Ch. 6.5b Hybrid Orbitals and Intermolecular Forces Chemical Bonding

POINT > Describe the concept of hybrid orbitals POINT > Describe 4 types of intermolecular forces POINT > Review polar covalent bonding POINT > Relate intermolecular forces to changes of state

POINT > Describe the concept of hybrid orbitals Previously you learned that methane (CH4) has a tetrahedral geometry with all bond angles = 109.5ᵒ You have also learned that the four valence e- of a carbon atom have a 2s22p2 arrangement This is a problem. How can 4 e- in different sublevels, with different energies, result in four identical bonding orbitals?

Linus Pauling (1916 – 1994) TWO Introduced: Hybrid orbitals Nobel Prizes!! Introduced: Hybrid orbitals Concept of electronegativity Idea that most covalent bonds have some ionic character Resonance structures

POINT > Describe the concept of hybrid orbitals Orbital hybridization: Several (different) atomic orbitals combine to form the same number of equivalent hybrid bonding orbitals Carbon  2s22p2 2s12p3  Mix to form four equal sp3 hybrid orbitals

POINT > Describe the concept of hybrid orbitals Methane (CH4)

WB CHECK: When a carbon atom is bonded to four atoms, as in carbon tetrachloride CCl4 the s and p orbitals of C each get 2 Cl the s and p orbitals of C form two hybrid orbitals the s and p orbitals of C form 4 hybrid orbitals the s and p orbitals of C overlap

Linus Pauling won his first Nobel Prize for his work WB CHECK: Linus Pauling won his first Nobel Prize for his work on DNA structure on chemical bonding on atomic structure on opposition to nuclear bomb testing

Linus Pauling won his second Nobel Prize for his work WB CHECK: Linus Pauling won his second Nobel Prize for his work on DNA structure on chemical bonding on atomic structure on opposition to nuclear bomb testing

POINT > Describe 4 types of intermolecular forces Covered so far: Ionic bonding in ionic compounds Metallic bonding in metals and alloys Covalent bonding that holds atoms together in molecules A variety of weaker forces exist between molecules and affect the properties of molecular compounds

POINT > Review polar covalent bonding Polar covalent bonds: atoms share electrons unequally due to electronegativity (EN) differences More EN atom: stronger pull on e- = slight (-) charge Less EN atom: slight (+) charge

POINT > Review polar covalent bonding We use delta (δ) to denote atoms that acquire partial charges in polar molecules

POINT > Review polar covalent bonding Ex. Hydrogen Chloride (HCl) δ+ δ- H Cl EN = 2.1 EN = 3.0

POINT > Review polar covalent bonding Hydrogen Chloride (HCl) H Cl EN = 2.1 EN = 3.0

POINT > Review polar covalent bonding Polar bonds result in a dipole: Any bond having two poles (charged regions)

POINT > Review polar covalent bonding Ex. Water (H2O) δ- EN = 3.5 O δ+ δ+ H H EN = 2.1 EN = 2.1

POINT > Review polar covalent bonding Ex. Water (H2O) EN = 3.5 O H H EN = 2.1 EN = 2.1

POINT > Review polar covalent bonding One end of a dipole is slightly negative, the other end slightly positive Dependent on shape of molecule

POINT > Review polar covalent bonding Ex. Carbon dioxide (CO2) vs. water (H2O) H O EN = 2.1 EN = 3.5 Dipoles Cancel O C Nonpolar Molecule Polar Molecule EN = 3.5 EN = 2.5 EN = 3.5

POINT > Describe 4 types of intermolecular forces Forces between molecules (from strongest to weakest) Ion-dipole forces: Ion-dipole forces occur when ionic compounds are dissolved in water The charged ions interact with the dipoles of the water molecules

Ion-dipole interactions POINT > Describe 4 types of intermolecular forces Ion-dipole interactions

POINT > Describe 4 types of intermolecular forces 2. Hydrogen bonding*: Compounds with O-H, N-H and F-H bonds can form hydrogen bonds The polarity of these bonds and the small size of hydrogen atoms causes strong attractions Hydrogen bonds create surface tension in water They hold molecules, like the two strands of DNA, together * These are not bonds. They are intermolecular forces

POINT > Describe 4 types of intermolecular forces Hydrogen bonding

WB CHECK: What type of intermolecular force would occur between H2O and MgCl2? What type of intermolecular force would occur between H2O and NH3?

Ex. HCl, HBr, H2S POINT > Describe 4 types of intermolecular forces 3. Dipole-dipole forces: These are interactions between polar molecules with out O-H, N-H or F-H bonds Ex. HCl, HBr, H2S

POINT > Describe 4 types of intermolecular forces 4. London dispersion forces (LDF): These are the weakest intermolecular forces LDFs occur when a polar molecule induces a temporary dipole in an otherwise nonpolar molecule

CHECK: Rank order the intermolecular forces, from strongest to weakest: (LDF = London dispersion force) a) dipole-dipole, H bonding, LDF, ion-dipole b) ion-dipole, dipole-dipole, H bonding, LDF c) LDF, ion-dipole, dipole-dipole, H bonding d) ion-dipole, dipole-dipole, H bonding, LDF e) ion-dipole, H bonding, dipole-dipole, LDFWB

POINT > Relate intermolecular forces to changes of state Intermolecular forces govern properties of substances (like boiling point) Interactions that tend to hold molecules together result in higher boiling and melting points

Bonding and intermolecular forces overview Strongest Weakest Metallic bonding Covalent bonding Ionic bonding Ion-dipole Hydrogen bonding Dipole-dipole London dispersion intermolecular forces

WB CHECK: What is the strongest intermolecular force that would be present in CH4? ion-dipole hydrogen bonding dipole-dipole LDF

WB CHECK: What is the strongest intermolecular force that would be present in Br2? ion-dipole hydrogen bonding dipole-dipole LDF

WB CHECK: What is the strongest intermolecular force that would be present in H2O? ion-dipole hydrogen bonding dipole-dipole LDF

WB CHECK: What is the strongest intermolecular force that would be present in HCl? ion-dipole hydrogen bonding dipole-dipole LDF

WB CHECK: What is the strongest intermolecular force that would be present in a mixture of ethanol (C2H5OH) and water? ion-dipole hydrogen bonding dipole-dipole LDF

Homework: Read and re-read 6.5 pages 187-197 F.A. #4-6 page 197