Chemistry SM-1232Week 3 Lesson 1

Dr. Jesse ReichAssistant Professor of Chemistry

Massachusetts Maritime AcademyFall 2008

Class Today• Chapter 12: Liquids Solids, and Intermolecular Forces• Evaporation and Condensation• Boiling• Energetics: Vaporization• Melting Freezing, Sublimation• Energetics: Meling Freezing• Heat of Fusion• Test Next Friday.• Finish reading Chapter 12. Short Quiz Wednesday, Test on

Friday.

Intermolecular Forces• Inter means in between 2 separate entities• Molecular means molecules• Forces means something affecting behavior

• The behavior caused by two molecules interacting with one another

Thermal EnergyThermal means heatEnergy in this case means motion

The motion caused by heat

Intermolecular Forces and Thermal Energy

• Strong intermolecular forces tend to make compounds bind together.

• Weak intermolecular forces tend to allow molecules to disperse

• Lots of intermolecuar forces makes compounds a liquid or gas at room temperature.

• Minimal intermolecular forces makes compounds a gas at room temperature.

Hydrocarbons

• Methane CH4 a gas mw=16

• Octane C8H18 a liquid

• Hexatriacontane C36H74 a wax

• Big things tend to be solids, small things tend to be gases

Properties: Reminder• Liquids: high density• Take the shape of their container• Definite volume and not easily compressed

Properties: Reminder• Solids:• High density• Definite shape• Definite volume• Crystalline or amorphous

Surface Tension• Surface tension is about the tendency of

liquids to minimize their surface areas.• Water has strong intermolecular forces

holding molecules together• In a test tube of water you’ll see the surface

isn’t flat, but curved because molecules are pulling closer together.

• This creates a surface tension that can hold objects that are more dense.

Viscosity• Viscosity is how thick a fuel is.• Lighter fluid is a very thin and not viscous at

all.• Molasses is a very viscous fluid• The stronger the intermolecular forces the

thicker the fluid or you’ll notice an increase in viscosity.

Evaporation and Condensation

• Evaporation is when liquids change phase into gases. This is often also called vaporization.

• Condensation is when gases change phase in liquids

Evaporation• The speed with which a liquid evaporates is

directly related to:• Surface area• Temperature• The speed with which a liquid evaporates is

inversely related to:• Strength of intermolecular forces

Volatile• Liquids that evaporate quickly or easily are

termed volatile (rubbing alcohol). Those that don’t evaporate easily are termed nonvolatile (grease).

Evaporation and Condensation

Equlibrium• Water is constantly evaporating, and

condensing all during the day. There are points during the day where the rate at which its condensing is the same as the rate at which its evaporating.

• We call this dynamic equilibrium.

Vapor Pressure• Vapor Pressure is the partial pressure of a gas

when the liquid is in dynamic equilibrium with its gas.

• Vapor Pressure is directly proportion to temperature.

• Vapor Pressure is inversely proportional to the strength of intermolecular forces.

Boiling• The boiling is when liquids start evaporating

quickly. This happens when the vapor pressure of a liquid is equal to the pressure above it (approximately 1 atm for your teapot).

• What happens when you use a water heater set to over 100C? What’s the temperature of the water if there is no pressure buildup?

Energetics of Evaporation and Condensation

• Evaporation is endothermic• Endo means in• Therm means heat• Endothermic means in heat, which means you

have to put heat into it to make it evaporate.

Energetics of evaporation and condensation

• Condensation is exothermic• Exo means out• Thermic means heat• Exothermic means heat out• When steam condenses back into a liquid heat

is released.

Heat of Vaporization• The amount of heat required to vaporize one

mole of a liquid is called the heat of vaporization

• DHvap

• H2O(l) –> H2O (g) = 40.7 kJ (per mole at 100C)• H is a positive value because it’s endothermic,

so that’s the amount of energy you have to ADD in

Heat of Condensation

• H2O(g) H2O (l) • Heat of condensation = -40.7kj ( per mole at

100C)• Since the value is negative that’s how much

energy was released by the process.

How do heats of vaporization work?

• Different liquids have different heats of vaporization.

• Heat of vaporization is also temperature depdenant. The higher the temp, the easier it is to vaporize a liquid into a gas.

How do we use heats of vaporization?

• THIS IS A CONVERSION FACTOR• If you know the number of moles and the heat

of vaporization you know how much energy is required.

• If you know the energy required you can figure out how many moles were present.

Example• You have 30g of water that you want to

vaporize. How much energy will be required?

• 67.8kj

Melting, Freezing and Sublimation

• At the melting point molecules don’t have enough energy to overcome intermolecular forces.

• They start clumping together as they are pulled together. This forms solids from liquids (freezing), liquids from gases (melting), or solids from gases (sublimation).

Energetics of melting and freezing

• To cool down your soda you drop ice in it. The process of melting is endothermic, it takes heat from the soda to melt the ice. That makes the temperature of the soda drop!

Freezing• Freezing is the opposite of melting. It’s exothermic,

which means heat is released as it’s turning from a liquid to a solid. As your water is turning into ice cubes heat is being generated.

• Think about a lake. Even though the temperature is below freezing the whole lake isn’t frozen immediately, that’s because as it’s freezing heat is being released so it takes more time to freeze over.

Heat of Fusion• The amount of heat required to melt 1 mol of

a solid is called the heat of fusion.• DHfus

• The heat of fusion of water is 6.02 kJ/mol.

• H2O(s) H2O(l)= 6.02 kJ• It’s a positive value, so you need to add in that

much energy to make 1 mol of ice melt.

Freezing• The amount of heat released when you freeze

water is -6.02 kJ/mol.• H2O(l)H2O(s)

How do we use heats of fusion?

• It’s a conversion factor. It relates moles to an amount of energy. So if want to know how much energy you have to put in to melt a volume of ice you can figure it out. If you know how much energy you used melting ice then you can tell how many moles there were.

Heat of fusion: an example

• You have 30g of ice that you want to melt. How much energy will it take knowing that the heat of fusion is 6.02 kJ/mol. Does the answer mean heat is released or absorbed? How can you tell by the number?

• 10 Kj

Sublimation• Sublimation is a physical change where a

molecule in the solid phase “skips” the liquid phase and goes straight to a gas.

Dry Ice

To Do• Test Next Friday.• Finish reading Chapter 12. Short Quiz

Wednesday, Test on Friday.