Intermolecular Forces

Properties of Solids and Liquids

• Liquid → solid, freezing
• solid → liquid melting

Collections of molecules are generally held together via intermolecular forces. These are forces of attraction between molecules due to the wya the electron cloud is oriented around the molecules.

Generally speaking, keeping temperature and pressure constant, solids → liquid → gas in order of strength of molecular force

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As you increase the pressure on the material sample, what will happen to the state of matter?

gas → liquid → solid (usually)

As you increase the temperature on the material sample, what will happen to the state of matter?

solid → liquid → gas

Triple point - The point where all three phases exist in equal probability for that given temperature and pressure

Critical Point - The point above which the liquid and gas phases are basically identical

• Super critical fluid

As pressure goes up it favors whatever is most dense, particles get squeezed closer togetehr because it gets the particles as close together as possible

Some materials are less dense as solids thna as liquids and therefore increasing pressure will actually favor the liquid phase rather than the solid phase because the molecules are closer together in the liquid phase rather than the solid phase. Water is the most famous example of this

Types of intermolecular forces

Recall from our chemical bonding unit that some molecules are polar and some are non polar

London Dispersion Forces

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Temporary dipole causes polaritzation on the molecule next to it causing a chain reaction of temporary dipoles + attractive force

All molecules will exhibit dispersion forces because they exist due to the random distribution of electrons around an atom or molecule. An orbital is the location where it is probable to find an electron so on average the charge of an eelctron is evenly distributed throughout the electron orbital but at any given time the electron will be found in a particular location and this makes that arrea more negative and the other region more positive

Orbital is just a probability but the position of the electron will be random inside that orbital at any given time

The uneven charge distrivution in the electron cloud due to randomness is called a temporary dipole. How easily these temporary dipoles form and influence the molecules around themselves is called the polarizability.

Generally speaking, dispersion forces typically contribute the most to the attraction between molecules even if the molecule has other forces of attraction

Dispersion also depends on the size and shape of the molecule. Bigger molecules have a larger electron cloud and are easier to polarize. We can see this ebcause the molecular mass of a molecule tends to be correlated to the boiling point of that molecule.

The longer shape has a larger dispersion force because the electron cloud is larger, despite having equal molar masses

Dispersion also depends on the size and shape of the molecule. Bigger molecules have a larger electron cloud and are easier to polarize.

Molar mass is only factor for strength of dispersion, should be size of electron cloud

Longer shaoe has larger dispersion because the electron cloud is larger, despite having equal molar masses

Dipole-Dipole forces

Temperoray dipoles have intermolecular attraction

Opposites attract

+→ +→

Creates a permanent electrostatic attraction between the partially negative parts of a molecule to the partially positive parts of a nearby molecule

In general dipole dipole forces are weaker than dispersion forces but are additive with dispersion force so molecules that have dipole dipole interactions have stronger intermolecular forces in total

Hydrogen Bonding

A partially covalent bond created out of really large polarizaiton where hydrogen is bonded to either a nitrogen, oxygen, or fluorine

The energy needed to separate molecules attracted to each other through hydrogen bonds is pretty large, around 4-25kj/mol. Still lass than the bonds themselves, but very measurable

If you do hydrogen bonding you also do dipole dipole bonding

Only one side of a hydrogen bond requires a H-X bond, the other side just needs to be a highly electronegative element

No, hydrogen bonding is not dipole-dipole attraction, it is something unique. Any molecule that underoges hydrogen bonding will also have a separate dipole-dipole interaction

Dipole-ion is between a polar molecule and an ion, can be very very strong due to the large charge of an ion

Phase Changes

Shows where there is a gap, when it changes state it stops heating up for a moment to change state then changes and keeps heating up

q=mcdT is heat of vaporization, studied this already

• slope is 1/mc freezing - melting is q=mdeltaHfus

condensation q = mdeltaHvap

Since intermolecular forces are all attractive there is a change in potential energy when a phase change happens. To go from solid to liquid or liquid to gas results in an increase in potential energy and the reverse direction is a reduction in potential energy

Find the energy needed to change 250g of ice at -10C to steam at 120 C

Use heats of vaporization and fusion into formulas

qtot = qice + qmelt + qliquid + qvap + qsteam

add up all of the heats it gives you total heat needed for it to go across

Cooling curve

Backwards heating curve, takes energy away

• One exception, liquids can be supercooled then frozen at a higher temperature
• Needs to supercool before it freezes, kinetic energy below freezing point to start freezing

To udnergo the phase change from liquid to solid you have to cool down to a temperature lower than the freezing point

An ice cube of mass 82.3g at -3.2 degrees celcius is added to a glass of water of mass 273.4g and a temperature of 21.2 degrees celcius. Find the final temperature.

• Guess and check all possible equations
• Answer has to be between -3.2 and 21.2
• Not all of the liquid is going to freeze, but all of the ice could melt

1. All the ice melted

qice = -qliquid

mCdT + mdH = -q…

add it all up and Tf = -2.5 degrees celcius, outside of the range it has to be in

2. Some ice melts, assume Tf = 0 degrees Celcius

82.3(2.108)(3.2) +M melt(334) = -273.4(4.184)(0-21.2)

555 + M melt (334) = 24300

Mmelt = 71.1g this works! everything is 0

the best thing to do is start with number 2, if you get an answer where more ice melts than possible you know your answer is above 0

Heat of Sublimation

dH sub = dH fus + dH vap

Vaporizaiton

q=mHvap, KE=1/2mv

Vapor pressure t=25c liquid turns into gas vapour also turns into liquid

Methanol vs water which has stronger forces of attraction

Dispersion > dispersion dipole dipole < dipole dipole H bond < h bond

Water has a higher boiling point so it has a higher strength of attraction

In any sample of a liquid there will be some particles with a high enough speed to escape the liquid and change phases

As temperature increases but remains below the boiling point, the vapor pressure should increase because

A liquid boils when the vapor pressure reaches the atmospheric presure because the molecules forming bubbles in the liquid will have a pressure equal to the external pressure and will not be compressed back into a liquid

T=BP then VP = 1atm

Collecting gas over water

reaction pipes under water and the gas bubbles into a test tube through the water P gas = PH2+PH2O

Distillation

A process of separating a mixture of liquids via boiling them. Whichever liquid has a lower boiling point will have a higher concentration in the gas mixture.

Liquid Chromatography

Mixture with dyes and ink, water moves up through ink spot, colors get pulled out of the ink.

Retention Factor = Distance chemical travelled / Distance solvent travelled

How far up the stationary phase each part of the mixture moves depends on how attracted to the moving phase it is. Something that is more attracted to the moving phase will move up the stationary phase for a larger distance

Rf is usually calculated as a measure of this (retention factor)

Structures of Ionic and Covalent Solids

When structures form, the atoms and ,olecules typically organize themselves into specific crystalline structures. There are some exceptions, like rubber, which form amorphous solids

• Molecular solid, held together by intermolecular forces not bonds
• Not a crystalline lattice

The crystalline structures are made up of repeating units called unit cells. Three types of unit cells are common

Four types of solids in order from weakest to strongest bonds

1. Molecular solid
   • Held together by intermolecular forces, like dispersion, ex dry ice
2. Metallic
   • Held together by metallic bonding, ex gold bar
3. Network Covalent
   • ex Diamond, graphite, covalent bonds to each molecule, crystalline
4. Ionic
   • The solid is held together by ionic bonds, ex KCl

Electrical charge is determined by free moving electrons or protons. Electricity is created by putting a charge across something

• Charges move across the gradient from high to low potential