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Chapter 11

Chapter 11. Intermolecular Forces (IMF) - the forces that exist between molecules Gases -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

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Chapter 11

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  1. Chapter 11 Intermolecular Forces (IMF)- the forces that exist between molecules Gases -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

  2. 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

  3. 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

  4. -states of matter can be changed by heating or cooling -changes the average kinetic energy EX- -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

  5. 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

  6. -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

  7. 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

  8. Ex: salt in water

  9. 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

  10. -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

  11. -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 -polarizabilityinc. as the # of e- in an atom or molecule inc. -strength inc. with inc. molecular weight

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

  13. 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

  14. -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

  15. -for molecules of about equal mass and size, the strength of IMF inc. with inc. polarity Ex: CH2CN vs. CH2CH2CH3 Boiling pt.= 355K and 231K Mass= 40amu and 43amu *close masses so dispersion forces are similar *higher bpin CH2CN due to dipole-dipole forces -page 431 Figure 11.8

  16. Which of the following have dipole-dipole forces? • 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

  17. 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 Hbonding have higher melting and boiling points -water exhibits very strong H-bonding and this explains its behavior

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

  19. In which of these substances is H bonding likely to play an important role in determining physical properties? methane CH4 methyl fluoride CH3F hydrazine H2NNH2 hydrogen sulfide H2S *consider Lewis structures **hydrazine**

  20. 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

  21. 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-bonding H2 < Ne < CO < HF < BaCℓ2

  22. Identify the IMF present in the following substances and select the substance with the highest boiling point. • CH3CH3 2) CH3OH 3) CH3CH2OH • dispersion, MW= 30amu • dispersion, H-bonding, MW= 32amu • dispersion, H-bonding, MW= 46amu *#3 has highest boiling point

  23. Results of IMF 1) 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

  24. 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-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)

  25. 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

  26. Phase Changes 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

  27. 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

  28. heat of sublimation(∆Hsub) -goes from solid to gas without passing through liquid phase ∆Hsub= ∆Hfus+ ∆Hvap -endothermic ex- frozen foods heat of deposition (∆Hdep) -gas to solid skipping liquid phase -exothermic -same magnitude as ∆Hsub, but opposite sign b/c energy is given off

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

  30. -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

  31. -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 compressed critical temp- the highest temp at which a distinct liquid phase can form critical pressure- pressure required to bring about liquefaction at this critical temp

  32. -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

  33. 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

  34. -equilibrium never occurs when vaporization happens in an open container volatile- liquids that evaporate easily -have high vapor pressure ex- nail polish remover nonvolatile- liquids that do not vaporize easily -have low vapor pressure ex- motor oil -vapor pressure inc with inc temp- particles move more and can escape to gas phase

  35. -a liquid boils when vapor pressure equals the external pressure acting on the liquid surface normal boiling point- boiling point of a liquid at 1 atm *the higher the temp the higher the boiling point *food takes longer to cook at higher elevations WHY? -pressure is lower, lowering boiling point of water below 100°C, taking longer to cook

  36. 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

  37. 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

  38. 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

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