Additional Aspects of Molecular Bonding & Structure Chapters 8 and 9 BLB 12 th.

35
Additional Aspects of Molecular Bonding & Structure Chapters 8 and 9 BLB 12 th

Transcript of Additional Aspects of Molecular Bonding & Structure Chapters 8 and 9 BLB 12 th.

Additional Aspects of Molecular Bonding & Structure

Chapters 8 and 9

BLB 12th

8.2 Ionic BondingEnergetics of Ionic Bond Formation

Na(s) + ½ Cl2(g) → NaCl(s) ΔHf° = −410.9 kJ

ΔH = 147 kJ/mol

• Lattice energy – energy required to completely separate the ions in one mole of an ionic compound

Na+(g) + Cl‾(g) → NaCl(s) ΔHlattice = −788 kJ

• Lattice energy ↑ as ion charges ↑ and size ↓`

Hess’s Law for lattice energy (Born-Haber cycle)

8.2 Ionic Bonding

• Electron Configurations of Ions (also 7.4)• Atoms will gain or lose electrons to

achieve a noble gas configuration.• Transition metal ions: s electron(s) are lost

first.

8.3 Covalent Bonding• Atoms share electrons.

8.4 Bond Polarity• Unequal sharing of electrons in a covalent

bond• Electronegativity – the ability of an atom in a

molecule (bonded) to attract electrons to itself

8.4 Bond Polarity• Nonpolar covalent bond – electrons shared

equally• Polar covalent bond – electrons shared

unequally due to different electronegativity values– Greater electronegativity difference, more polar

the bond (higher dipole moment) • Dipole moment – measured magnitude of a

dipole.– Predict direction– No calculations

The greater the difference in electronegativity, the more polar the bond.

Electronegativity

Electronegativity difference

• < 0.5 nonpolar• 0.5-2.0 polar• > 2.0 ionic

Rough guidelines only. See p. 304.

• Examples:

C–H

N–O

Na–Cl

Cl–Cl

9.3 Molecular Polarity

• Depends upon the polarities of the bonds and the molecular geometry of the molecule

• Bond dipole moments are vector quantities.• A molecular dipole moment is the vector sum

of its bond dipoles.• A molecule can be nonpolar, that is, have a

net dipole moment of zero, even if bond dipole(s) exist.

CO2 a nonpolar molecule

H2O a polar molecule

9.3 Molecular Polarity• Examples

KrF2

SO2

ICl3

XeO4

8.5 Drawing Lewis StructuresFormal Charges (p. 307)• There may be more than one valid Lewis Structure

for a given molecule.• Formal charges are used to determine the most

reasonable structure.• Calculate a formal charge (FC) for each atom:

FC = (# valence e¯) − (# e¯ belonging to atom)

8.5 Drawing Lewis StructuresFormal Charges• Best structure? The one with the formal charges

closest to zero and where the most negative charges reside on the most electronegative atoms.

• For H, Be, and B, formal charges indicate that the Lewis structure with an incomplete octet is more important than the ones with double bonds.

8.6 Resonance Structures

• Lewis structures and the VSEPR theory are means by which we try to mimic or predict the experimentally determined properties of molecules.

• When a combination of single and multiple bonds are used, it implies that the bond lengths are unequal.

• They’re not!

Resonance Structures• The measured bond lengths are an average of the

representative structures; somewhere between a single and double bond length.

Resonance• The organic compound

benzene, C6H6, has two resonance structures.

• It is commonly depicted as a hexagon with a circle inside to signify the delocalized electrons in the ring.

8.8 Strengths of Covalent Bonds

• Bond enthalpy – energy required to break a bond

• Enthalpy of reaction – estimate from difference of the bonds broken minus the bonds formed

413 kJ + 242 kJ 328 kJ + 431 kJ

−104 kJ

9.4 Orbital Overlap & 9.5 Hybrid Orbitals

• Electrons exist in orbitals.• Valence Bond Theory – bonding model

where orbitals overlap to form bonds• Hybridization combines orbitals into hybrid

orbital sets that match experimentally determined geometried.

• σ bond – one area of overlap • π bond – two areas of overlap

Orbital Overlapσ bonds – single area of overlap

Orbital Overlap in the formation of H2

sp Hybrid Orbitals

sp2 Hybrid Orbitals

sp3 Hybrid Orbitals

Hybrid Orbital Overlap

9.6 Multiple Bonds

C2H4 – the whole picture

C2H2 – the whole picture