Princeton Ch 5 - Bonding and Intermolecular Forces

Question 1

How do you calculate formal charge?

Answer 1

FC = (Valence e- ) - #single bonds - # e- in lone pairs

Question 2

For resonance structures, what do dotted lines resemble?

Answer 2

They indicate some double bond character.

Question 3

What are resonance structures?

Answer 3

Resonance structures are two or more sturctures where only nonbonding e-, and double and triple bonds may move around.

Question 4

Define bond dissociation energy.

Answer 4

BDE is the energy required to break a bond homolytically. In homolytic bond cleavage, one e- of the bond goes to each fragment of the molecule.

Question 5

Define heterolytic bond cleavage.

Answer 5

In heterolytic bond cleavage, also known as DISSOCIATION, both e- of the electron pair that make up the bond end up on the same atom; this forms a cation and anion.

Question 6

True or false. The higher the bond order, the shorter and stronger the bond.

Answer 6

True. Triple bonds are shorter and stronger than single bonds. But be aware! Bond length/BDE comparisons should only be made for similar bonds.

Question 7

Which bond is shorter and thus stronger: CC or CO?

Answer 7

The CO triple bond is stronger because of the EN attraction.

Question 8

When comparing the same type of bonds the number of bonds and the _ and _ character matters as well.

Answer 8

S and P character matters as well. The longer the bond the weaker the bond. But a greater percentage of p character results to a more directional hybrid orbital that is FARTHER from the nucleus.

Question 9

Distinguish between covalent and ionic bonds.

Answer 9

A covalent bond is formed between atoms when each contributes one or more of its unpaired e-. The e- are shared. A ionic bond is an electrostatic attraction between atoms that result from an atom losing/gaining an e- to form a cation and anion.

Question 10

What is a coordinate covalent bond?

Answer 10

When one atom donates BOTH of the shared e- in a bond. For ex, NH3 donates both e- in its lone pair to form a bond with boron in BF3. Since NH3 donates a pair of e-, it is known as a LEWIS BASE or NECLEOPHILE or LIGAND. When the coordinate bond breaks, the e- leaves with the ligand.

Question 11

What is a lewis acid?

Answer 11

A molecule that accepts a pair of e-.

Question 12

What is the prevailing rule or VSEPR theory?

Answer 12

One rule: since e- repel one another, e- pairs, whether bonding or non-bonding attempt to move as far apart as possible.

Question 13

Name the hybridization for the following e- groups: 2,3,4

Answer 13

2 = sp; 180 degrees; linear
3 = sp^2; 120 degrees; trigonal planar
4 = sp^3; 109.5 degrees; tetradhedral

Question 14

What is a sigma bond?

Answer 14

A sigma bond consists of two e- that are localized between two nuclei. Single bonds consists of sigma bonds.

Question 15

What is a pi bond?

Answer 15

A pi bond is composed of two e- that are localized to the region that lies on opp sides of the plane formed by the two bonded nuclei and immediately adjacent atoms, not directly between the two nuclei as with a sigma bond.

Question 16

True or false. In any multiple of bonds, there is only one σ bond. The remainder are pi bonds.

Answer 16

True.
A single bond = 1 σ bond
A double bond = 1 σ and 1 pi
A triple bond = σ and 2 pi

Question 17

Describe the polarity of OCS.

Answer 17

OCS is a polar molecule since it connects 2 diff atoms of unequal EN. The CO bond is more polar than the CS bond. This molecule is thus NOT symmetrical and the O and S don’t cancel each other out.

Question 18

What is an intermolecular force?

Answer 18

IM forces are relatively weak interactions that take place between neutral molecules.

Question 19

Distinguish ion-dipole, dipole-dipole, and dipole induced dipole forces.

Answer 19

Ion-dipole = polar molecules are attracted to ions, producing an Ion-dipole force.
Dipole-Dipole = attractions b/t the positive end of one polar molecule and the neg end of another polar molecule.
Dipole induced dipole = A permanent dipole in one moleule may induce a dipole in a neighboring nonpolar molecule, producing a momentary dipole.

Question 20

What is a London dispersion force?

Answer 20

An instantaneous dipole in a NONpolar molecule may induce a dipole in a neighboring nonpolar molecule. The resulting attraction is a LDF.

Question 21

What is the weakest intermolecular force?

Answer 21

London dispersion forces (instantaneous dipole between nonpolar molecules). Although, the bigger the molecule, the greater e-, which increases polarizability, The partial charges of rht induced dipole gets larger and the LDF strength increases.

Question 22

London dispersion forces, dipole forces, and hydrogen bonding are collecting known as ____.

Answer 22

Van der Waals Forces.

Question 23

Despite being weak, LDF can have a profound impact on the physical properties of a particular molecule. Describe how.

Answer 23

Substances with stronger IM forces will exhibit greater meling points, boiling points, viscosities, and lower vapor pressors than similar compounds with weaker IM forces. F2 is a gas under 1atm 25C but Br2 is a liquid because the strength of LDF increases with size.

Question 24

Describe what hydrogen bonding is.

Answer 24

H-bonding is the strongest type of IM force between neutral molecules. Two criteria: 1) a molecule must have a covalent bond between H and either N,O, or F, and 2) another molecule must have a lone pair of e- on an N,O, or F atom. This results in higher BP. H2S BP is < than H2O.

Question 25

What is vapor pressure. Why does H2O have a high VP?

Answer 25

VP is the pressure exerted by the gaseous phase of a liquid that evaporated from the exposed surface of the liquid. The weaker a substance’s IM forces, the HIGHER the VP. Since H2O undergoes H-bonding, it has a lower VP than other substances.

Question 26

What’s an ionic solid?

Answer 26

An ionic solid is held together by the electrostatic attraction between cations and anions in a lattice structure. Ionic bonds are strong; strength is dependent on magnitude of ion charges and size of the ions.

Question 27

What is a network solid?

Answer 27

In a network solid, atoms are connected in a lattice of covalent bonds, meaning that all interactions between atoms are covalent bonds. These are very strong at room temp. Diamonds are examples.

Question 28

Why are metallic solids good conductors?

Answer 28

Metal can be thought of as a covalently bound lattice of nuclei and their inner shell e-, surrounded by a “cloud” of e-. At least one valence e- per atom is not bound and is free to move throughout the lattice. These are called conduction e-.

Question 29

True or false. Molecular compounds usually have a lower melting point and boiling points that other types of solids.

Answer 29

True! Molecular compounds are held together by h-bonds, dipole-dipole, LDF forces. These are weaker than ionic, network, or metallic bonds.