6 - Shapes of molecules and intermolecular forces

Question 1

What is electron pair repulsion theory?

Answer 1

model used in chemistry for explaining & predicting shapes of molecules & polyatomic ions

• electron pairs surrounding central atom determine shape of molecule/ion
• electron pairs repel -> arranged as far apart as possible
• arrangement minimises repulsion & holds atoms in definite shape
• diff numbers of electron pairs results in different shapes

Question 2

Bonded-pair vs lone-pair repulsion

Answer 2

Lone pair of electrons slightly closer to central atom and occupies more space than bonded pair. So lone pair repels more strongly than bonding pair.

(Increasing repulsion)
bonded-pair/bonded-pair < bonded-pair/ lone pair < lone pair/lone pair

Question 3

Shape of molecule with 4 bonded pairs, 0 lone pairs and bond angle

Answer 3

Tetrahedral

109.5 degrees

Question 4

Bond angle and Shape of molecule with 3 bonded pairs, 1 lone pair

Answer 4

Pyramidal

107 degrees

Question 5

Bond angle and Shape of molecule with 2 bonded pairs, 2 lone pairs

Answer 5

Non linear

104.5 degrees

Question 6

Bond angle and Shape of molecule with 2 bonding regions, 0 lone pairs

Answer 6

Linear

180 degrees

Question 7

Bond angle and Shape of molecule with 3 electron regions, 0 lone pairs

Answer 7

Trigonal planar,

120 degrees

Question 8

Bond angle and Shape of molecule with 6 bonding regions, 0 lone pairs

Answer 8

Octahedral

90 degrees

Question 9

Bond angle and Shape of molecule with 5 bonding regions, 0 lone pairs

Answer 9

Trigonal Bipyramidal

90 degrees and 120 degrees

Question 10

Bond angle of CH4

Answer 10

109.5 degrees

Question 11

Bond angle of NH3

Answer 11

107 degrees

Question 12

Bond angle of H2O

Answer 12

104.5 degrees

Question 13

Define electronegativity

Answer 13

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

Question 14

How is electronegativity measured?

Answer 14

Using Pauling electronegativity values
Electronegativity increases towards F

• going right = more protons, Nuclear charge becomes more positive, more electronegative
  – going up = smaller radius, more electronegative

As you go left to right, atom size slightly decreases because more protons, and electrons are more attracted to them

Question 15

What makes an element more electronegative?

Answer 15

1) nuclear charge.
When there is more positive charge it is more electronegative

2)radius of atom.
Smaller radius atoms are more electronegative because electrons are closer, so there is less shielding

Question 16

Electronegativity difference = 0

Answer 16

(Pure) covalent

Question 17

Electronegativity difference = 0 to 1.8

Answer 17

Polar covalent

Question 18

Electronegativity difference > 1.8

Answer 18

Ionic bond

Question 19

What is a nonpolar bond

Answer 19

The bonded electron pairs are shared equally between the bonded atoms.

The bonded atoms are the same OR have same/similar electronegativity

Eg;
Cl2
Hydrocarbons

Question 20

What is a polar bond?

Answer 20

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

Electronegativity difference must be greater than 0.5.

Question 21

Dipoles in polar bonds

Answer 21

In H – Cl bond, H has a small partial positive charge and Cl has a small partial negative charge.

Separation of opposite charges is called a dipole

A dipole in a polar covalent bond does not change. PERMANENT DIPOLE.

Question 22

Polarity in polar molecules

Answer 22

For molecules with 2+ atoms there may be 2+ polar bonds

Polar molecule requires polar bonds with dipoles that do not cancel due to their direction. CO2 does not have a permanent dipole but H2O does

Question 23

Define intermolecular forces. What types?

Answer 23

An attractive force between molecules // weak interactions between dipoles of different molecules

Can be London forces, permanent dipole-dipole interactions or hydrogen bonding

Question 24

Covalent vs intermolecular forces

Answer 24

Covalent: strong, holds atoms in a molecule together. Determine identity and chemical reactions of molecules

Intermolecular: weak interactions between dipoles of different molecules. largely responsible for physical properties like melting & boiling points

Question 25

What are London (dispersion) forces / induced dipole-dipole interactions?

How do they arise?

Answer 25

1) Weak intermolecular forces that exist between ALL molecules (polar or non polar)
Act BETWEEN induced dipoles in different molecules

2) movement of electrons produces changing dipole in molecule. @ any moment, instantaneous dipole will exist (bits it’s position is always shifting). instantaneous dipole induces dipole on neighbouring molecule. induced dipole further induced dipoles on neighbouring molecules, which then attract one another.

