3. Bonding and IMFs

Question 1

What is the formula for formal charge?

Answer 1

FC = V - 1/2B - L

V = valence electrons 
B = bonding electrons 
L = lone electrons 

formal charge helps us determine the best Lewis structure for a given compound

Question 2

Which Lewis structure is best for any given compound?

Answer 2

1. the structure that minimizes FC

2. the structure that places negative FC’s on the most EN atoms.

Question 3

how does formal charge help with resonance structures?

Answer 3

In atoms with unequal resonance structures, the structure that minimizes FC will be the more dominant resonance structure.

Question 4

What is homolytic and heterolytic bond cleavage?

Answer 4

Homolytic cleavage: one electron goes to each atom when the bond is broken creating 2 radicals

Heterolytic cleavage: two electrons pair up onto one atom while the other atom gets no electrons, creating ions.

Question 5

What is bond dissociation energy?

Answer 5

BDE is the the amount of energy needed to perform homolytic bond cleavage

Question 6

t or f, the higher the bond order, the shorter and stronger the bond

Answer 6

true, triple bonds are shortest and strongest

Question 7

t or f, when comparing hybridized bonds, the one with more s-character is shorter and stronger.

Answer 7

true, s orbitals are closer to the nucleus and therefore the more s character the shorter the bond. (and the shorter the bond, the stronger it is). This makes sense in terms of hybridization

triple bond constituents are sp hybridized (50s 50p)
single bond constituents are sp3 hybridized (25s 75p)

Question 8

Covalent bonds: what determines bond polarity or a dipole moment?

Answer 8

Covalent bonds, each atom provides one or more electrons to form the bond. The EN of the respective atoms can pull electron density towards one direction of the bond, causing polarity.

when there is delta positive/delta negative relationship there is a dipole moment.

Question 9

What is a coordinate covalent bond?

Answer 9

This is when one atom provides both of the bonding electrons in a covalent bond

Lewis acid / Lewis base reactions cause this.

Question 10

What is an ionic bond? What causes ionic bond formation?

Answer 10

One atom fully gives up its electrons to another, forming two ions which are then held together by electrostatic forces. Ionic bonds are typically formed when there is a very large difference in EN between the two atoms. (so much EN that one atom pulls the e off the other).

Question 11

What two things determine ionic bond strength?

Answer 11

1. the size of the charge (larger charge = stronger)

2. the size of the atoms (larger atoms = greater bond length = weaker bond)

Question 12

What is an electron group?

Answer 12

any type of bond (double, single, triple) or lone pair of electrons.

double and triple bonds only count as one electron group. These help determine molecular geometry and hybridization.

Question 13

What is the molecular geometry of NH3 and H2O

Answer 13

NH3 - bonded to three hydrogen’s with one lone pair. Therefore, it is trigonal pyramidal
H2O: oxygen has two lone pairs, therefore, it is bent.

Question 14

what is the difference between orbital and molecular geometry?

Answer 14

orbital geometry refers to the distribution of electron orbitals, molecular geometry refers to the actual shape of the molecule. In molecules with no lone electrons, OG = MG.
However, in NH3, the shape (MG) is trigonal pyrimidal, but the OG is still tetrahedral.

Question 15

What is the hybridization of oxygen in water?

Answer 15

oxygen has 4 electron groups, and therefore, it is sp3 hybridized (25% s, 75%p)

Question 16

What are σ (sigma) bonds?

Answer 16

Sigma bonds represent end-to-end overlap of one hybridized orbital. Single bonds are created by one σ bond.

Question 17

How many bonding orbitals does an sp3 carbon have?

Answer 17

It has 4 sp3 bonding orbitals.

Question 18

What are π bonds?

Answer 18

π bonds represent side-to-side overlap (not end-to-end like sigma bonds) of two unhybridized p orbitals.

