2.6 Shapes of Molecules and Forces

Question 1

What is the valence shell repulsion theory ?

Answer 1

-Valence electrons exist in pairs.
-Electron pairs move away from each other to minimise electrostatic repulsion.
-Electron pairs are either bonding or lone pairs.

Question 2

What is the strongest bond - bond repulsion in order ?

Answer 2

Lone pair - Lone pair
Lone pair - Bonding pair
Bonding pair - Bonding pair

Question 3

Linear shape

Answer 3

-2 electron pairs (2bp).
-180 bond angle.
E.g. BeCl2

Question 4

Trigonal Planar

Answer 4

-3 electron pairs (3bp).
-120 bond angle.
E.g. BF3

Question 5

Tetrahedral

Answer 5

-4 electron pairs (4bp).
-109.5 bond angle.
E.g. CH4

Question 6

Pyramidal

Answer 6

-4 electron pair (3bp / 1lp).
-107.0 bond angle.
E.g. NH3

Question 7

Non-linear

Answer 7

-4 electron pairs (2bp / 2lp).
-104.5 bond angle.
E.g. H20

Question 8

Octahedral

Answer 8

-6 electron pairs (6bp).
-90 bond angle.
E.g. SF6

Question 9

What are multiple bonds classed as ?

Answer 9

Multiple bonds, close together, are classed as one area of electron bonding region.

Question 10

What is electronegativity ?

Answer 10

The ability of an atom to attract the shared electron pair in a covalent bond.

Question 11

What are 2 factors that affect electronegativity ?

Answer 11

-Size of the atom.
-Nuclear charges are different.

Question 12

What are Pauling values ?

Answer 12

Used to compare electronegativity of the atoms of different elements. Large value indicates the atoms are very electronegative.
-Covalent = o difference
-Polar covalent = less than 1.8 difference.
-Ionic = more than 1.8 difference.

Question 13

What is a non polar bond ?

Answer 13

Bonded electron pair is shared equally between bonded atoms.

Question 14

What is a polar bond ?

Answer 14

Bonded electron pair is shared unequally between bonded atoms. Atoms will have different electronegativity values resulting in a polar covalent bond.

Question 15

What are polar molecules ?

Answer 15

-Contain polar bonds.
-Not symmetrical.
-Dipoles do not cancel but act in different directions.

Question 16

What are intermolecular forces ?

Answer 16

Intermolecular forces are weak forces of attraction between molecules.

Question 17

London Forces

Answer 17

Temporary dipole - induced dipole.
Variation in external electrical field strength creates a temporary dipole. There is a shift in electron density. This causes a dipole in a neighbouring molecule.

Question 18

What is the typical bond enthalpy for London forces ?

Answer 18

10 KJ mol-1

Question 19

What is the permanent dipole-dipole interaction ?

Answer 19

Molecules with permanent dipoles interact. Creates a permanent dipole-dipole interaction between molecules.

Question 20

What is a typical bond enthalpy of permanent dipole-dipole interaction ?

Answer 20

25 KJ mol-1

Question 21

What is the melting/boiling point of simple molecular substances ?

Answer 21

Low melting and boiling point due to weak intermolecular forces between molecules.

Question 22

Can non polar simple molecular substances dissolve in polar solvents ?

Answer 22

No because non-polar substance only has London forces, whilst polar solvent has London forces and permanent dipole - dipole interactions.

Question 23

Can non polar simple molecular substances dissolve in non polar solvents ?

Answer 23

Yes because they will create London faces between each other as they have London forces between similar molecules.

Question 24

Can polar simple molecular substances dissolve in non polar solvents ?

Answer 24

No because non-polar substance only has London forces but polar substance has permanent dipole-dipole interactions.

Question 25

Can polar simple molecular substances dissolve in polar solvents ?

Answer 25

Yes because all have permanent dipole-dipole interactions so form the dipole-dipole attractions between each other.

Question 26

What is the electrical conductivity of simple molecular
substance ?

Answer 26

Doesn’t conduct as they are not charged

Question 27

What are hydrogen bonds ?

Answer 27

Attraction between an electron deficient hydrogen atoms on one molecule and a lone pair of electrons on an electronegative atom of a neighbouring molecule.

Question 28

How many hydrogen bonds can form to one water molecule ?

Answer 28

Water can form 4 hydrogen bonds.

Question 29

What is the anomalous property of water in terms of boiling point ?

Answer 29

Boiling point of H2O is relatively high due to hydrogen bonds that require more energy to break, increasing boiling point.

Question 30

What is the anomalous property of water in terms of density ?

Answer 30

Hydrogen bonds fix further apart from one another, which creates more space between molecules so is less dense. The volume in a solid is bigger than the volume in a liquid, so ice is less dense.

Question 31

What is the anomalous property of water in terms of surface tension ?

Answer 31

Hydrogen bonds hold water molecules together more than without bonds, which creates surface tension.