Question 26

Which London forces are stronger?

Link to BP

Answer 26

Bigger atoms = more electrons = bigger instantaneous dipoles = stronger London forces between particles = increased energy needed to overcome intermolecular forces = higher BP

Question 27

Define permanent dipole-dipole interaction

Answer 27

An attractive force between permanent dipoles in neighbouring POLAR molecules (polar -> electronegativity difference greater than 0.5)

Question 28

Define a simple molecular substance

Answer 28

Made up of simple molecules - small units containing a definite number of atoms with a definite molecular formula, such as neon Ne, hydrogen H2, water H2O and carbon dioxide CO2

Question 29

Define simple molecular lattice (structure, basically)

Structure of simple molecular lattice

Answer 29

Three dimensional structure of molecules, bonded together by weak intermolecular forces

In solid state, simple molecules form a regular structure called simple molecular lattice. Molecules held in place by WEAK intermolecular forces. Atoms within each molecules bonded together STRONGLY by covalent bonds

Question 30

Melting and boiling points of simple molecular substances (bonding properties)

Answer 30

All simple molecular substances are covalent, & can be solidified into simple molecular lattices by reducing temperature

In simple molecular lattice, weak intermolecular forces can be broken even by the energy present at low temperatures. Low melting and boiling points.

When a simple molecular lattice is broken apart during melting, only WEAK intermolecular forces BREAK, covalent bonds are strong and do not break.

Question 31

Electrical conductivity in simple molecular substances

Answer 31

There are no mobile charged particles and simple molecular structures. With no charge particles that can move there is nothing to complete an electrical circuit. Therefore, non-conductors of electricity

Question 32

Can polar solute dissolve in polar solvents?

Answer 32

Yes

Polar solute molecules and polar solvent molecules can attract each other. Process is are similar to dissolving of an ionic compound.

Question 33

Can nonpolar solute dissolve in nonpolar solvent?

Answer 33

Yes

When a simple molecular compound is added to a nonpolar solvent, intermolecular forces FORM BETWEEN the molecules and the solvent. The interactions WEAKEN the intermolecular forces in the simple molecular lattice. These BREAK and the compound DISSOLVES.

Question 34

Can polar solute dissolve in nonpolar solvents?

Answer 34

No

Question 35

Can nonpolar solute dissolve in polar solvent?

Answer 35

No

When a simple molecular substance is added to a polar solvent, there is little interaction between the molecules in the lattice and the solvent molecules. The intermolecular bonding within the polar solvent is too strong to be broken.

Question 36

Solubility of alcohols

Answer 36

The solubility depends on the strength of the dipole and can be hard to predict. Some compounds such as alcohol is contain both polar (O—H) and nonpolar (carbon chain) parts in their structure and can dissolve in both polar and nonpolar solvents

Question 37

Solid structure of simple molecular lattices

Answer 37

Covalently bonded molecules attracted by intermolecular forces

Question 38

Define Hydrogen bonding

Answer 38

Spec: intermolecular bonding between molecules containing N, O or F and the H atom of -NH, -OH or -FH

Glossary: A strong dipole-dipole attraction between an electron deficient hydrogen atom of -NH, -OH or -HF on one molecule and a lone pair of electrons on a highly electronegative atom containing N, O or F on a different molecule

Question 39

Describe anomalous properties of water resulting from hydrogen bonding, eg density of ice compared to water.

Answer 39

With two lone pairs on the oxygen atom and two hydrogen atoms, each water molecule can form for hydrogen bonds.
The hydrogen bonds extend outwards holding water molecules slightly apart and forming an open tetrahedral lattice full of holes.
The bond angle about the hydrogen atom involved in the hydrogen bond is close to 180°.
The holes in the open lattice structure decrease the density of water on freezing.

When ice melts, the ice lattice collapses and molecules move closer together. So liquid water is denser than solid ice.

Question 40

Describe anomalous properties of water resulting from hydrogen bonding, eg its relatively high MP & BP

Answer 40

Water has London forces between molecules. Hydrogen bonds are extra forces, over and above the London forces.
An appreciable quantity of energy is needed to break the hydrogen bonds in water, so water has much higher MP & BP than would be expected just from London forces.
When the ice lattice breaks, the rigid ARRANGEMENT of hydrogen bonds and ice is BROKEN. When water boils, the hydrogen BONDS BREAK completely.