Lecture 10-Chemical Bonding IV Flashcards
In an ionic bond, what do the +ve and -ve ions form from?
\+ve = metals -ve = non-metals
What is an ionic bond?
An electrostatic force between cations + anions
What is a covalent bond?
An electrostatic force between 2 nuclei + the shared e- pair
What is a dative covalent bond?
An electrostatic force between 2 nuclei + the shared e- pair
How strong is a dative covalent bond?
It’s indistinguishable from normal covalent once formed
Describe the polarity of the bonds?
Ionic = polar
Covalent = both
Dative covalent = often polar
Describe the solubility if the bonds in water?
Ionic = soluble
Covalent = depends on the molecule
Dative covalent = Depends on the molecule (often reacts with water)
Describe metallic bonding?
Electrostatic attraction between metal ions + mobile delocalised e-
What are delocalised e-?
E- not associated with a particular atom
What requirements are needed for metallic bonding?
The metal = low ionisation energies
(metal readily gives up their e- —> positive metal ions)
The metal = vacant valence orbitals
(e- move freely between overlapping orbitals)
What does the great strength of metals depend on?
- N.O of e-
- Packing of cations
Why are metals malleable + ductile?
+ve ions move over each other because of the e- between
Give 2 characteristics of metals?
- Good conductors of heat + electricity in solid,liquid state
- Lustrous
What type of forces determine melting + boiling points?
Intermolecular forces
Give an example of intramolecular forces?
- Polar covalent
- Non-polar
Give examples of intermolecular forces?
Dipole-dipole
Hydrogen bonds
When and where do attractive + repulsive operate?
Between e- + protons
when atoms / molecules approach each other
Can attraction be between permanent charges (ions) , permanent dipoles or induced dipoles?
Yes
List the intermolecular forces from strongest to weakest
Ion-dipole
H bonds
Dipole-dipole
Dispersion forces
What is the only non-polar intermolecular force?
Dispersion forces
How do london dispersion forces form?
1) e- = constantly moving in their orbitals
2) at any given moment = more e- on one side of the atom / molecule —> instantaneous temporary dipole
3) this dipole induces anther in a neighbouring molecule
4) Attraction between these dipoles = a weak intermolecular force
Where do london dispersion forces occur?
Between all types of molecules
What does the strength of london dispersion forces depend on?
Strength increases with the n.o of e-
Describe the significance of london dispersion molecules in boiling points?
Noble gases
Halogens
Alkanes
Describe the significance of london dispersion molecules in solids?
Hardness
Describe the significance of london dispersion molecules in behaviour of gases?
At low temperatures
Describe the significance of london dispersion molecules in cell membrane?
Stabilizing lipid bilayers
Describe the significance of london dispersion molecules in PROTEINS?
Protein structure + self-recognition
What are the requirements for permanent molecular dipole-dipole?
- Electronegative difference
- A non-uniform charge distribution in 3-D
What is a dipole-dipole bond?
Electrostatic attraction between permanent dipoles
What do dipole-dipole interactions result in?
Higher than expected boiling pints vs mass of the molecule
Describe the significance of dipole-dipole reactions?
Carbonyls
Protein folding
What are ion-dipole forces?
Electrostatic interaction between an ion and a uncharged polar molecule
What does the strength of ion-dipoles depend on?
The charge of the ion + magnitude of the dipole
Where are ion-dipoles most common + example?
In solution
e.g. KCL dissolving in H20
Requirements for H bonding?
O- , N- , F-
(small size , high charge density ,lone pair in small orbital)
A H+ atom (donor) in a POLAR BOND
What are the properties of H bonds?
Weak + easily broken
Directional
Additive
Give an example of a positional isomer with H bonds?
o-nitrophenol Mp = 43-45
p-nitrophenol Mp = 110-115
Name the hydrides?
H20
HF
NH3
CH4
Why does water form an extensive network of H bonds?
O = 2 lone pairs
H2O molecules = 2H atoms
Compare the structure of ice to H2O normal structure?
Each H2O = H bonded to 4 others in a tetrahedral formation
Ice = diamond-like crystal structure
What does water do due to optimal H bonding?
Molecules are further apart in liquid water
Does ice have a fixed arrangement of H bonds?
Yeah
What allows the correct pairing of DNA bases?
Matching number of H bond donors + acceptors
In an alpha helix + beta sheet structure what are the strongest interactions?
Between amide N-H and C=O
Describe the bonding in cellulose?
Multiple H bonds within and between sugar chains (held rigidly)
-Flat sheets of H bonded chains held together by london dispersion
What is the strongest intermolecular forces between acetone (ketone with carbonyl) + H2O?
Dipole-dipole
