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Intermolecular Forces of Attraction. Attractive forces that cause atoms or molecules to stick together. Mixtures. Elements or compounds blended together but not chemically combined. Mixtures. Can be solid, liquid, or gaseous mixtures Solid mixture examples:

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Intermolecular forces of attraction l.jpg

Intermolecular Forces of Attraction

Attractive forces that cause atoms or molecules to stick together


Mixtures l.jpg
Mixtures

  • Elements or compounds blended together but not chemically combined


Mixtures3 l.jpg
Mixtures

  • Can be solid, liquid, or gaseous mixtures

  • Solid mixture examples:

    • Jewelry gold, most rocks, steel, alloys

  • Liquid mixture examples

    • Beverages, sea water, solutions

  • Gaseous mixture examples

    • Air, car exhaust


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Mixtures

  • Why do they form? What allows the parts to stay mixed? Why do they stay together?

  • The components of a mixture are held together by IMF’s – intermolecular forces of attraction


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Intermolecular Forces

All intermolecular forces are:

  • attractive forces

  • between molecules

  • do not make new compounds

  • Makes the molecules “sticky”

  • weaker than true “bonds”


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Types of IMF’s

  • London (dispersion) forces

    • all molecules

    • weakest interaction

  • dipole-dipole forces

    • polar molecules

  • ion-dipole forces

    • Between polar molecules and ions

  • hydrogen bonding

    • H atoms w/ O, N, F not covalently bonded to it

    • strongest interaction


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Polar forces

  • “Dipole - Dipole” interactions

  • positive ends attract negative ends of other molecules

  • about 1% as strong as a covalent bond



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London forces

  • Momentary dipole-dipole interaction

  • “instantaneous” dipole “induces” a dipole in a neighboring molecule

  • brief attractive force results





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London forces

  • Strength depends on:

  • size of electron cloud

    • bigger = stronger

  • number of atoms in molecule

    • more atoms = more electrons = stronger


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Ion – dipole forces

  • Between polar molecules and ions

    • Aqueous solutions of ions

  • Positive ends of polar molecules attract negative ions

  • Negative ends attract positive ions

  • Only important in aqueous solutions


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Hydrogen Bonds

  • Special case of polar attraction

  • H atom is

    • 1)bonded to high EN atom and

    • 2)attracted to a high EN atom it is not bonded to (N, O, F)

  • 5 times stronger than regular polar attractions



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Physical property effects of Intermolecular Forces of Attraction

…all come down to how sticky the molecules are toward each other…


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The Key Idea…

  • The stronger the IMF’s, the tighter the molecules cling to each other, the harder it is to separate them.


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The Key Idea…

  • In other words, the harder it is to separate molecules, the stronger their IMF’s


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What does that mean…”separate the molecules”?

Pull them apart:

  • Melting

  • Boiling

  • Physically move them apart

    • Peel them away. Wipe them off, etc…





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Intermolecular Forces of Attraction

Attractive forces that cause atoms or molecules to stick together


Types of imf s26 l.jpg
Types of IMF’s

  • London (dispersion) forces

    • all molecules

    • weakest interaction

  • dipole-dipole forces

    • polar molecules

  • ion-dipole forces

    • Between polar molecules and ions

  • hydrogen bonding

    • H atoms w/ O, N, F not covalently bonded to it

    • strongest interaction


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The Key Idea…

  • The stronger the IMF’s, the tighter the molecules cling to each other, the harder it is to separate them.


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Solubility of a solid in liquid

General rule: Like dissolves Like

  • Polar solutes dissolve in polar solvents

    • Sugar (polar solute) in H2O(polar solvent)

  • Ionic solids dissolve in polar solvents

    • NaCl (ionic solid) in H2O (polar solvent)

  • Nonpolar solutes dissolve in nonpolar solvents

    • Dried paint (nonpolar solute) in paint stripper – turpentine, etc. (nonpolar solvent)


  • Colligative properties of solutions l.jpg
    Colligative properties of Solutions

    -Properties of solutions that are affected by the number of dissolved particles, not the identity of the particles

