Chapter 11Intermolecular Forces (IMF)- the forces that exist between moleculesGases-IMF not strong enough to hold particles together-assumes shape and volume of container-expands to fill container-compressible-flow readily-diffusion within a gas occurs readily

Liquids-IMF strong enough to hold particles close together-assumes shape of container-does not expand to fill container-incompressible-flows readily-diffusion within a liquid occurs slowly-known as a condensed phase b/c particles are fairly close together

Solids-IMF very strong- virtually locks particles in place-has definite shape and volume-incompressible-does not flow-diffusion within a solid occurs slowly-known as a condensed phase

-states of matter can be changed by heating or cooling-changes the average kinetic energyEX--dec the temp of a gas will dec KE and form a liquid and then lock particles in place to form a solid-inc pressure of a gas can bring molecules closer together to form a liquid and then a solid

IMF-usually much weaker than intramolecular forces (ionic, metallic or covalent bonds)-less energy is required to change a state of matter than to break a bond-boiling points increase with stronger IMF-melting points increase with stronger IMF-page 428 Table 11.2

-all IMF are electrostatic- attractions between + and -

-weaker than bond b/c distances between molecules are often larger than the distances between atoms held together by bonds

Ion-Dipole Forces-exists between an ion and a polar molecule-cations are attracted to - end of a dipole -anions are attracted to + end-magnitude of attraction increases as either the ionic charge or the magnitude of the dipole increases-important for solutions of ionic substances in polar liquids-strongest IMF*only found in ionic compounds dissolved in solution

Ex: salt in water

Dispersion Forces (London Dispersion)-present in all molecules and atoms-weakest IMF-caused by motion of e- and instantaneous dipole moment -can cause an instantaneous dipole moment on an adjacent atom, causing the atoms to be attracted -page 429 Fig 11.4

-fluctuations in the electron distribution in atoms and molecules result in a temporary dipole– region with excess electron density has

partial (─) charge– region with depleted electron density has

partial (+) charge

-strength depends on ease with which the charge distribution can be distorted-called polarizability-the greater the polarizability the more easily an e- cloud can be distorted to give an instantaneous dipole-more polarizable molecules have larger dispersion forces-polarizability inc. as the # of e- in an atom or molecule inc.-strength inc. with inc. molecular weight

-shape also influences- longer straight chains are stronger b/c molecules can come into contact along entire molecule-more compact = less contact

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

-effective only when moleules are very close together

-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

-for molecules of about equal mass and size, the strength of IMF inc. with inc. polarityEx:CH2CN vs. CH2CH2CH3

Boiling pt.= 355K and 231KMass= 40amu and 43amu*close masses so dispersion forces are similar*higher bp in CH2CN due to dipole-dipole forces-page 431 Figure 11.8

Which of the following have dipole-dipole forces?1) CO2

-has polar bonds, but is nonpolar b/c it is linear no dipole-dipole forces

2) CH2Cℓ2

-has polar bonds and net dipole has dipole-dipole forces

3) CH4

-has nonpolar bonds no dipole-dipole forces

Hydrogen Bonding-occurs when polar molecules have H atoms bonded

directly to very electronegative atoms (F, O, and N)

-H atom is attracted to nonbonding e- pair on another similar molecule

-causes strong interaction between H and other atoms (O, F and N) in other molecules

-second strongest IMF-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

In which of these substances is H bonding likely to play an important role in determining physical properties?methane CH4 methyl fluoride CH3Fhydrazine H2NNH2 hydrogen sulfide H2S

*consider Lewis structures**hydrazine**

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

List the substances BaCℓ2, H2, CO, HF, and Ne in order of inc boiling point.* BaCℓ2 is ionic*all others have dispersion forces*MW= 2, 28, 19.9, 20.2*CO and HF have dipole-dipole b/c polar*HF has H-bondingH2 < Ne < CO < HF < BaCℓ2

Identify the IMF present in the following substances and select the substance with the highest boiling point.1) CH3CH3 2) CH3OH3) CH3CH2OH

