Intermolecular Forces

Question 1

Electronegativity

Answer 1

The ability of an atom to attract a BONDING PAIR of electrons in a covalent bond towards ITSELF.

Question 2

Factor affecting electronegativity 1

Answer 2

SIZE OF THE POSITIVE CHARGE ON THE NUCLEUS:

• moving left to right across a period, the number of PROTONS in the nucleus INCREASES.
• This increased positive charge, increases the ATTRACTION between the NUCLEUS and the pair of electrons in the covalent bond.
• Therefore, elements on the right of the periodic table are more electronegative than those on the left.

( The two electrons in the first shell of an atom CANCEL OUT the effect of TWO PROTONS in each element)

Question 3

Factor affecting electronegativity - Atomic radius

Answer 3

ATOMIC RADIUS:
- The SMALLER the atomic radius , the CLOSER the bonding electrons will be to the nucleus of an atom.

• As we move from left to right across a period, the atomic radius DECREASES.
• Therefore elements on the right of the periodic table are more electronegative than those on the left.

Question 4

Factor affecting electronegativity - number of inner shells

Answer 4

THE NUMBER OF INNER SHELLS:

• Electrons in the inner shell SHIELD electrons in the outer shell from the positive charge of the nucleus.
• Therefore , the greater number of inner shells, the LOWER the electronegativity.

Question 5

Covalent bonds between two IDENTICAL atoms

Answer 5

• The NUCLEUS of each atom attracts the bonding pair of electrons to the EXACT SAME extent.
• The bonding pair will be SHARED EQUALLY between the two atoms and will be found on average HALF WAY between the two nuclei.
• The bond between two identical atoms is NON-POLAR and 100% covalent.

Question 6

Covalent bonds between two DIFFERENT ATOMS

Answer 6

• The more ELECTRONEGATIVE atom will cause its nucleus to attract the bonding pair of electrons MORE STRONGLY than the other atoms nucleus.
• The bonding pair will NOT BE SHARED EQUALLY between the atoms and will on average found CLOSER to the more electronegative atom.
• The bond between two different atoms is said to be POLAR

Question 7

Representing Polar covalent bonds

Answer 7

• Write DELTA + and a DELTA - to show the charges.
• Delta means slight and this is because the electron pair has only SHIFTED towards the more electronegative atom.
• The DELTA - always goes on the MORE ELECTRONEGATIVE atom.
• Alternatively , we use a CROSSED ARROW pointing towards the MORE ELECTRONEGATIVE element.

Question 8

Polar covalent bonds

Answer 8

A separation of OPPOSITE CHARGES.

The separation of opposite charges ACROSS A BOND is called a DIPOLE.

If the difference in charge across the polar covalent bond does not change, it is called a PERMANENT DIPOLE.

Question 9

Polar molecule

Answer 9

When a PERMANENT DIPOLE exists over an UNSYMMETRICAL MOLECULE.

Question 10

Non-polar molecule

Answer 10

• The molecule is SYMMETRICAL.
• The PERMANENT DIPOLES in the bonds CANCEL OUT with each other
• This is because they OPPOSE each other EQUALLY due to the SYMMETRY of the molecule.

Question 11

The polar nature of WATER

Answer 11

• Both O-H bonds have a PERMANENT DIPOLE.
• The molecule is UNSYMMETRICAL with a bond angle of 104.5 degrees and NON- LINEAR shape.
• The dipoles act in different directions and do NOT oppose each other equally and cancel each other out.
• Overall the OXYGEN end has a DELTA - charge and the HYDROGEN end of the molecule has a DELTA + charge.

Question 12

The non-polar molecule CARBON DIOXIDE

Answer 12

• Both C=O bonds have a PERMANENT DIPOLE.
• The molecule is SYMMETRICAL with a bond angle of 180 degrees and a LINEAR shape.
• The dipoles act in opposite directions and exactly oppose each other.
• The overall dipole over the molecule is ZERO because the dipoles CANCEL OUT.
• Therefore, a carbon dioxide molecule has NO OVERALL POLARITY.

Question 13

The Pauling scale of Electronegativity

Answer 13

The scale compares the electronegativity of atoms of different elements.

NON-METALS:
- nitrogen
- oxygen
- fluorine
- chlorine
Are the MOST electronegative atoms.

GROUP 1 METALS:
The LEAST electronegative atoms.

Question 14

Type of bonding and electronegativity

Answer 14

The DIFFERENCE in electronegativity between two atoms indicates the type of BONDING.

PURE COVALENT:
Non-polar bond due to identical atoms bonded together with NO difference in electronegativity.
Electronegativity difference is 0

POLAR COVALENT:
SMALL difference in electronegativity due to NON-IDENTICAL atoms bonded together.

Therefore, PARTIAL charges exist on atoms.
Electronegativity is between 0 and 1.80.

IONIC:
LARGE differences in electronegativity and so ions with FULL charges form.

Electronegativity difference is ABOVE 1.80

Question 15

INTERmolecular forces

Answer 15

• WEAK interactions BETWEEN the dipoles of different molecules.
• Occur because molecules are attracted to each other.
• Determine the PHYSICAL properties of molecules.

Question 16

INTRAmolecular forces

Answer 16

• STRONG interactions that occur WITHIN molecules.
• Refer to either COVALENT or IONIC bonds
• Determine the CHEMICAL REACTIVITY of molecules.

Question 17

Simple molecular substances

Answer 17

• Consist of relatively SMALL molecules and each molecule has a FIXED number of atoms.

LOW boiling point ~ takes very little energy to break intermolecular forces.

• HEATING a simple molecular substance causes the molecules to move faster.
• At a certain temperature, the intermolecular forces break and this allows the molecules to move away from each other.

