C6 - Shapes of Molecules and Intermolecular Forces

Question 1

What do the different lines on a diagram of a molecule represent?

Answer 1

A solid line = a bond in the plane of the paper

A solid wedge = comes out of the plane of the paper

A dotted wedge = goes into the paper

Question 2

What determines the shape of a molecule?

Answer 2

The number of electron pairs around a central atom.
The electron pairs are as far apart as possible to minimize repulsion between them.
Different numbers of electron pairs result in different shapes.

Question 3

How do you calculate the shape of a molecule?

Answer 3

1) Work out the # of electrons in the outer energy level of the central atom
2) Consider any charge on the substance e.g. add an electron for each negative charge and vise versa
3) Calculate the number of electrons from the central atom involved in covalent bonding (# of covalent bonds made) and add all the electrons
4) Divide the total number of electrons around the central atom by 2 to get the # of electron pairs.
5) Determine the basic shape

Question 4

What molecular shape is made from 2 electron pairs?

What is the angle between atoms?

Answer 4

Linear shape

180 degrees

Question 5

What molecular shape is made from 3 electron pairs?

What is the angle between atoms?

Answer 5

Trigonal planar

120 degrees

Question 6

What molecular shape is made from 4 electron pairs?

What is the angle between atoms?

Answer 6

Tetrahedral

109.5 degrees

Question 7

What molecular shape is made from 6 electron pairs?

What is the angle between atoms?

Answer 7

Octahedral

90 degrees

Question 8

What are double bonds?

How are they treated?

Answer 8

2 bonding pairs of electrons which are treated as electron pairs known as regions of electron density.
Multiple bonds are therefore treated the same as single bonds.

Question 9

What is the structure of ammonia?

NH3

Answer 9

There are 4 electron pairs around the N atom however one is a lone pair (which has a stronger force of mutual repulsion as opposed to a L-B bond) so it’s structure is not tetrahedral.
Its structure will be “pyramidal”.
Its angle will be reduced by 2.5 degrees so is 107 (rather than 109.5 degrees.)

Question 10

What is the structure of water?

Answer 10

There are 4 electron pairs but 2 of them are lone pairs (which have a stronger force of mutual repulsion as opposed to a L-B bond) so its structure is not tetrahedral.
Its structure will be “non-linear”.
Its angle will be reduced by 5 degrees (2 * 2.5) so is 104.5.

Question 11

What is electronegativity?

Answer 11

The ability of an atom to attract electrons in a covalent bond.
It is affected by atomic radius, the number of unshielded protons and nuclear charge.

Question 12

Why does electronegativity increase across periods?

Answer 12

Because, going left to right, nuclear charge increases (as a proton is added each time) and atomic radius decreases (as the stronger nuclear charge brings the electrons closer to the nucleus) which attracts electrons closer the the nuclei, increasing negativity.

Question 13

Why does electronegativity decrease going down the group?

Answer 13

Atomic radius increases and full energy levels shield the electrons in the bond from the increased attraction of the greater nuclear charge, reducing electronegativity.

Question 14

Why does atomic radius decrease across a period?

Answer 14

The stronger positive charge pulls electrons closer.
Charge increases without significant extra shielding by the e- and new e- don’t contribute greatly to shielding as they’re added to the same principal energy level.

Question 15

What happens if the electronegativity difference is greater than 1.8?

Answer 15

The more electronegative atom will have gained control of the electrons so is considered ionic rather than covalent.

Question 16

What happens if the electronegativity of both atoms in a covalent bond are identical?

Answer 16

The e- in the bond will be equally attracted to both of them, resulting in a symmetrical distribution of electron density around the atoms.

Question 17

What happens to the polar bonds in non-polar molecules?

Answer 17

If the polar bonds are arranged symmetrically, the partial charges cancel each other out, becoming non-polar. e.g. CO2
This also occurs when the bonded atoms are the same or have similar electro-negativities, e.g. H2, producing pure covalent bonds.

Question 18

What happens to the polar bonds in non-polar molecules?

Answer 18

If the polar bonds are arranged asymmetrically, the partial charges do not cancel out and the molecule is polar e.g. H2O.
This also occurs when bonded atoms are different or have different electro-negativities e.g. HCl

Question 19

Why do electrons arrange themselves as far apart as possible?

Answer 19

To minimize repulsion

Question 20

What’s the order of repulsion between lone and bonding pairs of electrons?

Answer 20

(Highest repulsion)

Lone pair & Lone pair

Lone pair & Bonded pair

Bonded pair & Bonded pair

(Lowest repulsion)

Question 21

What are intermolecular forces?

Answer 21

Weak interactions between dipoles of different molecules.

Question 22

What does bond enthalpy measure?

Answer 22

Covalent bond strength

Question 23

What molecule shapes have bond angles of 109.5, 107 and 104.5 degrees?

Answer 23

109.5 - tetrahedral e.g. CH4
107 - pyramidal e.g. NH3
104.5 - non linear e.g. H2O

Question 24

What are the 3 types of intermolecular forces?

Answer 24

Induced dipole-dipole interactions (London forces)

Permanent dipole-dipole interactions

Hydrogen bonding

Question 25

What are London forces also known as?

Answer 25

Induced dipole-dipole forces

Question 26

What are induced dipole-dipole interactions?

Answer 26

[London forces] are weak intermolecular forces which exist between ALL molecules, polar and non-polar.

They act between induced dipoles in different molecules.

