2.2

Question 1

what is a discrete molecule

Answer 1

an electrically neutral group of two or more atoms held together by chemical bonds

Question 2

state some exceptions to the octet rule

Answer 2

beryllium chloride
boron trichloride
phosphorus(V) chloride
sulfur hexafluoride

Question 3

what is a displayed formula

Answer 3

shows each bonding pair as a line drawn between the atoms involved (H-O-H)

Question 4

what is a dative bond

Answer 4

a dative bond is formed when an empty orbital from one atom overlaps with an orbital containing a lone pair of electrons of another atom

Question 5

what is the electron pair repulsion theory

Answer 5

the shape of a molecule or ion is caused by repulsion between bonding pairs and lone pairs of electrons that surround the central atom
the electron pair arrange themselves around the central atom so that the repulsion between them is a minimum

Question 6

what shape is a molecule with 2 bond pairs and its bond angle

Answer 6

linear
180

Question 7

what shape is a molecule with 3 bond pairs and its bond angle

Answer 7

trigonal planar
120

Question 8

what shape is a molecule with 4 bond pairs and its bond angle

Answer 8

tetrahedral
109.5

Question 9

what shape is a molecule with 5 bond pairs and its bond angles

Answer 9

trigonal bipyramidal
90 and 120

Question 10

what shape is a molecule with 6 bond pairs and its bond angle

Answer 10

octahedral
90

Question 11

what shape is a molecule with 3 bond pairs and 1 lone pair and its bond angle

Answer 11

trigonal pyramidal
107

Question 12

what shape is a molecule with 2 bond pairs and 2 lone pairs and its bond angle

Answer 12

V-shaped
104.5

Question 13

define dipole

Answer 13

exists when two charges of equal magnitude but opposite signs are separated by a small distance

Question 14

what is a dipole caused by

Answer 14

the more electronegative element attracts the electrons resulting in separation of charge

Question 15

what molecule shapes are symmetrical (therefor dipoles cancel out)

Answer 15

linear
trigonal planar
tetrahedral (some exceptions such as CHCl3)

Question 16

explain london forces

Answer 16

electron density fluctuates over time in a molecule
at any instant the electron density can become unsymmetrical generating an instantaneous dipole
which then induces a dipole in a neighboring molecule
these dipoles then attract one another

Question 17

state 3 features of london forces

Answer 17

attractive force increases with increasing number of electrons in a molecule
the strength of these attractions depends upon the shape and size of the molecules (the more points of contact between molecules the greater the london forces)
they’re always present with any molecules

Question 18

explain permanent dipoles

Answer 18

if 2 permanent dipoles are aligned correctly they will attract one another

Question 19

are london forces or permanent dipoles stronger and why

Answer 19

london forces are stronger because in order for permanent dipoles to occur the molecules must be correctly aligned

Question 20

define hydrogen bond

Answer 20

an intermolecular interaction between a hydrogen atom of a molecule bonded to an atom which is more electronegative than hydrogen and another atom in the same or a different molecule

Question 21

what are the 3 most common atoms hydrogen bonds form with

Answer 21

oxygen
nitrogen
fluorine

Question 22

what is the rough bond angle of hydrogen bonds

Answer 22

180

Question 23

how do hydrogen bonds form between water molecules

Answer 23

partial bond formation occurs between the lone pairs from an oxygen and a hydrogen atoms of another water molecule

Question 24

what groups can form hydrogen bonds

Answer 24

amines
alcohols

Question 25

what is the only significant interaction between alkane molecules

Answer 25

london forces

Question 26

why does boiling temperature increase as relative molecular mass (chain length) of alkanes increases

Answer 26

number of electrons per molecule increases so london forces are increased number of points of contact increases so london forces increase

Question 27

how does branching of alkanes effect boiling temperature and why

Answer 27

lowers boiling temperature as there are fewer points of contact for london forces

Question 28

why do alcohols have higher boiling temperatures than alkanes

Answer 28

because alcohols have an OH group and can therefor form hydrogen bonds on top of london forces which require more energy to overcome than just london forces

Question 29

why does boiling temperature increase from HCl -> HBr -> HI

Answer 29

due to increased number of electrons per molecule so there is an increase in london forces

Question 30

why does water have a relatively high mp & bp

Answer 30

water has hydrogen bonds which have strong intermolecular forces of attraction that require a lot of energy to overcome

Question 31

why does HF have a lower boiling point than water even though they have the same number of electrons

Answer 31

- both form the same london forces (roughly) - both form hydrogen bonds however HF on average only forms on hydrogen bond per molecule whereas water forms 2 not all of the hydrogen bonds are broken in the vaporization of HF whereas they are in water

Question 32

why is ice less dense than water

Answer 32

ice is arranged in rings of 6 held by hydrogen bonds creating large areas of space inside the rings so when ice melts the ring structure is destroyed and distance between molecules decreases

Question 33

how do solutes dissolve in solvents

Answer 33

the solute particles must particles must separate from each other and become surrounded by solvent particles the force of attraction between solute and solvent must be strong enough to overcome solute-solute attractions and solvent-solvent attractions

Question 34

where does the energy come from when dissolving ionic solids

Answer 34

hydration of ions

Question 35

explain the hydration of an ionic solid

Answer 35

the dipole negative attracts the anions sufficiently to remove them from the lattice and the anion becomes surrounded by water molecules the dipole positive attracts the cations and surrounds the cation in water molecules

Question 36

how is solubility of alcohols effected by chain length

Answer 36

solubility decreases as chain length increases as london forces predominate between the alcohol molecules

Question 37

why may a substance not dissolve in water

Answer 37

it is a non-polar molecule therefor can't form hydrogen bonds with water molecules attractions between the molecule and water is not strong enough to disrupt the hydrogen bond system