at each pressure is along this line.

Phase Diagram of H2O Note the high critical temperature and critical pressure: These are due to the strong van der Waals forces between water molecules.

Phase Diagram of CO2 Phase Diagram of Iodine