4. & 14. Chemical Bonding and Structure

Question 1

Volatility

Answer 1

the tendency of a substance to vaporize

Question 2

Working method to deduce molecular polarity

Answer 2

1. Determine molecular geometry
2. For each bond present, using electronegativity differences, deduce bond polarity for each bond present and draw the dipole moments as vectors
3. Using vector addition, sum all the dipole moments present to establish whether or not there is a net dipole moment for the molecular. If so, it is polar.

Question 3

Resonance

Answer 3

Using multiple structures for a molecule or ion that cannot be described fully with one.

Question 4

Delocalisation

Answer 4

electrons are shared by more than two atoms in a molecule or ion, drawn in resonance

Question 5

Allotropes

Answer 5

difference structural modifications of the same element

Question 6

Properties of covalent network solids

Answer 6

Atoms are held together by covalent bonds in a giant 3d lattice structure

• High melting points
• Poor electrical conductors (exceptions: graphite & graphene)
• Typically insoluble in common solvents
• Generally hard, though in graphite the layers can slide past one another

Question 7

Graphene’s unique properties

Answer 7

• Thinnest and strongest of known materials, first two-dimensional crystal ever discovered
• 300 times more efficient than copper as a thermal and electrical conducter
• When rolled up into a sphere, it becomes fullerene
• Future applications: graphene plastic composite materials to replace metals in aerospace industry & LCD & screens

Question 8

Coordinate covalent bonding

Answer 8

The shared pair of electrons originate from one of the two bonding atoms

Question 9

The order of intermolecular forces

Answer 9

London forces < dipole-dipole forces < hydrogen-bonds

Question 10

Unique properties of + what is C60 fullerene

Answer 10

Carbon allotrope, atoms arranged in closed shells (trigonal pyramidal structure). Similar shape to soccerball, buckminsterfullerene or buckyballs. C60 is composed of individual molecules with strong covalent bonds but with weak London forces between. Can be used as gene or drug carriers, “cages”.

Question 11

What is silicon dioxide

Answer 11

SiO2. Amorphous (solid with no ordered structure) as sand. Called quartz in its crystalline form. Si-O-Si bonding in a lattice. High melting point due to strong covalent bonds

Question 12

What are London Forces

Answer 12

Aka dispersion force or instantaneous induced dipole-induced dipole forces. At a given instant of time, a non-polar molecule might have slightly more electron density than another part. This is the instantaneous dipole and will influence adjacent molecules.
The magnitude is dependent on 
● number of electrons
● size (volume) of the electron cloud
● shapes of molecules.

Question 13

Polarizability

Answer 13

The ease of distortion of the electron cloud of a molecular entity by an electric field.

Question 14

How is the strength of the London force related with the atomic radii, and thus amount of electrons?

Answer 14

V ∝ 1/r^6
Where V is potential energy associated with the interactions.
Also related to size (volume) of the electron cloud and the shapes of molecules

Question 15

Dipole-dipole moments

Answer 15

In this type of intermolecular force, there is an attraction between the positive end of one permanent dipole and the negative end of another permanent dipole on an adjacent molecule

Question 16

Hydrogen bonding (occur with N-H, O-H & F-H)

Answer 16

he hydrogen bond is an attractive interaction between a hydrogen atom from a molecule or a molecular fragment X–H in which X is more electronegative than H, and an atom or a group of atoms in the same or a different molecule, in which there is evidence of bond formation.

Question 17

Differentiate between electronegativity values

Answer 17

0 < Δχ < 0.4 = NP
0.5 ≤ Δχ ≤ 1.8 = P
Δχ > 1.8 = ionic

Question 18

Graphite’s properties

Answer 18

Has flat layers of hexagonal carbon rings (trigonal planar) attracted by weak intermolecular forces. The london forces are weak, hence the layers can slide past each other - hence it is a lubricant.
Good electrical conductor as it has delocalised pi electrons (the carbons only have 3 bonds).

Question 19

Diamond’s properties

Answer 19

Carbons form 4 covalent bonds in tetrahedral structures. No delocalised electrons, hence not conductive. Extremely high melting point and very hard.

Question 20

Coordination number

Answer 20

Number of ligands in a transition metal complex

Question 21

Ammine

Answer 21

Any of a class of inorganic coordination compounds of ammonia and a metallic salt.

Question 22

Calculate formal charge

Answer 22

FC = (#valence electrons) - 1/2(#bonding electrons) - (#non-bonding electrons).

Question 23

Why is formal charge useful?

Answer 23

Used to determine the preferred Lewis structure

Question 24

How to decide for the most stable Lewis structure

Answer 24

• The one with difference in formal charge closest to zero.

- The one with the negative charge closest to the most electronegative ion.

Question 25

Outline which orbitals go into sigma and pi bonds

Answer 25

Sigma:
s + s
s + px
px + px

Pi:
py+py
pz+pz

Question 26

Bond order

Answer 26

Calculated by total number of bonds / total number of resonance structures.

Question 27

What determines strength of a metallic bond

Answer 27

• charge of cation
• # delocalised electrons
• radius of cation

Question 28

F = K_c * (q1*q2)/r^2

Answer 28

Force = constant * (charge 1 * charge 2)/radius^2

Question 29

In terms of CHn
sp^3
sp^2
sp

Answer 29

sp^3: CH4
sp^2: C2H4
sp: C2H2