F321: Molecules and IM forces

Question 0

Covalent bond

Answer 0

A shared pair of electrons

Question 1

Molecules

Answer 1

Atoms covalently bonded together

Question 2

Dative covalent bond

Answer 2

Both electrons Ina shared pair are donated/originate from one atom

Question 3

Electronegativity

Answer 3

The ability of an atom to attract the bonding electrons in a covalent bond

Question 4

Trends in electronegativity

Answer 4

Increases across period
Decreases down the group
(Cl < O)

Question 5

Dipole/polar

Answer 5

Charge separation within a species leading to two regions (poles) of opposite charge

Question 6

Non-polar

Answer 6

Symmetry in distribution of electron density around atom/molecule

(Remember a molecule with polar bond can be non-polar if the molecule is symmetrical so the bond dipoles cancel out)

Question 7

Permanent bond dipole/polar covalent bond

Answer 7

Arises when the atoms sharing a pair of electrons have sufficient difference in electronegativities
(C-H usually considered close to non-polar)

Question 8

Shape with 6 bonding pairs (eg. SF6)

Answer 8

Octahedral

90 degree bond angles

Question 9

Shape with 4 bonding pairs (eg. CH4)

Answer 9

Tetrahedral

109.5 degree

Question 10

Shape with 3 bonding pairs, 1 lone pair (eg. NH3)

Answer 10

Pyramidal

107 degrees

Question 11

Shape with 2 bonding pairs, 2 lone pairs (eg. H2O)

Answer 11

Non-linear

104.5 degrees

Question 12

Shape with 3 bonding pairs (eg. BF3)

Answer 12

Trigonal planar

120 degrees

Question 13

Shape with 2 bonding pairs (eg. CO2)

Answer 13

Linear

180 degrees

Question 14

Instantaneous dipole- induced dipole (Van Der Waals’)

Answer 14

Electrons are constantly in motion and at any one time an uneven distribution of electrons causes an instantaneous dipole in one molecule (temporary), this induces and dipole in a neighbouring molecules.

Question 15

What effects strength of VDW

Answer 15

VDW strength increases with number of electrons (explains increasing bpts down gp.7) and/or greater surface area contact between molecules.

Question 16

Permanent dipole- dipole

Answer 16

Occurs between polar molecules, stronger than VDW

Question 17

Hydrogen bonding

Answer 17

Occurs between the lone pair on N, O, F in one molecule and electron deficient H of N-H/O-H/F-H. N,O,F more electronegative than H leading to bond dipoles.

Strongest intermolecular forces
H bonds in water stronger than in ammonia as O more electronegative
O has two lone pairs so can from twice as many H bonds

Question 18

Anomalous properties of water due to H bonds

Answer 18

Higher mpt than expected
High surface tension
Ice less dense than water (2 hydrogen bonds per molecule of water form lattice holding molecules further apart)