L10 Metallic & Intermolecular bonding

Question 1

What is metallic bonding?

Answer 1

= electrostatic attraction between metal ions and mobile (aren’t associated with a particular atom) delocalised e-
- Has a crystaline structure

Question 2

What are the requirements for metallic bonding?

Answer 2

• The metals should have ow ionisation energies - metals readily give up their e- to form metal cations
• The metal should have vacant valence orbitals - the e- should move freely between the overlapping orbitals

Question 3

What are the characteristics of metal?

- why?

Answer 3

1. Great strength - depends on
   - No. of e- (alkali metals are weak - 1/2 valence e-)
   - Packing of cations (how large they are)
2. Good conductors of heat & electricity in liquid or solid state (electrons can move)
3. Malleable & ductile (the ions can slide over each other as the e- are in between countering the repulsion)
4. Lustrous (conduction band of e- reflects light back)

Question 4

What is an intramolecular force?

Answer 4

= force that holds atoms together within a molecule

e. g. ionic, covalent
* stronger than intermolecular forces

Question 5

What is intermolecular force?

Answer 5

= attractive forces between molecules = electrostatic interactions (attraction/repulsion between e-/protons in ions, permanent dipoles or induced dipoles)

e. g. H bonds
* weaker than intramolecular forces

Question 6

What are the biological uses for intermolecular forces?

Answer 6

= controls recognition processes & molecular interactions between cells

• Antibody-antigen interaction
• Protein-protein
• Enzyme-substrate
• Needs to be weak & reversible

Question 7

What are the different types of intermolcular forces?

• which is the strongest?
• what are they between?

Answer 7

Strongest -> weakest:

• Ion-dipole forces - polar molecules
• Hydrogen bonds - polar molecules
• Dipole-dipole forces - polar molecule
• London Dispersion forces (Van der Waals/London forces) - non-polar molecules

Question 8

What are London Dispersion forces?

Answer 8

= a weak intermolecular/electrostatic force between temporarily induced dipoles (formed by in a non-polar molecule when a instantaneous dipole is formed by more e- being on one side that then induces a dipole in the neighbouring molecules/atoms)

• Occurs in all types of molecule all the time
• Strength increases with the no. of e-/size of the molecule (the magnitude of the temporary dipole is larger)

Question 9

What is the significance of London Dispersion forces?

Answer 9

• Present in all molecules - important in non-polar ones
• Stabilises lipid bilaylers - cell membranes
• Protein structure & protein-protein recognition
  Affects:
• The bp of Nobel gases, halogen & alkanes
• The hardness of some solids
• The behavior of gases at low temp - deviation from the ideal gas laws

Question 10

What are dipole-dipole interactions?

Answer 10

= electrostatic attraction between permanent dipoles (found in bonds with a electronegativity difference) = the partial + end of one is attracted to the partially - end of another
* Can act in addition to London dispersion forces

Question 11

What is the significance of dipole-dipole interactions?

Answer 11

• Carbonyls C=O
• Result in higher boiling points than expected from the mass of the molecule
• Protein folding

Question 12

What is an ion-dipole force?

Answer 12

= electrostatic interaction between an in and an uncharged polar molecule
e.g. a negative end of a dipole is attracted to a positive ion
- Strength depends on charge of the ion & the magnitude of the dipole
- Most common in solution
Significance:
- Allows for the dissolving of ionic compounds in water and in other polar solvents
e.g. KCl dissolving in water

Question 13

What is a hydrogen bond?

Answer 13

= electrostatic interaction between a:
1. Electronegative atom (O, N, F) [small size, high charge density, lp of e- in a small orbital = hydrogen bond acceptor)
AND
2. An electropositive hydrogen atom (= hydrogen bond donor) in a polar bond (usually N-H, O-H or F-H)

Question 14

What are the properties of hydrogen bonds?

Answer 14

• Weak & easily broken compared to covalent bonds
• Directional
• Additive

Question 15

What is the significance of hydrogen bonds?

Answer 15

• Account for the physical state of water
• Strong enough to stabilise macro molecules
• DNA base pairing: C/G = 3 H-bonds, A/T = 2 H-bonds
• Protein secondary structure: alpha helices & beta sheets (strongest interactions between amide N-H and C=O)
• Enzyme-substrate and antibody-antigen interactions
• Solubility of compounds in water
• The more H-bonds the higher the bp

Question 16

What is the hydrogen bonding like in water?

Answer 16

Extensive network of hydrogen bonds (they break and reform constantly):

• each O atom has 2 lp
• each water molecule has 2 H atoms

Question 17

What is the hydrogen bonding like in ice?

Answer 17

Crystaline lattice = each H2O molecule is hydrogen bonded to four other in a tetrahedral formation (diamond-like structure)
- molecules are further apart than in liquid water

Question 18

What is the bonding in cellulose?

Answer 18

• H-bonds: hold glucose polymers together between chains

- London dispersion: hold together flat sheets of hydrogen-bonded chains