C6- Shapes of Molecules and Intermolecular forces

Question 1

Electron Pair repulsion theory

Answer 1

Each electron region takes up a position to minimise repulsion

Question 2

Molecular shape

2 Areas of electron density

Answer 2

180

Linear

Question 3

Molecular shape

3 Areas of electron density

Answer 3

Trigonal planar

120

Question 4

Molecular shape

6 areas of electron density

Answer 4

Octahedral

90

Question 5

Molecular shape

5 areas of electron density

Answer 5

Trigonal bipyramidal

Two bonds at 180
other 3 in a plane at 120

Question 6

Molecular shape

4 areas of electron density

Answer 6

depends on number of lone pairs present on central atom

lone pairs repel more than bonded pairs

2.5 degrees lost per lone pair gained

Question 7

Molecular shapes

4 AED 0 lone pairs

Answer 7

Tetrahedral

109.5

Question 8

Molecular shapes

4 AED 1 Lone pair

Answer 8

Pyramidal

107

Question 9

Molecular shapes

4 AED 2 Lone pairs

Answer 9

Non Linear

104.5

Question 10

Molecular shape

wedge/ dash

Answer 10

wedge= towards

dash= away

Question 11

Electronegativity

Answer 11

ability of an atom to attract a pair of electrons in its own covalent bonds

Question 12

Pauling scale

Answer 12

Compare electronegativity of atoms of different elements

Large value= Very electronegative

As nuclear charge increases, atomic radius decreases

Question 13

Most electronegative atoms

Answer 13

F O N Cl

( and group 7)

Question 14

Polar bond

Answer 14

Asymmetric distribution of charge across the bond

Polar covalent bond

Delta= Slight charge

Question 15

Non Polar bond

Answer 15

Pure covalent bond

Bonded electron pair is shared between bonded atoms

Usually:
- Atoms are the same e.g. Br2
- Atoms have the same/ similar electronegativity values

Question 16

Dipole

Answer 16

Permanent dipole

Separation of opposite charges

Question 17

Polar molecule

Answer 17

There is a difference in electronegativities/ the bonds are polar

Molecule is not symmetrical therefore dipole do not cancel out

Question 18

Polar solvents and solubility

Answer 18

Polar solvent molecules attract ions

Ionic lattice breaks down as it dissolves

Solution surrounds ions

Question 19

Permanent Dipole- Dipole interactions

Answer 19

Permanent dipoles attract one and other

Molecules must be polar

Question 20

London Forces

’ induced dipole-dipole interactions ‘

Answer 20

ALL molecules

Movement of electrons produces changing dipoles

At one moment instantaneous dipole exists, position shifting

Instantaneous dipole induces dipole on neighbouring molecule

Induced dipole induces further dipoles, attract one and other

Question 21

Strength of london forces

’ induced dipole-dipole interactions’

Answer 21

More Electrons=

Larger instantaneous and induced dipoles

Greater the induced dipole-dipole interactions

stronger attractive forces between molecules

Question 22

Explain how an instantaneous dipole formed

Answer 22

Fluctuation in electron density in electron density around a molecule creates an instantaneous dipole in a molecule

The instantaneous dipole induces a dipole in a neighbouring molecule

Question 23

Why does I have a higher boiling point than Br

group 7 = F, Cl, Br, I, At

Answer 23

I2 molecules have more electrons

Form stronger London dispersion forces

Require more energy to overcome

Question 24

Why does HCl have a higher melting point than Cl2

Answer 24

HCl is a polar molecule, so forms permanent dipole forces

These are stronger than London dispersion forces formed by non polar molecules like Cl2

HCl requires more energy to overcome forces

Question 25

H bonding

Answer 25

Specific stronger example of a permanent dipole dipole interaction Small electronegative atom with lone pair interacts with a H atom which is bonded at 180 to N, O or F

Question 26

Criteria for drawing H bonds

Answer 26

Draw all lone pairs Draw all partial charges H bond from atom to lone pair, drawn at 180

Question 27

Simple molecular substance Low melting point and boiling point

Answer 27

In a simple molecular lattice (solid) weak intermolecular forces can be overcome by energy present at low temperatures Covalent bonds are not overcome

Question 28

Non polar simple molecular substance in non polar solvent

Answer 28

Soluble Intermolecular forces form between molecules and the solvent Intermolecular forces overcome and compound dissolves

Question 29

Non polar simple molecular substance in a polar solvent

Answer 29

Insoluble Little interaction between molecules in lattice and solvent molecules The intermolecular bonding within polar solvent is too strong to be broken

Question 30

Simple molecular substances Solubility

Answer 30

Polar --> dissolves similar to an ionic compound Solubility depends on strength of dipole

Question 31

Why does water have a relativley high melting point and boiling point

Answer 31

H bonds are extra forces over and above London forces More energy needed to break than with just London forces --> appreciable amount H bonds broken upon boiling

Question 32

Why does ice float in water

Answer 32

Hydrogen bonds hold water molecules apart in an open lattice structure Water molecules are further apart in ice than in water Ice is less dense than water, so it floats

Question 33

Ionic compound Solubility

Answer 33

Charged ions can interact with the polar solvent Breaking down the ionic lattice. The polar molecules attracting and surrounding the ions

Question 34

exam technique for 3 marker on name and explain the molecular shape

Answer 34

Name the shape State number of Bonding pairs and lone pairs Electrons repel as far apart as possible AND lone pairs repel more than bonding pairs