ap chemistry notes chapter 10- liquids, solids, and intermolecular forces

8/7/2019 AP Chemistry Notes Chapter 10- Liquids, solids, and intermolecular forces

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AP Chemistry Notes Chapter 10

Liquids, Solids and Intermolecular Forces

Any substance can exist as a solid, liquid or gas under the proper conditions.

The phase in which matter exists at standard conditions depends on intermolecular

forces of attraction or the attractive forces between individual particles of the substance.

Dont be fooled into thinking that boiling points are high or freezing points are lowbased on our familiarity with water, which is really one of a few substances that undergo

both those phase changes under normal conditions. Elemental nitrogen boils

vigorously at 196C, and elemental iron freezes at nearly 1500C.

It is assumed that you already know how solids, liquids and gases compare with regard to

definite shape and volume. Together, liquids and gases are known as fluids.

There are several important intermolecular forces. Some are stronger, others weaker, and

with a little knowledge and common sense, one can usually determine what forces are atwork between particles by making some simple observations about what phase a

substance is in at a particular set of conditions and what its freezing and boiling

temperatures are.

Categories of intermolecular forces:

1. Ion-ionattractions: Electrostatic attractions are very strong and virtually all ioniccompounds are solids at STP. Such compounds are collectively called salts.

2. Ion-dipole: Some molecules (of which water is the classic and most importantexample) contain a permanent dipole, as discussed in Chapter 9. Such moleculesare strongly attracted to ionic charges, which is why water dissolves so many

ionic compounds well.

3. Dipole-dipole: For molecules with a permanent dipole, the center of positivecharge of one molecule is attracted to the center of negative charge of another.

Such attractions are not particularly strong unless the molecules are fairly closetogether.

4. Hydrogen bonding: In molecules where H is bonded to O, N or F, there is a highdegree of polarization, resulting in particularly highly charged positive and

negative poles. In such molecules, the attraction between molecules is stronger

than simple dipole-dipole attractions. Hydrogen bonding explains many unusualphenomena that will be discussed shortly.

5. London dispersion forces: Sometimes called van der Waals forces (althoughtechnically, van der Waals forces also include dipole-dipole and hydrogen

bonding), these are the weakest of the intermolecular forces and exist between all

molecules, even non-polar ones. They are the result of induced dipolar charges.

Draw a diagram below to illustrate this.


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kinetic energy to overcome the intermolecular forces and escape to the vapor phase. Therelationship between vapor pressure and temperature can be represented graphically and

mathematically by this simple formula:

Ln(Pvap) = -Hvap/R (1/T) + C wherey = ln(Pvap),x = 1/T (in Kelvins), m = slope = -

Hvap/R. and b = y intercept = C. C is a constant particular to the liquid in question.

Probably a more useful version of this equation involves 2 pressures and 2 temperatures.

It allows a scientist to calculate the new vapor pressure at a new temperature, or thetemperature needed to achieve a certain vapor pressure (such as calculating a boiling

point at a different pressure or elevation.) The equation looks like this:

Ln PvapT1 = Hvap 1 - 1PvapT2 T2 T1

Phase Changes

You should know the names for all possible phase changes between solid, liquid and gas

phases.

Heat of Fusion: Hfus is the heat required to melt one mole of a solid. Melting is alwaysendothermic, leaving Hfus positive. Conversely, freezing is exothermic and Hfus isnegative.

Heat of Vaporization:Hvap is the heat required to vaporize one mole of liquid to gas.Vaporization is endothermic, and condensing is exothermic. This is one of the reasons

that steam burns can be so severe.

Specific Heat Capacity: c is the heat required to change the temperature of 1 gram of asubstance by 1C. It is different for different substances, and is even different fordifferent phases of the same substance. For example, c for water is 1.0 cal/gC but for

steam and ice, c is only about 0.5cal/gC.

To calculate heat energy change within a given phase for a substance H = mcT.

A Heating Curve (or Cooling Curve) has this basic profile. Fill in the missing

information.


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Triple Point Diagrams tell us a lot about a substances phase change conditions.

Triple point diagrams look like this. Fill in the missing information.

Two important features of the diagram are the triple point and the critical point.

Critical Temperature is the highest temperature at which a substance can exist as aliquid. Beyond that temperature, the gas can no longer be compressed to liquid phase

regardless of pressure.

Critical Pressure is the pressure required to liquefy the gas at the critical temperature.

Compare triple point diagrams for water and carbon dioxide.


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Vapor Pressure: All substances possess a vapor pressure, that is, pressure caused by

evaporating (or subliming) molecules. Vapor pressure depends on intermolecularattractions (viscosity) and temperature. Non-polar liquids like ether or gasoline have

higher vapor pressures than polar liquids like water. The vapor pressure of solids is

lower still.

Tungsten metal is said to have a vapor pressure of

Vapor pressure is directly proportional to temperature.

When a liquid is heated until its vapor pressure equals the pressure of theatmosphere above the liquid, the boiling point is reached.

The molecules have so much kinetic energy and are moving so fast that they cannot

remain in the liquid phase, even if they are not at the liquid surface. The result is the

formation of vapor bubbles in the liquid that rise to the surface and burst.

A liquids normal boiling point is the boiling temperature at 1 atmosphere ofpressure.

Change the pressure, change the boiling point.Solids

When molecules slow down enough so that the intermolecular forces of attraction arestrong enough to keep the molecules from moving past each other, the substance freezes

to solid form.

Non-polar substances (like the components of air) have very low freezing (and boiling)

points.

Polar and ionic substances have higher freezing/melting points.


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If allowed to cool slowly enough, most substances will freeze into some sort of crystals.Crystals are regular geometric arrangements of particles that have flat cleavable surfaces

and straight edges. There are many different crystal shapes.

If substances are cooled too rapidly for crystals to form, the material becomes an

amorphous solid. Amorphous literally means without form, but here is simply meansnon-crystalline.

Glass is an example of an amorphous solid.

Some solids are classified as metallic or covalent network solids. Here, there are no

simple intermolecular forces holding particles together.

1. Metallic bonding: This type of bonding occurs in metals and metal alloys(solutions). Think of this as a group of positive nuclei all sharing a collective

group of electrons. Scientists call this the sea of electrons or electron gas

bonding model. Since electrons do not adhere to a particular nucleus, but are freeto move around from one nucleus to another, most metals have the properties of

electrical conductivity and malleability. Metallic bonding is typically quite

strong, making metals hard solids at room temperature.

A second theory of metallic bonding is the MO (molecular orbital) theory,

which suggests that the s or s and p orbitals of neighboring metallic atoms form

overlapping molecular orbitals, allowing the collective sharing of outer electronsby groups of metallic atoms. It may be thought of as being similar to the sharing

of delocalized electrons in molecules through the formation of resonance orbitals.

2.

Covalent network solids: Within molecules there are covalent bonds holding theatoms of the molecule together. Covalent bonds are extremely strong; much

stronger than intermolecular forces. In covalent network solids, atoms are bondedcovalently in a 3-dimensional array, making for a very strong crystalline solid.

The hardest substances known to man are covalent network solids. They includediamond, silica (silicon dioxide or quartz) and silicon carbide. A diamond may be

thought of as one huge molecule of carbon atoms.

ap chemistry notes chapter 10- liquids, solids, and intermolecular forces

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