Condensed Phases and Intermolecular Forces

Let’s look at particle diagrams of liquids & solids and compare them to particle diagrams of gases

Describe & compare the relative positions and motions of particles in each of 3 phases:

The Question: Why do some substances exist as gases, some as liquids, and some as solids at room temp?

part of answer has to do with attractive forces between separate but neighboring molecules

2 broad categories of attractive forces come into play: 1. INTRAmolecular forces of attraction 2. INTERmolecular forces of attraction

Forces INTERMOLECULAR INTRAMOLECULAR Dispersion Dipole-Dipole Hydrogen Bonding Covalent Ionic Metallic

#1: Intramolecular Forces Intra means “within” Intramolecular attractive forces are chem bonding forces Definition: attractive forces that hold particles together in chemical bonds 3 types: ionic, covalent, or metallic bonds

# 2: Intermolecular Forces (IMF) (aka: van der Waals forces) Inter means “between” or “among” Intermolecular forces:  attractive forces between neighboring molecules Intermolecular forces are weaker than Intramolecular forces

IMF: weaker than intramolecular (bonding) forces  ≈ 5% to 15% of strength of intramolecular forces account for phase of matter at room temp account for phase of matter at room temp  strong IMF  condensed phase (solid/liquid)  weak IMF  gas phase

IMF determine phase of matter phase is determined by: “competition” between strength of  IMF and  KE

 If IMF are strong, substance will be solid or liquid at room temp  strong attractive forces keep particles close together  If IMF are weak, substance will be gas at room temp  weak attractive forces allow particles to spread far apart & be free to move

It’s all a balancing act! IMF KE [this substance = a gas at room temperature]

Intermolecular Forces vs. Kinetic Energy IMF KE [this substance = a condensed phase (solid/liquid)]

since T is measure of average KE, changing T can change phase changing T changes average KE of particles  T change can allow change in phase

3 types of IMF: 1. Dispersion forces 2. Dipole-Dipole forces 3. Hydrogen bonds

1. Dispersion Forces 1. Dispersion Forces: ● weakest IMF ● occur between non-polar molecules

dispersion forces & non-polar molecules instantaneous and momentary change electron cloud will fluctuate results from motion of electrons due to attractive forces

Non-polar atoms/molecules non-no non-polar means no poles can’t tell one end of atom/molecule from other end electrons are evenly distributed charge is evenly distributed atom/molecule: symmetrical

Non-polar Atoms/Molecules: monatomic atoms:  He, Ne, Ar, Kr, Xe, Rn 0 diatomic elements:  H 2, N 2, O 2, Cl 2, F 2, I 2, Br 2 0 small symmetrical molecules:  CO 2, CX 4 0 carbon-hydrogen molecules:  CH 4, C 2 H 6, C 3 H 8

Dispersion Forces and Size larger the electron cloud, the greater the fluctuations in charge can be strength of attractive dispersion forces ↑ with increrasing molecule size increasing strength of dispersion forces:  Rn > Xe > Kr > Ar > Ne > He  I 2 > Br 2 > Cl 2 > F 2  C 8 H 18 > C 5 H 12 > C 3 H 8 > CH 4

2. Dipole-Dipole forces 2. Dipole-Dipole forces: intermediate IMF occur between polar molecules

What do you know about charge? Opposites Attract!  this time, uneven distribution of electrons (charge) is permanent! examples: non-polar polar H 2 HI CH 4 CH 3 Cl

Polar Molecules:  geometry NOT symmetrical (asymmetrical)  uneven electron distribution permanent separation of charge  has poles: one end partly (-) and one end partly (+)

neighboring molecules orient themselves according to their opposing attractive charges

3. Hydrogen Bonding forces 3. Hydrogen Bonding forces: strongest IMF subtype of dipole-dipole attractive forces attractive force occurs between H in one molecule and F, O or N in neighboring molecule H-F H-O or H-N

Hydrogen Bonding Force H-O N-H 0 this attractive force occurs between molecules with FON!!!

Hydrogen Bonding Force hydrogen bonding force is special subtype of dipole-dipole attractive forces F, O, and N are all small and electronegative  strong electron attraction  H has only 1 electron, so if being pulled away H proton is almost “naked” H end of molecule is always positive & F, O, or N end is always negative

Strength of Hydrogen Bonding Force fluorine most electronegative element, so  H-F bonds are most polar and exhibit strongest hydrogen bonding attractive forces  H-F > H-O > H-N

IMF vs. Physical Properties If strength of IMF  then:  boiling point   melting point   heat of fusion   heat of vaporization  while:  evaporation rate  If strength of IMF  then:  boiling point   melting point   heat of fusion   heat of vaporization  while:  evaporation rate 

boiling point of N 2 is 77 K (-196˚C) IMF are very, very weak forces (dispersion)

Hydrogen bonding: strongest IMF influences physical props a great deal

IMF vs. Temp IMF more important as temp is lowered  low temperature = low evaporation rate  high temperature = high evaporation rate

Indicate type of IMF for each molecule: NH 3 Ar N 2 HCl HF Ne O 2 HBr CH 3 NH 2 hydrogen bonding dispersion forces dipole-dipole forces hydrogen bonding dispersion dipole-dipole hydrogen bonding