    Example: vapor pressure lowering

    The more particles (with little tendency to vaporize) that are added to a solvent, the lower the vapor pressure of the solution becomes


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    Colligative properties of Solutions

    Density of a solution

    The presence of dissolved particles(whose mass is greater than the mass of the solvent particles)in a solution causes the density of a solution to increase

    • Sugar water is more dense than pure water

    • Sugary sodas are more dense than diet sodas

      • it takes more sugar molecules than artificial sweetener molecules



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    Colligative properties of Solutions

    Boiling point elevation:

    The presence of dissolved particles tends to increase the boiling point of a liquid

    Salt added to water before boiling to prepare food increases the temperature the water boils at, and thus cooks the food faster, as the water is hotter


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    Colligative properties of Solutions

    Freezing point depression

    The presence of dissolved particles tends to lower the freezing point of a liquid

    Salt or other substances are used to melt ice, because as they dissolve into the ice, they lower the freezing point of the ice, and thus even more of the ice melts


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    Compressibility

    • gases are compressible

      • empty space between molecules

      • Weak IMF’s won’t keep molecules together

    • liquids, solids aren’t

      • closely packed


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    Diffusion

    • The intermingling of molecules with each other

    • gases = rapid  much empty space

    • liquids = slowly  more closely packed

    • solids = negligible  locked in place


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    Cohesion/Adhesion

    • Cohesion = molecules sticking together

    • Adhesion = molecules sticking to a substrate


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    Viscosity

    • The resistance of a fluid to flow.

    • The stronger the IMF’s, the higher the viscosity


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    Surface Tension

    • Intermolecular attractions at the surface are only inward towards the substance


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    Surface Tension, continued

    • Molecules at the surface of a liquid act as a “skin”

    • the greater the IMF’s, the more pronounced the effect

    • liquids seek to lower their surface tension

    • water drops are spherical


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    Surface Tension, continued


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    Surface Wetting

    • The spreading of a liquid across a surface

    • must have adhesion  cohesion


    Gasoline a nonpolar hydrocarbon l.jpg
    Gasoline – a nonpolar hydrocarbon

    • Low surface tension

      • only London forces

    • low cohesion

      • spreads easily across most surfaces

    • does not effectively “wet” glass


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    H2O – a polar molecule that H- bonds

    • High surface tension

      • hydrogen bonding

    • High cohesive forces

    • High adhesive forces

      • will H-bond with O’s in glass

    • Good wetting agent for glass


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    Slide45 l.jpg

    • Surfactants, used in soaps and detergents, drastically lower the surface tension of water

      • soapy water spreads more easily over dirty surfaces


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    Vaporization

    • The change of a liquid → gas is called vaporization

      • an example of separating molecules

    • Evaporation: happens at the exposed surface of a liquid

    • Boiling: happens within the liquid itself


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    Evaporation

    • Liquids exert vapor pressure due to the evaporated liquid

    • The stronger the IMF’s, the slower the evaporation rate, the lower the vapor pressure for a given temperature


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    Evaporation

    • The higher the T of a substance, the more molecules with enough energy to overcome cohesive forces/surface tension and evaporate

    • As liquid T , vapor P 


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    Temperature is ~ the average KE of a sample

    coolest

    warmest

    Abundance

    Energy

    Minimum E to evaporate


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    What happens...

    • To the evaporation rate

      • if the temperature = the boiling point ?

    • All the molecules, on average, have enough energy to vaporize

      • the liquid boils


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    What happens...

    • If the vapor pressure equals the atmospheric pressure ?

    • All the molecules, on average, have enough energy to vaporize

      • the liquid boils


    T vs vapor pressure l.jpg
    T vs Vapor Pressure

    Vap

    Pressure

    1.00 atm

    100oC

    Temperature


    How can you boil a liquid l.jpg
    How can you boil a liquid?

    • Heat it to its boiling point

      vapor P = air P

    • Reduce the air P above itto equal the vapor P of the liquid at that temperature

      • air P = vapor P

    • all done!


    Intermolecular forces of attraction54 l.jpg

    Intermolecular Forces of Attraction

    Forces of attraction between molecules


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