An sp2 atom has 3 sp2 hybridized orbitals. It ALSO has one unhybridized p-orbital. This can overlap with another atoms unhybridized p-oribital to create a π bond.

Question 19

how many sigma and π bonds are in a single, double, and triple bond?

Answer 19

All bonds only have 1 sigma bond

single: 1 σ
double: 1 σ 1 π
triple: 1 σ 2 π

Question 20

When counting how many bonds in a molecule, DON’T FORGET the non-drawn hydrogens bonded to carbon atoms.

Answer 20

true

Question 21

What is an ion-dipole force? What is a dipole-dipole force?

Answer 21

Ion-dipole: in a polar molecule, there are delta + and delta - sides. An ion-dipole force exists between the ion and the opposing delta charge.

dipole-dipole force: two dipole compounds (polar compounds) interact with each-other

Question 22

What is an induced dipole?

Answer 22

A dipole compound can induce a momentary dipole moment in a non-polar compound.

Question 23

What are London dispersion forces?

Answer 23

multiple transient dipole with momentary attractions in non-polar molecules. These are the most basic force, with every compound experiencing them (a compound only needs electrons to experience them)

Question 24

Are London dispersion forces strong?

Answer 24

LD force are the weakest IMF’s. However, LD gets stronger with atomic size. This is why I2 exists as a solid but F2 exists as a gas.

Question 25

List the order of the strength of intermolecular forces.

Answer 25

Ion-dipole > H-bonds > dipole-dipole > induced-dipole > London dispersion

Question 26

What are Van der Waals forces?

Answer 26

Van der Waals forces are a general term to explain all intermolecular forces. However, it is often used to only refer to dispersion forces.

Question 27

T or f, increasing a substances IMFs will

increase boiling point, melting point, viscosity, and decrease vapor pressure.

Answer 27

True

Question 28

What is hydrogen bonding? What is the criteria for this to occur?

Answer 28

H-bonds are the strongest IMF between neutral molecules.

1. H bonded to NOF
2. Another molecule must have a lone pair of electrons on an NOF atom.

H2O ||||| H2O

the delta negative oxygen’s lone electrons are attracted to the delta positive hydrogen of the other water molecule.

Question 29

What is vapor pressure?

Answer 29

VP is the pressure exerted on a liquid by the gaseous state of that liquid that has been evaporated. Substances with weak IMFs have high vapor pressure because its material is evaporating more. Thus boiling point and VP are inversely related.

Question 30

What are volatile liquids?

Answer 30

liquids with high vapor pressure (liquids that evaporate easily) are volatile.

Question 31

What is an ionic solid?

Answer 31

A solid lattice structure held together by ionic bonding (strong EN difference between bonding atoms pulls the electrons off of one onto the other). Almost all ionic compounds are solid at 298K.

Question 32

What is a network solid?

Answer 32

A solid held in lattice by only covalent bonds. Similar to ionic solids, the ONLY force involved is intramolecular bonding (no IMFs).

Question 33

T or f, network solids tend to be very strong.

Answer 33

True, this is because they only consist of intramolecular covalent bonds (they are essentially one large molecule). diamonds are network solids.

Question 34

What are metallic and molecular solids?

Answer 34

metallic solid –> electrons can move freely throughout the solid; they conduct electricity

molecular solids –> uses IMFs to hold the solid together. These are the weakest solids, often as gases or liquids at 298K

Question 35

t or f, all metals, except mercury are solid at 298K

Answer 35

true

Question 36

Explain which has the lowest melting point out of MgO, CH4, Cr, HF

Answer 36

MgO: ionic compound = strong
CH4: covalent compound uses dispersion forces = weakest (unless in huge quantities)
Cr: metals are solid at room temperature
HF: hydrogen bonding increases melting temp.

thus, CH4 will have the lowest melting point

Question 37

t or f, larger charges and smaller ions make for the weakest ionic bonds.

Answer 37

false. they make the strongest (decreased size = shorter bond length = stronger bond).