Question 27

How are induced dipole-dipole forces produced?

Answer 27

Random movement of electrons produces a changing dipole in a molecule. (interactions of e-)

At any instant, an ‘instantaneous’ dipole will exist however its position is continuously changing.

The instantaneous dipole induces a dipole on a neighbouring molecule. (The e- of one molecule repel the e- of another molecule so there is a slight positive charge nearer to the electrons and repulsion is reduced, forming another dipole)

The ‘induced’ dipole induces further dipoles on further neighbouring molecules which then attract one another (+ - + - + -)

They are the weakest of intermolecular forces. This is due to how the electrons are constantly moving so the interaction is only brief. This then results in the formation of London forces elsewhere.

Question 28

What are Van der Waals’ forces?

Answer 28

Van der Waals’ forces implies both induced and permanent dipole-dipole interactions whereas London forces is just for induced interactions.

Question 29

What affects the strength of London forces?

Induced d.d. interactions

Answer 29

The number of e-
The more e- in each molecule:
- the larger the instantaneous and induced dipoles
- the greater the induced interactions
- the stronger the attractive forces between molecules

Question 30

What is a permanent dipole-dipole interaction?

Answer 30

Interactions between the permanent dipoles in different polar molecules. (the slight charges present in polar molecules only).
They are stronger than induced dipole-dipole interactions therefore molecules with induced d.d. forces AND permanent d.d. forces require MORE energy for the interaction to be broken.
e.g HCl (requires more energy) compared to simple molecules like F2.

Question 31

What is a hydrogen bond?

Answer 31

A special type of permanent dipole-dipole interaction found between molecules that contain a hydrogen atom attached to a very electronegative atom with a lone pair of electrons e.g. O, N or F

It is represented as a dashed line.

They are the strongest type of intermolecular force (not as strong as single covalent bonds)

Question 32

What are the anomalous properties of water?

Answer 32

• Ice is less dense than water
• Water has relatively high melting/boiling points
• Surface tension

Question 33

Why is ice less dense than water?

Answer 33

Each water molecule can form 4 hydrogen bonds which hold water molecules apart in an open tetrahedral lattice structure when frozen therefore the water molecules in ice are further apart than in water.

The gaps/holes within the structure therefore decrease the density of ice so it is able to float.

This enables ice to act as an insulating layer and doesn’t destroy marine habitats

Question 34

Why does water have relatively high melting/boiling temperatures?

Answer 34

Water has many hydrogen bonds as well as London forces.

The hydrogen bonds are much stronger than London forces so require much more energy for these forces to be overcome.
When ice lattices break, the rigid arrangement of H bonds are broken and when water boils, the bonds are broken completely.

Question 35

What is surface tension?

Answer 35

Surface tension is a result of the forces present at the surface of a liquid due to cohesion.
The water molecules at the surface of water have no other water molecules above them so only have hydrogen bonds below them which bind the molecules together in cohesion.

Cohesion therefore causes the molecules to be pulled down producing a force or ‘tension’ across the surface of the water.
This appears as though the water has an elastic ‘skin’ which has been stretched across the water. This is all as a result of the hydrogen bonds formed between water molecules.

Question 36

How does hydrogen bonding link to DNA?

Answer 36

DNAs double helix structure is held together by H bonds which enables a single DNA strand to create a perfect copy of itself by replication.

Replication depends on the four bases, A, T, G and C.

• A & T pair by forming 2 H bonds
• C & G pair by forming 3 H bonds

The shape and chemical structure of these bases ensure they pair up correctly.

A & G are both purines with double ringed structures and T & C are single ringed pyrimidines.

H bonding in the double helix can only take place between a purine and pyrimidine.
The bases must fit together so that a H atom from one molecule and an electronegative atom (O or N) from the other molecule are aligned correctly.

Question 37

What are simple molecules?

Answer 37

Small units containing a definite number of atoms with a definite molecular formula e.g. Ne, H2, H2O and CO2.
Simple molecular substances are made up of these simple molecules.
When solid, they form simple molecular lattices held by weak intermolecular forces and atoms within each molecule are bonded together strongly by covalent bonds.
They’re all covalently bonded and have no charged particles so aren’t conductive.

Question 38

What is the melting and boiling point of simple molecules?

Answer 38

They have low melting and boiling points.
In a simple molecular lattice, weak intermolecular forces can be broken by energy present at low temperatures.

Only the weak intermolecular forces break, not the strong covalent bonds.

Question 39

What is the solubility of simple molecules?

Answer 39

• Non-polar simple substances are soluble in non-polar substances as intermolecular forces are formed between the molecules and the solvent.
  The interactions weaken the intermolecular forces in the simple lattice and break the compound.
  (Similarly, polar substances dissolve in polar solvents)

Simple molecules aren’t soluble in polar substances as there is little interaction between the molecules in the lattice and the solvent and the permanent d.d. bonds in the polar substance are too strong to be broken

E.g. iodine (n.p.) is soluble in cyclohexane (n.p.) but not water

Question 40

What is the solubility of polar simple molecular substances?

Answer 40

They dissolve in polar solvents as the polar solute and solvent molecules attract each other.
E.g. HCl is soluble in water
Solubility depends on the strength of the dipole. Some compounds have polar (hydrophilic) and non-polar (hydrophobic) parts to their structure

Question 41

What two dipoles are created during induced dipole dipole attractions?

Answer 41

Instantaneous dipoles followed by induced dipoles