MCAT Study Guide Chemistry Ch. 5 – Chemical Bonds 2017-08-15T06:45:06+00:00

I.          5.1:  LEWIIS DOT STRUCTURE

A.     FORMAL CHARGE

1.     Determining FC of a molecule gives us the ideal configuration (FC should be close to 0)

2.     FC = V – ½B – L   → V = valence electrons, B = bonded electrons, L = lone pairs

3.     Best Lewis structures have octet of electrons and FC of 0

B.     RESONANCE

Resonance structures used when there are double/triple bonds with lone pair(s) of electrons

1.     May be equivalent or nonequivalent (major and minor form)

 

II.          5.2:  BOND LENGTH

A.     BDE (bond dissociation energy)

Energy required to break a bond homolytically (one electron from each pair goes to each fragment of the broken molecule, forming 2 radicals)

1.     Heterolytic bond cleavage (dissociation) – both electrons go to the same atom, forming a cation and anion

2.     See O Chem 3.3 notes for further detail about relative bond length

3.     Long bond = weak bond

4.     Short bond = strong bond

 

III.          5.3:  TYPES OF BONDS

A.     COVALENT BONDS

A bond formed between 2 nonmetals; when each contributes one or more of its unpaired valence electrons (these are shared to create an octet)

1.     Polarity of Covalent Bonds

a)     Electronegativity of each atom will determine polarity

b)     Dipole moment – a molecule that has polarity due to difference in electronegativity of atoms involved in covalent bonding (electron density is uneven)

2.     Coordinate Covalent Bonds – one atom will donate both of the shared electrons in a bond

a)     NH3 donates a lone pair to BF3 → F3BNH3

b)     Lewis acid (electrophile) – electron pair acceptor (BF3 in above)

c)     Lewis base (nucleophile, ligand) – electron pair donor (NH3 in above); when this bond breaks, the electron pair will go back with this molecule

(1)   Ligand sounds like lye, which is a base

B.     IONIC BONDS

Bond between metal and nonmetal (forming a cation and an anion), then the two atoms are attracted to each other by electrostatic attraction

1.     Ionic bonds between metals and nonmetals must have high difference in electronegativity between atoms

2.     Strength of the bond is proportional to the charges on the ions (by distance2)

a)     EX:  MgS, NaCl → Mg2+ and S2- bond is 4x as strong as Na+ and Cl

3.     Bonds are ionic if electronegativity is very different!

 

IV.          5.4:  VESPR THEORY

A.     This predicts shape of molecule

 

V.          5.5:  HYBRIDIZATION (see O chem 3.1)

 

VI.          5.6:  MOLECULAR POLARITY

A.     If a molecule contains no polar bonds, it cannot be polar; also, if it contains two symmetrically oriented polar bonds, the dipoles cancel each other out

 

VII.          5.7:  INTERMOLECULAR FORCES

Weak interactions that take place between neutral molecules

A.     Van der Waals forces – all intermolecular forces

1.     Ion-dipole forces – polar molecules attracted to ions

2.     Dipole-dipole forces – attractions between the positive end of one molecule and the negative end of another

a)     Hydrogen bonding – the strongest dipole-dipole force

(1)   Must have covalent bond between H and N, O, or F on one atom

(2)   Must have a lone pair on N, O, or F of the bonding atom

3.     London dispersion forces – very weak, transient interactions between instantaneous dipoles in nonpolar molecules

a)     Weakest of all forces

b)     The only thing that needs to be present is electrons on an atom

c)     ↑ # of electrons → ↑ polarizability (and therefore the London dispersion forces)

B.     Vapor Pressure (VP)

This is a physical property determined by the strength of the intermolecular forces

1.     VP – the pressure exerted by he gaseous phase of a liquid that evaporated from the surface of the exposed liquid

a)     Weak intermolecular forces = high VP

(1)   Volatile – easily vaporized liquids

b)     ↑ T = ↑ VP

(1)   ↑ KEav of a molecule will allow it to overcome IM forces and escape into vapor

VIII.          5.8:  TYPES OF SOLIDS

A.     CRYSTAL SOLIDS

Have a sharp melting point and highly regular arrangement of atoms or molecules (repeating)

1.     IONIC SOLIDS – held together by electrostatic attraction between cations and anions in crystal lattice structure

a)     Usually a metal and nonmetal

b)     Strong bonds, most ionic solids are solid at room temp

c)     Smaller ions with greater charges = stronger bonds

d)     Inter molecular forces = intramolecular forces

2.     NETWORK SOLIDS – atoms are connected in a lattice of covalent bonds, meaning every interaction between atoms are covalent bonds

a)     Can be thought of as one giant molecule

b)     Very strong (diamonds, quartz)

c)     Inter molecular forces = intramolecular forces

3.     METALLIC SOLIDS – covalently bound lattice of nuclei and their inner shells surrounded by a “cloud” or “sea” of electrons

a)     Conduction electron – at least 1 valence electron per atom that is not bound to any one particular atom and is free to move throughout the lattice

4.     MOLECULAR SOLIDS – solids composed of molecules in crystal lattice structure (may be all one element, for example, like tetrahedral P4, forming the solid)

a)     Molecules held together by weak IM forces only → lowest boiling and melting point

B.     NONCRYSTAL SOLIDS

1.     AMORPHOROUS SOLIDS – only solids that do not exhibit crystalline structure; molecules are irregularly bonded

2.     Can be soft and rubbery (formed by long, tangled molecules)

3.     Can be glassy (irregularly covalently bonded, hard and brittle)

MCAT Study Guide Chemistry - Kim Matsumoto


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