Question 18

Induced dipole-dipole interactions

Answer 18

Also referred to as LONDON or DISPERSION forces.

ATOM 1 ~
- electrons are randomly moving from place to place and are not evenly spread out.
- For a fraction of a second , more electrons are on the RIGHT hand side of the atom.
- Forms an INSTANTANEOUS DIPOLE ~ right hand side has a delta negative charge and the left hand side has a delta positive charge.

ATOM 2 ~
- Because the right hand side of atom 1 has a delta negative charge, it will REPEL the electrons in atom 2.
- This causes the electrons in atom 2 to move towards the right hand side.
- Forms an INDUCED DIPOLE as it was caused by the dipole in atom 1.

Induced dipoles form in further neighbouring molecules.

ALL of these dipoles now experience a force of attraction known as LONDON forces.

• Typically 0.001 the strength of a covalent bond.

Question 19

Properties of LONDON forces

Answer 19

• Weakest IMF and easily broken
• Caused by RANDOM electron movement.
• Every single atom or molecule will experience London forces even if they experience other IMF as well.
• The STRENGTH depends on the number of ELECTRONS.
  This means atoms with a GREATER number of electrons have a HIGHER BP as they experience strong London forces.

Question 20

PERMANENT dipole-dipole interactions

Answer 20

When two molecules that have a PERMANENT DIPOLE get near enough then their permanent dipoles can lead to an ATTRACTION.

H-F —— H-F ——- H-F—— H-F—–H-F ——–

• POLAR molecules like H-F have permanent dipoles.
• The delta negative charge on the fluorine atom is attracted to the delta positive charge on the hydrogen atom on the neighbouring molecule.
• This results in a WEAK permanent dipole-dipole force.
• This force is typically 0.01 the strength of a covalent bond.

Question 21

The TWO types of intermolecular forces

Answer 21

VAN DER WAALS FORCES:
Dipole-dipole interactions & induced dipole-dipole interactions.

HYDROGEN BONDING

Question 22

PERMANENT dipole-INDUCED forces

Answer 22

When a PERMANENT DIPOLE in a molecule induces a dipole in another molecule.

EXAMPLE:
H-F —– F-F

• The delta negative charge on the F atom of H-F repels the electrons in F2 , moving the electrons in F2 to the right side of the atom.
• This makes the right side delta negative and the left side delta positive.
• The permanent dipole in HF has induced a dipole in F2 and results in a weak permanent dipole-induced force.
• Typically 0.01 the strength of a covalent bond.

Question 23

Hydrogen bonding

Answer 23

• The strongest type of intermolecular force
• Around one tenth the strength of a covalent bond
• They are a specialised type of permanent dipole-dipole force.

Question 24

TWO conditions for hydrogen bonds to form

Answer 24

• A hydrogen atom must be bonded to a STRONGLY ELECTRONEGATIVE element.
• The electronegative atom must have at least one LONE PAIR of electrons.

This means the only elements that can from hydrogen bonds are:

• NITROGEN
• OXYGEN
• FLUORINE

Question 25

The formation of a hydrogen bond in HYDROGEN FLUORIDE

Answer 25

- The hydrogen atom is highly electron deficient d+, due to being covalently bonded to the highly electronegative fluorine atom d- - The Hd+ is attracted to the LONE PAIR of electrons on the fluorine atom in a neighbouring molecule. - This attraction is known as a hydrogen bond and is indicated using a HASHED LINE. - The hydrogen atom is found along the straight line drawn between the two fluorine atoms - LINEAR SHAPE.

Question 26

Properties of water : DENSITY

Answer 26

- In its LIQUID form, the water molecules are moving RANDOMLY , sometimes close together, other times further apart. - This means as a liquid , hydrogen bonds are constantly being FORMED and BROKEN. - In its solid form, ICE, the water molecules arrange themselves into an ordered structure , stabilised by hydrogen bonds. - This means the water molecules are further apart. - Therefore, the solid form of water, is LESS DENSE than the liquid form. - This means that ice can float on the surface of water.

Question 27

Properties of water : BOILING POINT

Answer 27

- HIGHER THAN EXPECTED temperatures are required . - This is because EXTRA ENERGY is required to overcome the strength of the hydrogen bonds , IN ADDITION to the permanent dipole-dipole and induced dipole-dipole forces. - This means water has a RELATIVELY high melting and boiling point.

Question 28

Properties of water : SURFACE TENSION

Answer 28

- The presence of hydrogen bonding gives water a HIGHER THAN EXPECTED surface tension. - This means water ha a RELATIVELY high surface tension.

Question 29

Properties of water : VISCOSITY

Answer 29

- Liquid water flows LESS easy than expected. - This means water has a RELATIVELY HIGH viscosity.

Question 30

The structure of ice

Answer 30

- Forms an OPEN TETRAHEDRAL LATTICE structure of water molecules. - Each water molecule forms FOUR HYDRGEN BONDS due to having TWO lone pairs and TWO hydrogen atoms.

Question 31

How to figure out if a molecule is POLAR or NON-POLAR

Answer 31

Identify the CHARGES on each element: - If the elements are different , there will be a difference in electronegativity. - The more electronegative element (further right) will have a delta negative charge, whilst the other element has a delta positive charge. - The bonds have a PERMANENT DIPOLE Is the molecule SYMMETRICAL or NOT : - Are the charges evenly distributed - Draw the molecule and identify the shape and bond angle. POLAR: - Unsymmetrical - The dipoles act in different directions and DO NOT oppose each other equally and cancel out. - The molecule has an OVERALL DIPOLE. NON-POLAR: -Symmetrical - The permanent dipoles in the bonds oppose each other equally and CANCEL OUT.