Intermolecular Attractions

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Intermolecular Attractions. Bonding and VSEPR Theory Structures of Solids and liquids. Electron Dot (Lewis) Diagrams Explain Chemical Bonding. Chemical bonds occur when electrons are transferred or shared by elements so that they each become more stable. - PowerPoint PPT Presentation

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Intermolecular Attractions

Bonding and VSEPR Theory

Structures of Solids and liquidsIntermolecular AttractionsChemical bonds occur when electrons are transferred or shared by elements so that they each become more stable

Electron Dot (Lewis) Diagrams Explain Chemical BondingHow many electrons do most atoms want in their outer energy level to be stable?

0126818Bonds that form between two nonmetals are which type?

IonicCovalentMetallicDrawing Electron Dot DiagramsElectrons usually stay in pairs when bonded.

Bonding pairs pair of electrons that form the bond - can be represented as a line segmentLone (or unbonded) pairs pairs of electrons that are not involved in bonds and are shown as dots

How many bonding pairs are in the following compound?

12346918 How many lone pairs are in the following compound?

12346918

How many bonding pairs and lone pairs are in the following compound?

6 bonding pairs, 18 lone pairs12 bonding pairs, 18 lone pairs12 bonding pairs, 36 lone pairs6bonding pairs, 6 lone pairsDrawing Electron Dot Formulas for CompoundsExceptions: Hydrogen only needs 2 electrons (1 bond)

Boron tends to need only 6 electrons (3 bonds)

Single atoms go in the center

If more than one single atom, middle atom central atom

Draw the electron dot formula. Then state how many bonding and unbonding pairs are present.A) NBr3

B) Water

C) Chlorite ion (ClO2- )

D) CF2Cl2

Multiple BondsIf there are not enough electrons to form full octets, multiple bonds may need to be formed.

Draw the electron dot formulaE) O2

F) CO2

Resonance StructuresIf there are more than one possibility, resonance structures are drawn.

Resonance structures show possible locations of the bonds. In reality the electrons exist as an average of the two structures splitting time equally between them.

Resonance ExampleEach resonance structure is shown followed by the combination with the double bonds shown with a dotted line as one of the bonds.

Draw the electron dot formulas including resonance structuresG) SO2

H) N2O

What is the name of the property that describes the tendency of an atom to attract electrons when bonded to another atom?

Ionization energyConductivityElectronegativityMetallic CharacterBond lengthClassifying Bond TypesChemical bonds can be classified by how much the bonded electrons are shared or are not shared by the elements involved.

Electronegativity: The ability of one atoms in a molecule to attract electrons to itself.Wolfgang Pauling set electronegativities on a scale from 0.7 (Cs) to 4.0 (F).Electronegativity increases across a period and down a group.

Electronegativities of ElementsElectronegativity

Bond Classification based on Electronegativity DifferenceAs the difference in electronegativity increases, electrons are less equally shared and become more polar.

Type of BondsElectronegativity DifferenceNonpolar covalent

Polar covalent

Ionicx 0.4

0.4 < x < 1.8

x 1.8Bond Classification based on Electronegativity Difference

Classify the bond between the following elements: Cl and Cs

IonicPolar CovalentNonpolar CovalentClassify the bond between the following elements: C and H

IonicPolar CovalentNonpolar CovalentClassify the bond between the following elements: N and O

IonicPolar CovalentNonpolar CovalentIntermolecular ForcesInteractions between MoleculesPolarity of a CompoundLike bonds, compounds themselves can also be classified as polar or nonpolar.

Polarity is based on:Difference in electronegativity of atoms within a compoundSymmetry of the compoundNonpolar Compounds- Diatomic molecules are always nonpolar. (ex. F2)

Also, compounds that are totally symmetric may be nonpolar as well. (ex. CCl4)

Nonpolar Compound the bonds are polar but the dipoles cancel out since the compound is symmetrical (tetrahedral)

Nonpolar Compound the bonds are polar but the dipoles cancel out since the compound is symmetrical (linear)

Polar CompoundsPolar compounds have one side of the compound that is more positive and another side that is more negative.

Polar Compounds

Polar Compounds

Polar Compounds

BF3 = Polar or Nonpolar

PolarNonpolar

CH3F = Polar or Nonpolar?

PolarNonpolar

CF4 = Polar or Nonpolar?

PolarNonpolarBr2 = Polar or Nonpolar

PolarNonpolarPBr3 = Polar or Nonpolar

PolarNonpolarIntermolecular ForcesIntermolecular Forces are forces that exist between two molecules that hold them together.

Intermolecular Forces are caused by charge differences and polarity (because positive and negatives attract)

The stronger the polarity, the stronger the attraction between molecules.Intermolecular ForcesThe stronger the polarity, the stronger the attraction between molecules.

The strength of the attraction between molecules determines properties such as:Boiling pointMelting pointSurface tensionCohesionCapillary action Types of Intermolecular ForcesThree major types of intermolecular forces:Dipole-Dipole Interactions

Hydrogen Bonds

Dispersion ForcesDipole-Dipole InteractionOccurs in polar molecules.

Positive pole of one molecule is attracted to the negative pole of the next molecule.

Hydrogen BondsOccurs in polar molecules when the hydrogen atom is attracted to the more electronegative nitrogen, oxygen, or fluorine atom of another molecule.

Dispersion ForcesDispersion forces are the weakest type of intermolecular forces because they exist between nonpolar molecules.

Usually, the electrons are shared equally. But because electrons are constantly moving, sometimes a temporary dipole forms when all the electrons are on one side of the molecule.

This temporary dipole would cause an attraction with another temporary dipole.Summary of Intermolecular Forces (from strongest to weakest)

What kind of intermolecular force would exist in H2O?

Hydrogen bondingDipole-DipoleDispersion What kind of intermolecular force would exist in PCl3?

Hydrogen bondingDipole-DipoleDispersion What kind of intermolecular force would exist in Br2?

Hydrogen bondingDipole-DipoleDispersion What kind of intermolecular force would exist in NH3?

Hydrogen bondingDipole-DipoleDispersion Melting and Boiling PointsIn order to melt or boil a substance, intermolecular forces must be broken.

Therefore, weaker intermolecular forces require less energy and have lower melting and boiling points.

Therefore, stronger intermolecular forces require more energy and have higher melting and boiling points.

Which type of intermolecular force would have the lowest boiling point and exist as a gas at room temperature?

Ionic BondsDispersion ForcesHydrogen BondsCovalent BondsDipole-Dipole attractionsWhich type of intermolecular force would have the highest melting point and always exist as a solid at room temperature?

Ionic BondsDispersion ForcesHydrogen BondsCovalent BondsDipole-Dipole attractionsWhich compound would have the lowest boiling point and exist as a gas at room temperature?

H2ON2CuI2CO2LiFWhich compound would have the highest melting point?

NH3NaBrF2CO2NH3Effects of Molecular Mass on Melting and Boiling PointsThe higher molecular masses will have higher melting and boiling points because they have more electrons that form temporary dipoles.

Acts as the tiebreaker if the forces are the same types highest mass has the strongest force (highest melting and boiling point).