Unit 2: Polar Covalent Bonds; Acids and Bases

Question 1

Describe how differences in electronegativity give rise to bond polarity

Answer 1

A polar bond is a covalent bond in which there is a separation of charge between one end and the other
- one end is slightly positive, the other slightly negative

Question 2

Electronegativity inductive effect

Question 3

Polar covalent bond

Answer 3

A polar bond is a covalent bond in which there is a separation of charge between one end and the other
- one end is slightly positive, the other slightly negative

Question 4

What if two atoms of equal electronegativity bond together?

Answer 4

If the atoms are equally electronegative (aka, they’re the same atom) both have the same tendency to attract the bonding pair of electrons so it will be found on average halfway btw the 2 atoms (still in molecular orbital)
- This bond can be thought of as a “pure” covalent bond- where electrons are shared evenly btw the 2 atoms

Question 5

When is an ionic bond, opposed to a covalent bond, formed?

Answer 5

If B is a lot more electronegative than A, then the electron pair is dragged right over to B’s end of the bond; A has lost control of its electron, and B has complete control over both = ions have been formed

Question 6

Dipole moment

Answer 6

Mathematically, dipole moments are vectors; they possess both a magnitude and a direction. The dipole moment of a molecule is therefore the vector sum of the dipole moments of the individual bonds in the molecule. If the individual bond dipole moments cancel one another, there is no net dipole moment

Question 7

Explain how dipole moments depend on both molecular shape and bond polarity

Question 8

Formal charge

Answer 8

Compares the number of electrons around a “neutral atom” (an atom not in a molecule) versus the number of electrons around an atom in a molecule. Assigned to an atom in a molecule by assuming that electrons in all bonds are shared EQUALLY, regardless of electronegativity. The sum of the formal charges of each atom must be equal to the overall charge of the molecule or ion.

We assign electrons in the molecule to individual atoms according to these rules:
1. Non-bonding electrons are assigned to the atom on which they are located
2. Bonding electrons are divided equally btw the 2 bonded atoms

FC = (# of valence electrons in free atom) - (# of LP electrons) - (1/2 # of bond pair electrons)

Question 9

Valence electrons

Question 10

Bonding electrons

Question 11

Non-bonding electrons

Question 12

Carbocations, Carbanions, Carbon Radical

Answer 12

Carbocations: occur when a C has only 3 bonds, and no LPs. Have only 6 valence electrons and a FC of +1

Carbanions: occur when a C has 3 bonds plus 1 LP. Have 8 valence electrons and a FC of -1

Carbon Radical: has 3 bonds and a single, unpaired electron. They have 7 valence electrons and a FC of 0

Question 13

Tetravalent

Answer 13

Carbon is tetravalent, meaning that it commonly forms four bonds

Question 14

Resonance form

Answer 14

Sets of Lewis structures that describe the delocalization of electrons in a polyatomic ion or a molecule
- based on molecular orbitals NOT averages of different bonds btw atoms
- we describe the electrons in such molecular orbitals as being delocalized, that is they cannot be assigned to a bond btw 2 atoms

Question 15

6 rules for estimating stability of resonance structures

Answer 15

1. The greater the number of covalent bonds, the greater the stability since more atoms will have complete octets
2. The structure with the least number of formal charges is more stable
3. The structure with the least separation of formal charge is more stable
4. A structure with a negative charge on the more electronegative atom will be more stable
5. Positive charges on the least electronegative atom (most electropositive) is more stable
6. Resonance forms that are equivalent have no difference in stability and contribute equally (eg. benzene

Question 15

When are resonance structures used?

Answer 15

When one Lewis structure for a single molecule cannot fully describe the bonding that takes place between neighboring atoms relative to the empirical data for the actual bond lengths between those atoms

Question 16

Resonance hybrid

Answer 16

The net sum of valid resonance structures is defined as a resonance hybrid, which represents the overall delocalization of electrons within the molecule
- A molecule that has several resonance structures is more stable than one with fewer

Question 17

Delocalization

Answer 17

In resonance structures, the electrons are able to move to help stabilize the molecule, this movement of the electrons is called delocalization

Question 18

Resonance Hybrid

Answer 18

The combination of all resonance structures
- Though the FC closest to zero is the most accepted structure, in reality the correct Lewis structure is a combination of all the resonance structures

Question 19

Bronsted-Lowry Acid

Answer 19

Acids are defined as being able to donate protons in the form of hydrogen ions

Question 20

Bronsted-Lowry Base

Answer 20

Bases are defined as being able to accept protons

Question 21

Conjugate Acid

Answer 21

Substance on the right side of the equation; base turns into conjugate acid

Question 22

Conjugate Base

Answer 22

Substance on the right side of the equation; acid turns into a conjugate base

Question 23

Acidity constant, Ka

Answer 23

*Also known as the acid dissociation constant

The relative acidity of different compounds or functional groups, aka their relative capacity to donate a proton to a common base under identical conditions, is quantified by a number called the dissociation constant

Any particular acid will always have the same pKa (assuming we are talking about an aqueous solution at room temp) but different aq solutions of the acid could have different pH values, depending on how much acid is added to how much water.

Question 24

Equilibrium constant, Keq

Question 25

Lewis acid

Answer 25

An electron-pair acceptor

Question 26

Lewis base

Answer 26

An electron-pair donor

Question 27

Lewis theory of acids and bases

Answer 27

A Lewis acid is an electron-pair acceptor and a Lewis base is an electron-pair donor. This covers Bronsted-Lowry proton transfer rxns, but also includes rxns in which no proton transfer is involved

Question 28

Coordinate or Dative Bonds

Answer 28

Any covalent bond that arose bc one atom brought a pair of its electrons and donated them with another

Question 29

Nucleophile

Answer 29

Electron donor (aka a Lewis base)

Question 30

Electrophile

Answer 30

Electron acceptor (aka Lewis acid)

Question 31

Dipole-dipole forces

Question 32

London dispersion forces

Question 33

Hydrogen bond

Question 34

Intermolecular forces

Answer 34

Hold molecules together in a liquid or solid, 3 types of forces

Question 35

Noncovalent interaction

Question 36

Van der Waals forces

Question 37

How do changes between the solid, liquid, and gaseous states almost invariably occur for molecular substances without breaking covalent bonds?

Answer 37

Due to the large difference in the strengths of intra- and intermolecular forces

Question 38

Intramolecular vs Intermolecular forces

Answer 38

Intramolecular forces
- such as covalent bonds that hold atoms together in molecules and polyatomic ions

Intermolecular forces
- hold molecules together in a liquid or solid
- generally much weather than covalent bonds
- determine bulk properties such as melting points and boiling points
- electrostatic = they arise from the interaction btw positively and negatively charged species; bc electrostatic interactions fall off rapidly with increasing distance btw molecules, intermolecular interactions are most important for solids and liquids where molecules are close

Question 39

3 kinds of intermolecular interactions

Answer 39

1. dipole-dipole - van der Waals force
2. London dispersion - van der Waals force
3. hydrogen bond