2) dispersion, MW= 30amu2) dispersion, H-bonding, MW= 32amu3) dispersion, H-bonding, MW= 46amu*#3 has highest boiling point

Results of IMF1) viscosity- the resistance of a liquid to flow-the higher the viscosity, the more slowly it flows -increases with greater IMF (or molar mass) 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- higher KE can overcome the IMF

2) surface tension- inward force or pull that tends to minimize the surface area of a liquid

-energy required to inc the surface area of a liquid by a unit amount-molecules with increased IMF have increased

surface tension-water has an extremely high surface tension b/c of

the strong H-bondingex- 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)

3) capillary action- ability of liquid to flow against gravity up a narrow tube

cohesive forces- bind similar molecules to one another

adhesive forces- bind a substance to a surface -if adhesive are greater than cohesive then the

liquid will be drawn up -if cohesive are greater than adhesive than liquid

does not rise-shape of meniscus determined by cohesion and

adhesion

Phase Changesheat 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

heat of vaporization (∆Hvap)-amount of heat required to vaporize one mole of

a liquid- endothermic heat of condensation (∆Hcond)-amount of heat required to condense one mole

of a gas to a liquid-exothermic-same magnitude as ∆Hvap, but opposite sign b/c

energy is given off

heat of sublimation (∆Hsub)-goes from solid to gas without passing through

liquid phase∆Hsub= ∆Hfus + ∆Hvap

-endothermicex- frozen foodsheat of deposition (∆Hdep)-gas to solid skipping liquid phase-exothermic-same magnitude as ∆Hsub, but opposite sign b/c energy is given off

Heating Curves-graph of temp vs. amount of heat added

-can calculate enthalpy change of a system for each segment of the heating curve

-in AB, CD, and EF a single phase is heated from one phase to another use ∆H = mC∆T

-for BC use ∆Hfus

-for DE use ∆Hvap

*energy used up to increase distance between particles

-a gas normally liquefies at some point when pressure is applied-if temp inc, pressure must inc even more-if temp reaches a certain point no amount of pressure can cause a liquid to form-at that point, as pressure inc and the gas becomes more steadily compressedcritical temp- the highest temp at which a distinct liquid phase can formcritical pressure- pressure required to bring about liquefaction at this critical temp

-above the critical temp, the KE of the molecules is greater than the attractive forces that lead to the liquid state

-the greater the IMF, the higher the critical temp

supercritical fluid- occurs when critical temp and pressure are exceeded and the liquid and gas phases are indistinguishable from each other

vapor pressure- pressure above the surface of the liquid, causes evaporation

-will be lower with stronger intermolecular forces-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

-vapor pressure of a liquid is the pressure exerted by its vapor when liquid and vapor are in dynamic equilibrium

-equilibrium never occurs when vaporization happens in an open container

volatile- liquids that evaporate easily-have high vapor pressure

ex- nail polish removernonvolatile- liquids that do not vaporize easily

-have low vapor pressureex- motor oil-vapor pressure inc with inc temp- particles

move more and can escape to gas phase

-a liquid boils when vapor pressure equals the external pressure acting on the liquid surfacenormal boiling point- boiling point of a liquid at 1 atm*the higher the pressure the higher the boiling pt*food takes longer to cook at higher elevationsWHY?-pressure is lower, lowering boiling point of water below 100°C, taking longer to cook

phase diagram- summarizes conditions under which equilibria exists between the different states of matter

-can be used to predict which phase of a substance is present at any given temp and pressure

-page 445 Fig 11.27

1) red curve is vapor pressure curve of liquid-equilibrium between liquid and gas phase-@ 1 atm is the normal boiling point-ends at the critical point (C)- critical temp and pressure-supercritical fluid beyond critical point

2) green curve is sublimation curve-separates the solid from the gas phase

3) blue curve is melting curve-separates solid phase from liquid phase-usually slopes to the right as pressure inc b/c most solids are denser than their liquids-@ 1 atm is the normal melting point

triple point-point T, where the three curves intersect-all three phases exist in equilibrium