6 - Shapes of molecules and intermolecular forces

Question 1

electron pair repulsion theory

Answer 1

• e- surrounding a central atom determine the shape of the molecule
• the e- pairs repel one another so they are arranged as far as possible
• the arrangement of e- pairs minimises repulsion and holds the atoms in a definite shape

Question 2

state the trend of increasing repulsion in molecules
why

Answer 2

increasing repulsion downwards

bonded pair and bonded pair

bonded pair and lone pair

lone pair and lone pair

• lone pair of electrons are slightly closer to central atom and occupy more space, so the lone pair repels more strongly than a bonding pair

Question 3

BP: 2
LP: 0

Answer 3

linear
180

Question 4

BP: 3
LP: 0

Answer 4

trigonal planar
120

Question 5

BP: 4
LP: 0

Answer 5

tetrahedral
109.5

Question 6

BP: 6
LP: 0

Answer 6

octahedral
90

Question 7

BP: 3
LP: 1

Answer 7

pyramidal
107

Question 8

BP: 4
LP: 3

Answer 8

square planar

Question 9

BP: 5
LP: 0

Answer 9

trigonal bipyramidal

Question 10

explanation for shape of a molecule

Answer 10

State bonding pairs and lone pairs in molecule

Electron pairs around the central atom repel as far apart as possible and lone pairs repel more than bonding pairs and decrease the bond angle (by 2.5)

greater the electron pairs the smaller the bond angle

Question 11

electronegativity

Answer 11

the attraction of a bonded atom for the pair of electrons in a covalent bond

Question 12

how does EN increase in the periodic table

Answer 12

across the table nuclear charge increases and atomic radius decreases

therefore

en increases across the table and up the table

therefore

most EN elements are O, F and Cl

Question 13

non-polar bond

Answer 13

bonded electron pair is shared equally between the bonded atoms

• happens when bonded atoms have the same of similar EN

Question 14

polar bonds

Answer 14

the bonded electron pair is shared unequally between the bonded atoms

• happens when bonded atoms have different EN

Question 15

polar molecules

Answer 15

• have one polar bond (permanent dipole)
• or has more than one permanent dipole that act in different directions but don’t oppose each other so don’t cancel each other
• molecule is not symmetrical

Question 16

non-polar molecules

Answer 16

dipoles oppose each other and cancel each other out
- the molecule is symmetrical

Question 17

three intermolecular forces

Answer 17

induced dipole-dipole
permanent dipole-dipole
hydrogen bonding

Question 18

intermolecular forces

Answer 18

weak interactions between dipoles of different molecules

Question 19

induced dipole-dipole

Answer 19

Movement of electrons produces a changing
dipole in a molecule, creating an instantaneous
dipole

The instantaneous dipole induces a dipole on a
neighbouring molecule

The induced dipole induces further dipoles on
neighbouring molecules, which then attract one
another

Question 20

the strenght of induced

Answer 20

• the larger the instantaneous and induced dipole, the greater the induced dipole-dipole so stronger attractive forces between molecules
• larger number of e- means larger induced dipoles
  -> more energy is needed to overcome the IF

Question 21

permanent dipole-dipole

Answer 21

interactions between the permanent dipoles in different polar molecules

Question 22

simple molecular substances

Answer 22

substances made of simple molecules - small units containing a definite number of atoms with a definite molecular formula

Question 23

simple molecular lattice

Answer 23

• molecules are held in place by weak IF
• atoms in each molecule are bonded by covalent bonds strongly

Question 24

simple molecular substances Tm and Tb

Answer 24

• low Tm
• as IF are weak, so little energy is needed to overcome them

Question 25

Simple molecular substances solubility

Answer 25

Non-polar
-> in a non-polar solvent = IF forms between the simple substance and the solvent and this weakens the IF in the simple molecular lattice. the compound dissolves
-> in polar solvent = little interactions between simple substance and the solvent so IF in the simple molecular lattice is too strong

Polar
-> may dissolve in polar solvents as they can attract each other
—- sugar, HCL
-> it does depend on the strength of the dipole. eg ethanol contains polar OH and non-polar carbon chain so can dissolve in both non-polar and polar

Question 26

electrical conductivity of simple molecular substances

Answer 26

• there are no mobile charged particles in simple molecular structures
• so there is nothing to complete an electrical circuit

Question 27

hydrogen bond

Answer 27

A hydrogen bond is an attraction between a
lone pair of electrons on an electronegative
atom in one molecule and a hydrogen atom in
another molecule attached to an electronegative
atom

strongest IF

Question 28

why is ice less dense than water

Answer 28

Ice is less dense than water
-Because H2O molecules in ice are held apart by
hydrogen bonds in open lattice structure
- so water molecules in ice are further apart in ice than water. so it is less dense

-

Question 29

water has a high Tm and Tb

Answer 29

Because hydrogen bonds are stronger than
other intermolecular forces so more energy is
needed to break the hydrogen bonds