Download - Intermolecular Forces
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Intermolecular Forces
Mr. Nelson AP Chem
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Intermolecular ForcesForces that exist between
moleculesAlso called Van der Waals forces
Compare to intramolecular forces, which are the forces that hold atoms togetherHolds individual molecules together, but does not determine macro properties (aka boiling point)
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Intermolecular ForcesLargely determines physical
properties of solids and liquids Intermolecular forces are
weaker than intramolecular forces (chemical bonding)
Boiling points and melting points are an indication of the strength of intermolecular forces
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Ion-Dipole forcesInteraction between an ion and a
partial charge in a polar moleculeNot technically a van der Waals force
Positive ions are attracted to negative end of a dipoleForce depends on dipole moment of polar molecule and charge of the ion
Example: NaCl in Water
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Dipole-dipole forceAttractive forces between polar
moleculesWeaker than ion-dipole forces
Positive end of one molecule is near negative end of another
In liquids, molecules of equal size and mass have increasing intermolecular attractions with increasing polarity
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London Dispersion Forces
London proposed that the motion of electrons in an atom or molecule can create an instantaneous dipole momentExample: If both electrons on an He atom are on the same side of a nucleus, you have an instantaneous dipole moment
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London Dispersion Forces
A temporary dipole on one atom can induce a similar dipole on an adjacent atom (called an induced dipole)
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London Dispersion Forces
Because of induced dipoles, molecules attract one another
Dispersion forces exist in all types of molecules (polar or non-polar, charged or not)
In many cases, dispersion forces can be stronger than dipole-dipoleExample: CH3F (B.P. -78.9 °C) vs. CCl4 (B.P. 76.5 °C)
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London Dispersion Forces
Dispersion forces increase with molar mass of the moleculeMore electrons allows for greater chances of dispersion
When comparing molecules of similar weights and shapes, dipole-dipole forces tend to be the decisive factor
When comparing differing weights, dispersion forces is the decisive factor
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Intermolecular Forces Recap
ExampleOf Br2, Ne, HCl, HBr, and N2, which has:The largest intermolecular dispersion forces?
The largest dipole-dipole attractive forces?
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Hydrogen BondingA special type of dipole-
dipole attraction that exists between hydrogen atoms in a polar bond and small electronegative ions
Can only form between hydrogen and N, O, or F
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Hydrogen BondingWater has a
lower molar mass and a higher boiling point
Dispersion forces do not account for this
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Hydrogen Bonding Explains density of ice
Solid is less dense than liquid (not common)
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Review of IM forcesIon-Dipole Forces•Exist when ions and dipole present•Stronger than simply dipole-dipole
Dipole-Dipole Forces•Between polar molecules only•Larger dipole = larger force
Dispersion Forces•Between all molecules•Higher molar mass = larger force
Hydrogen Bonding•Strongest IM Force•Exists only between H with either N, O, or F
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Phase ChangesChanges in state of matter
In general, each state of matter can change into either of the two other states
Phase changes require energy
When becoming a more disordered state, requires energy to overcome intermolecular forces that hold them together
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Phase Changes
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Phase ChangesMelting, vaporization, and sublimation are all endothermic processes
Freezing, condensation, and deposition are all exothermic processes
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Phase Changes Fusion
Melting of a solidMolar heat of fusion or enthalpy of fusion (∆Hfus)
∆Hfusion = -∆Hsolidification
VaporizationVaporizing of a liquidMolar heat of vaporization or enthalpy of vaporization (∆Hvap)
∆Hvaporization = -∆Hcondensation
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Heating CurvesGraph of temperature of the
system versus heat added to the system
Some segments of the graph are heating as single phase others are converting one phase to another
Heat change when temp. inc. is given by q=mC∆T. During phase change, it is q=n∆H (n is moles of substance)
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Heating Curve of Water
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Phase Change Problem Freon-11 (CCl3F) has a normal boiling
point of 23.8 °C. The specific heat of CCl3F (l) is 0.87 J/g K and CCl3F (g) is 0.59 J/g K. The heat of vaporization is 24.75 kJ/mole. Calculate the heat required to convert 10.0 g of Freon-11 from liquid at -50.0 °C to gas at 50.0 °C.
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Vapor PressureIn a closed system, liquid will
initially evaporate and then condense back into its liquid form
The pressure exerted by this liquid/gas equilibrium is called vapor pressure
When evaporation occurs in an open system, no equilibrium can be established
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Vapor Pressure Substances with high vapor pressure
evaporate more easily than those with a low vapor pressure
Volatile liquids (aka liquids that evaporate easily) have high vapor pressures
Water can vaporize at room temp. because molecules in liquid move at different speeds and some can overcome IM forces (think space shuttles/escape velocity)
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Vapor PressureAs the number of gas
molecules increases in a closed system, the probability that gas molecules will strike the liquid increases.
This allows the molecules to become “trapped” by the IM forces of the liquid
http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/vaporv3.swf
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Temperature & Vapor Pressure
Vapor pressure increases nonlinearly with temperature
More molecules have enough KE to escape into gas phase
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Vapor Pressure & Boiling Point
Boiling point is when the vapor pressure equals the external pressure acting on the surfaceA “normal” boiling point is measured at 1 atm
A more volatile liquid has a lower boiling point and a higher vapor pressure (and vice versa)
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Phase Diagrams A graph that displays the conditions
under which an equilibrium exists between different states of matter
Allows prediction of a substance’s state of matter at a given temperature and pressure
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Phase DiagramsCritical Temperature (Tc) is the
highest temperature a substance can exist as a liquidMeaning above Tc molecular motion breaks IM attraction (Tc measures strength of IM forces)
Critical Pressure (Pc) is pressure required to bring about liquefaction at the critical temperature
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Phase DiagramsTriple Point is the place at
which all three phases exist at equilibrium