Chapter 11 intermolecular forces IMF attractive forces between

Chapter 11 intermolecular forces (IMF)- attractive forces between all molecules and atoms -these forces hold phases of matter together -strength of IMF determines the phase of a substance at a given temperature -IMF are weaker than bonding forces b/c of distance between molecules -it takes a much higher temp to break a bond than to overcome IMF

Properties of Phases of Matter Phase Density Shape Volume IMF Solid definite strong definite moderate high Liquid moderate indefinite Gas low indefinite weak

Changes Between Phases

A C B Which best depicts vapor emitted from a pot of boiling water? BOX A

Types of IMF 1) dispersion force (London force) -caused by motion of e-weakest of all IMF -the more e-, the stronger the dispersion force -ex- helium

-higher molar mass = greater dispersion forces = higher boiling points -molecular shape also plays a part in the strength of dispersion forces -page 467

2) dipole-dipole forces -exist in all polar molecules -polar molecules have permanent dipoles that interact with the permanent dipoles of neighboring molecules *+ end of polar molecule is attracted to – end of its neighbor -this attraction is the dipole-dipole force

-all molecules (polar and non-polar) have dispersion forces -only polar also have dipole-dipole forces -this extra force raises melting and boiling points compared to nonpolar molecules with similar molar masses

Which of the following have dipole-dipole forces? 1) CO 2 -has polar bonds, but is nonpolar b/c it is linear no dipole-dipole forces 2) CH 2 Cℓ 2 -has polar bonds and net dipole even though tetrahedral has dipole-dipole forces 3) CH 4 -has “slightly” polar bonds, but tetrahedral no dipole-dipole forces

-polarity of molecules helps determine miscibility (ability to mix) between liquids -polar is miscible with other polar molecules, nonpolar with nonpolar -“like dissolves like”

3) hydrogen bonding -occurs when polar molecules have H atoms bonded directly to very electronegative atoms (F, N, O) -causes strong interaction between H and other atoms in other molecules -strongest of the first three intermolecular forces -substances with H-bonding have higher melting and boiling points -water exhibits very strong H-bonding and this explains its behavior

-these two compounds have the same formula, but different structures which results in one having strong H-bonding and one not

Which one of the following compounds has a higher boiling point and why? -all have similar molar masses- similar dispersion forces -all are polar- all have dipole-dipole forces **hydrogen peroxide b/c it has H-bonding, harder to break apart

4) ion-dipole force -occurs when an ionic compound is mixed with a polar compound -strongest of all IMF -common in aqueous solutions

-ex- salt and water

Summary of Intermolecular Forces

Results of Intermolecular Forces 1) surface tension -inward force or pull that tends to minimize the surface area of a liquid -molecules with increased IMF have increased surface tension -water has an extremely high surface tension b/c of the strong H-bonding ex- forming spherical droplets, things being able to float on water -it is difficult to break the H-bonds -will decrease by adding a surfactant (soap)

2) viscosity -the resistance of a liquid to flow -increases with greater IMF b/c molecules cannot flow as easily -also depends on molecular shape- higher in longer molecules b/c they can become entangled -temp plays a part- the higher the temp, the lower the viscosity

3) capillary action -ability of liquid to flow against gravity up a narrow tube cohesive forces- attraction between molecules of the same substance adhesive forces- attraction between molecules of different substances -if adhesive are greater than cohesive then the liquid will be drawn up -if cohesive are greater than adhesive than liquid does not rise -page 476

Vaporization -going from liquid to gas -need adequate energy to break free from liquid -the higher the temp, the more the liquid will vaporize -some escaped gas molecules with low energy will plunge back into molecule -this is called: Condensation -gas to liquid -opposite of vaporization

volatile- liquids that vaporize easily ex- nail polish remover nonvolatile- liquids that do not vaporize easily ex- motor oil Rate of vaporization will increase with: -increasing temp -increasing surface area -decreasing IMF

heat of vaporization (∆Hvap) -amount of heat required to vaporize one mole of a liquid - ∆Hvap is always positive b/c energy is absorbed heat of condensation (∆Hcond) -amount of heat required to condense one mole of a gas to a liquid -same magnitude as ∆Hvap, but opposite sign b/c energy is given off

vapor pressure- pressure above the surface of the liquid, causes vaporization -will be lower with stronger IMF -if a container is sealed, the liquid will still vaporize and condense, just not into the atmosphere -when rate of condensation equals rate of vaporization the liquid has reached dynamic equilibrium

boiling point- temp at which vapor pressure equals external pressure normal boiling point- temp at which vapor pressure equals 1 atm -at lower pressure, water boils at lower temp -will take longer to cook foods at locations with higher altitudes *Ex- Mt. Whitney, CA water boils at 87°C -once boiling, the liquid will not rise above boiling point until all of the water has been converted to steam

critical temp- the temp at which a liquid cannot exist supercritical fluid- has properties of both liquids and gases critical pressure- pressure needed to bring the liquid back at the critical temp **can be used as a solvent to extract things- CO 2 extracts caffeine from coffee beans **can be easily removed by lowering the pressure below the critical pressure and it will evaporate away

heat of sublimation (∆Hsub) -goes from solid to gas without passing through liquid phase ex- frozen foods deposition- gas to solid skipping liquid phase melting point- temp at which solid turns to liquid -melting is also called fusion freezing point- temp at which liquid turns to solid

heat of fusion (∆Hfus) -amount of heat required to melt one mole of a solid - endothermic= + heat of solidification (∆Hsolid) -amount of heat required to solidify one mole of a liquid -exothermic= -same magnitude as ∆Hfus, but opposite sign b/c energy is given off -page 488

How much heat is required to melt 32. 30 g of acetone at its melting point? 32. 3 g C 3 H 6 O x 1 mole x 5. 69 k. J = 3. 17 k. J 1 58 g 1 mole -look at heat curve page 489 *there is no temp change during conversion

Phase Diagrams page 491 -493 -a map of the phase of a substance as a function of pressure (y-axis) and temp (x-axis) Features of a phase diagram regions- represents conditions where solid, liquid or gas is stable lines- each line or curve represents a set of temp and pressures at which the substance is at equilibrium between the two phases on either side of the line

triple point- unique set of conditions where all three phases exist in equilibrium critical point- where supercritical fluids exist