Intermolecular Forces

Intermolecular Forces• Van der Waals Forces—the two weakest

attractions between molecules.– Dipole interaction—polar molecules are

attracted to one another (kind of like magnets)

Intermolecular Forces• Van der Waals Forces—the two weakest attractions

between molecules.– Dispersion forces—when an atom’s electrons are

concentrated in one place, it causes its neighbor’s electrons to move

Intermolecular Forces• Hydrogen Bonding—when hydrogen is bonded

to an electronegative atom(O,N,F), it creates polarity. This polarity creates an attraction between molecules. Much stronger than the other two, but NOT a true bond.

Kinetic Molecular Theory (KMT)• Solid, liquid or gas, the particles that

make up matter are in constant motion.

• Particles in a liquid or a solid are attracted to each other.

Liquids• The disruptive motion of particles

of a liquid versus the attractive forces between them causes the liquid to flow and have volume.

All liquids possess the following:

• Vapor Pressure• Boiling Point• Viscosity• Surface Tension• The amount of each that a liquid

possesses is due to the attractive forces between the particles of that liquid.

Vapor Pressure• Vaporization-conversion of a liquid to a gas.• Evaporation is the conversion to a gas at the

surface of a liquid (NOT boiling).DEMO

• Vapor pressure is the pressure caused by the evaporated particles above the surface of a liquid.

Atmospheric Pressure

• The air is ~7 miles high.

• That air piled upon you exerts a pressure.

• That pressure is 14.7 lb/in2

Atmospheric Pressure

• This air pressure has a HUGE effect on what things on this planet look like and how they work.

Would These Things Work the Same At Zero Atmospheric

Pressure?suction cup parachute drinking straw

candle pogo stick broom

match swing alarm clock

flashlight automobile shotgun

vacuum cleaner air bag radio

paper airplane balloon bow and arrow

helicopter paint star

rocket blow dryer bicycle pump

aerosol spray can Frisbee flag

baseball and bat TNT golf

plant siphon refrigerator

syringe smoke detector magnet

Boiling Point• When a liquid is heated enough (at a particular

atmospheric pressure), particles everywhere in the liquid start to vaporize. This is the boiling point of that liquid.

• At this point, vapor pressure = atmospheric pressure. DEMO

• Every pure liquid has its own bp.

Viscosity• Resistance to flow.DEMO

• Again, every pure liquid has its own.

Surface Tension• Results from an imbalance of

attraction between liquid particles at the surface of the liquid.

• Liquid acts as if it has a skin.

Surface Tension

• Allows things like this:

Water• As a result of water’s polarity, it

exhibits hydrogen bonding.

• Hydrogen bonding is the cause of almost all of water’s unique and important properties.

Hydrogen Bonding in Water• Gives water a high surface tension.DEMO

• Give it a low vapor pressure/high boiling point.• Make it denser as a liquid than as a solid.DEMO

• #1. Using the rule of LIKE DISSOLVES LIKE, which of the following left outside would get dissolved by rain and end up in your well water.

• Candle waxSugarSalt (sodium chloride)Drywall (CaSO4) (hint Ca=+2 charge, SO4=-2 charge)Lead ChlorideSodium FluoridePlastic bottlesGlass bottlesAlcohol

#2: Soap is made from a chain of carbon and hydrogen atoms (zig-zag line) connected to a

charged group of atoms like SO4. The chains of carbons have no charge so they are ignored by

water as they migrate through the water

The oil in the oil droplet has no charge and blends nicely with the long carbon chain end of

soap, which also has no charge. The water molecules, however, are attracted to the charged

end of the soap molecule.

Eventually, the soap molecules will align themselves so the non-charged end will stay dissolved in the oil drop, but the charged SO4 end well be held on to by water.

Instead of the oil drop getting squeezed out of the way of water, it gets attracted by water because it has these

charged SO4 groups sticking out of it. So in essence, the soap allows water to dissolve oil. Question: Is the plus or

negative side of water attracted to the SO4 groups?

3. Since water has a partially charged plus and minus ends, it aligns itself like a crosslinked net (or skin) at the surface of water. They call this

surface tension. Mosquito larva take advantage of the strength of surface tension by suspending

themselves from the water's surface.

This represents what water molecules would do at the surface. They line up + to -, which gives it

strength. However, if soap is added to the water...

...the alignment of water is disrupted. This soap molecule has a chain of carbons and hydrogens that water ignores, and a

negatively charged SO4 group that attracts the positive end of water molecules. This disrupts the previous chain of water

molecules (the net) but the water still locks onto the negative SO4 group. The long chain of carbons and hydrogens have no charge

so water is not attracted to these chains. This weakens the

surface tension. Why?

Soap added to water causes the mosquito larvae to sink thereby dooming the adult mosquito from

ever making it to the air. As this remarkable picture shows, with water surface tension intact,

the adult mosquito can escape into the air.

Solids• If the particles of all liquids are

attracted to each other, then the particles of solids are even more so.

• Particles of most solids are closely & orderly packed and have vibration as their only form of motion.

Amorphous Solids

• Some solids, when they form, do not have their particles in an orderly arrangement. These are called amorphous solids.

• Examples: rubber, glass

Motion of Particles

• The particles of a substance have three types of motion, depending on the phase they are in:

–Translational

–Rotational

–Vibrational

Phase Diagrams• A phase diagram lets you work out exactly what

phases are present at any given temperature and pressure.

• Here is the phase diagram for water:

Here’s one for CO2:

Heating Curves• The graph of temperature against time is called a heating

curve. Here’s one for water:

Notice that, in general, the temperature goes up the longer the heating continues. However, there are two horizontal flat parts to the graph. These happen when there is a change of state.

• Here is a heating curve for iron:

• At what temp. does iron melt?• At what temp. does iron boil?• These heating curves are accurate,

assuming what state of conditions?