Condensed Phases and Intermolecular Forces
Condensed Phases and
Intermolecular Forces
Fundamentals
How do particle diagrams of liquids & solids compare to those of gases?
Describe the relative positions and motions of particles in each of the 3 phases.
The Question
Why do some substances exist as gases at room temperature, some as liquids, and some as solids?
2 kinds of forces
Part of the answer has to do with the forces between separate molecules.
There are 2 broad categories of forces you need to be aware of.
Intramolecular ForcesIntramolecular forces = attractive
forces that hold particles together in ionic, covalent, or metallic bonds.
Intra means “within.”
Intramolecular forces = bonding forces.
Intermolecular Forces
Inter means “between” or “among.”Intermolecular forces are forces
between molecules.Intermolecular forces are weaker than
intramolecular forces.
There are 3 types of intermolecular forces.
Intermolecular forces determine phase.Intermolecular forces determine phase.
“Competition” between strength of intermolecular forces and kinetic energy determines phase.
If intermolecular forces are strong, substance will be a solid or liquid at room temperature. Particles want to clump together.
If intermolecular forces are weak, substance will be a gas at room temperature. Particles spread apart.
It’s a balancing act!
Intermolecular Forces
Kinetic Energy
This substance = a gas at room temperature.
Intermolecular Forces vs. Kinetic Energy
Intermolecular Forces
Kinetic Energy
This substance = a condensed phase.
Why Temperature Changes Affect Phase
Since temperature is a measure of avg kinetic energy, changing temperature changes phase.
Changing temperature changes the average kinetic energy
Changing the temperature
Intermolecular Intermolecular ForcesForces
KineticEnergy
Intermolecular Forces
A few % the strength of the intramolecular or bonding forces
Account for phase at room temperature.Account for phase at room temperature.
Strong intermolecular forces condensed phase.
Weak intermolecular forces gas phase
Intermolecular Forces & Phase
Strong Intermolecular Forces = SolidsSolids have high melting points, high
boiling points.
Weak Intermolecular Forces = GasesGases have low melting points, low boiling
points.
3 Types of intermolecular forces
Dispersion ForcesDispersion Forces: These are the weakest. Occur between all molecules but are most important for nonpolar molecules.
Dipole-dipole forcesDipole-dipole forces: These are intermediate. Occur between polar molecules.
Hydrogen bondsHydrogen bonds: These are the strongest. Occur between molecules containing an H-F, H-O, or H-N bond.
Dispersion Forces
Instantaneous and momentary; fluctuating. Results from motion of electrons. Avg = fig. a.
If the charge cloud is not symmetrical it will induceinduce an asymmetry in its neighbor’s charge cloud!
Nonpolar molecules
NonNonpolar means nono poles. Can’t tell one end of the molecule from the other.
Symmetrical.
Examples of Nonpolar Molecules
Monatomic gas molecules: He, Ne, Ar, Kr, Xe, Rn
Diatomics where both atoms are the same element: H2, N2, O2, Cl2, F2, I2, Br2
Larger molecules if they are very symmetrical molecules: CH4, C2H6, C3H8
Dispersion Forces and Size
Dispersion forces increase with the size of the molecule.
The larger the electron cloud, the greater the fluctuations in charge can be.
Rn > Xe > Kr > Ar > Ne > HeI2 > Br2 > Cl2 > F2
C8H18 > C5H12 > C3H8 > CH4
Boiling point of N2 is 77 K. Intermolec-ular forces are very weak dispersion forces.
Dipole-dipole Forces & Polar Molecules
Molecule shows permanent separation of charge. It has poles – one end is somewhat negative & one end is somewhat positive.
Polar means the molecule has poles, +’ve & -’ve. The geometry and the charge distribution are not symmetrical.
Polar Molecules
Source: http://wps.prenhall.com/wps/media/objects/602/616516/Media_Assets/Chapter10/Text_Images/FG10_01-01UN.JPG
What do you know about charge?
Opposites Attract!
This time, the situation is permanent!
Examples: HI, CH3Cl
Hydrogen Bonding
Occurs between molecules containing an H-F, H-O, or H-N bond. (FON!!!)
Hydrogen Bonding
Hydrogen bonding is the extreme case of dipole-dipole bonding.
F, O, and N are all small and electronegativeelectronegative. They really attract electrons. H has only 1 electron, so if it’s spending time somewhere else, the proton is almost “naked.”
The H end is always positive and the F, O, or N is always negative.
The bonding electrons spend more time by the oxygen atom than by the hydrogen atoms.
Oxygen end – bit negative
Hydrogens – bit positive Source: http://ghs.gresham.k12.or.us/science/ps/sci/ibbio/chem/notes/chpt2/water.gif
Hydrogen Bonding in Water
Source: http://images.google.com/imgres?imgurl=http://food.oregonstate.edu/images/learni/w6.jpg&imgrefurl=http://food.oregonstate.edu/learn/water.html&h=231&w=216&sz=19&tbnid=niAcgd-QCTMJ:&tbnh=103&tbnw=96&hl=en&start=7&prev=/images?q=hydrogen+bonding+in%
Source: http://images.google.com/imgres?imgurl=http://food.oregonstate.edu/images/learni/w6.jpg&imgrefurl=http://food.oregonstate.edu/learn/water.html&h=231&w=216&sz=19&tbnid=niAcgd-QCTMJ:&tbnh=103&tbnw=96&hl=en&start=7&prev=/images?q=hydrogen+bonding+in%
Hydrogen bonding is the strongest intermolecular force and influences the physical properties of the substance a great deal.
Result of Hydrogen Bonding
Increased Boiling Point over predicted value based on molecular size & comparison with similar compounds.
Strength of Hydrogen Bonding
Fluorine is the most electronegative element, so H-F bonds are the most polar and exhibit the strongest hydrogen bonding.
H-F > H-O > H-N
Intermolecular Forces vs.
Physical Properties If intermolecular forces increase, Boiling point Melting point Heat of Fusion Heat of Vaporization Evaporation Rate Vapor Pressure
Intermolecular Forces vs. Temperature
Intermolecular forces become more important as the temperature is lowered.
EX: Low temperature – low evaporation rate High temperature – high evaporation rate
Indicate the kind of intermolecular forces for each molecule below.NH3ArN2HClHFNeO2HBrCH3NH2
• Hydrogen bonding• Dispersion forces• Dispersion forces• Dipole-dipole forces• Hydrogen bonding• Dispersion• Dispersion• Dipole-dipole• Hydrogen bonding
ForcesForces
ForcesForces
INTERMOLECULARINTERMOLECULAR INTRAMOLECULARINTRAMOLECULAR
DispersionDispersion
Dipole-DipoleDipole-Dipole
Hydrogen BondingHydrogen Bonding
CovalentCovalent
IonicIonic
MetallicMetallic