1d – Intermolecular Forces

 To examine the effects of intermolecular forces on properties of a substance  To know how Van der Waals forces arise and their effect  To Know how permanent dipole/dipole interactions occur and their effect on properties  To know how hydrogen bonding arises and it’s importance for properties of some substances

Noble gases have full outer electron shells They do not need to combine with other atoms. They are said to be monatomic. However, the monatomic gases do form weak inter-atomic bonds at very low temperatures. Group 0 are all gases and exist as individual atoms. He ++

Sometimes the electrons can end up on one side of the atom, i.e. the electron cloud can wobble This means that one side of the atom is more negative than the other side. i.e. 2 ‘electric poles’ are formed, otherwise called a dipole. A temporary dipole is therefore formed. Van der Waals forces These charges are given the symbol δ ‘delta’ A dipole can induce other atoms to form dipoles, resulting in dipole –dipole attraction. ++ δ-δ- δ+δ+ δ-δ- δ+δ+ δ-δ- δ+δ+

b.p / K B.p.’s increase as the size of the atom increases This happens because the Van der Waals’ forces increases with increasing size of atoms Helium Neon Argon Krypton Xeon

A SMALL MOLECULE COVALENT SUBSTANCE

covalent bonds (strong)

intermolecular attractions (weak) covalent bonds (strong) what causes them?

intermolecular bonds (weak) covalent bonds (strong) van der Waal’s forces = MUCH WEAKER THAN

boiling point

boiling point WHY?

wee, light molecule weak attraction big, heavy molecule strong attraction lower boiling point higher boiling point

the heavier the molecule the stronger the intermolecular attraction the higher the boiling point.

F F F F Strong covalent bond Weak Van der Vaals force Strong intra-molecular bonding and weak inter- molecular bonding exist in this diatomic molecule. F 2 m.p o C or 53 K A covalent bond is formed when a pair of electrons are shared. The atoms in a covalent bond are held together by electrostatic forces between positively charged nuclei and negatively charged electrons. 9+ Fluorine atom 9+ Fluorine molecule F 2 diatomic

Halogens b.p.’s b.p./ K As the size of the halogen atom increases, so does the size of the van der waals’ forces between the halogen molecule Fluorine Chlorine Bromine Iodine

The differing electronegativities of different atoms in a molecule and the spatial arrangement of polar covalent bonds can cause a molecule to form a permanent dipole. ++ -- -- -- -- 4 polar covalent C-Cl bonds in CCl 4 tetrahedral shape -- -- -- ++ 3 polar covalent C–Cl bonds and 1 polar covalent C-H bond in CHCl 3 No permanent dipole Symmetrical molecule Permanent dipole Asymmetrical molecule NON-POLAR molecule POLAR molecule e.g. also CO 2 e.g. also H 2 O

Both propanone and butane have the same formula mass of 58 however, butane boils at – 1 o C while propanone boils at 56 o C Butane has no permanent dipoles, so only Van der Waals forces between molecules. So has a lower boiling point. Propanone is a polar molecule as it has a permanent dipole, so has polar- polar attraction as well as Van der Waals’ forces between molecules. ++ -- b.p. 56 o C b.p. -1 o C

Hydrogen bonding is a special type of dipole-dipole attraction in which hydrogen atom acts as a bridge between two very electronegative atoms. It is the strongest of the weak inter-molecular forces. A  - H  + B  - A and B are electronegative atoms, such as F, O or N. Such atoms possess one or more lone pair of electrons. Proteins consist of long chain atoms containing polar > C=O and H-N < bonds. Hydrogen bonds help give proteins their shape. The Hydrogen atom is in a straight line between A  - and B  -. The bond strength is stronger than other forms of permanent dipole-permanent dipole interaction but weaker than a covalent bond. H F ++ --

Group

1 st row 2 nd row 3 rd row 4 th row Group

The intermolecular attraction is much stronger than you would expect it to be. This extra strong attraction is given the name THE HYDROGEN BOND. But what causes it?

AA pure covalent bond

AA AB -- ++ polar covalent bond pure covalent bond

HX -- ++ A special type of polar covalent bond X = a highly electronegative element

++ -- ++ -- ++ -- HYDROGEN BONDS (HYDROGEN BONDING) X = a very electronegative atom H  X  covalent bonds

Covalent substances that have hydrogen bonding have higher boiling points have higher melting points often have harder crystals than you would otherwise expect.

O H H -- -- ++ ++ WATER

Oxygen has 2 lone pairs of electrons which can form a hydrogen bonds with two hydrogen atoms. Water has a high surface tension. The molecules on the surface have in effect, hydrogen bonds. This has the effect of pulling the surface molecules closer together. Water has a its greatest density at a temperature of 4 o C. When, as water cools further, the molecules start to move further apart, due to the hydrogen bonding, until a more open structure formed at its freezing point. So ice floats!! Pond Skater Ice Skater: Each water molecule, in theory, could be surrounded by 4 hydrogen bonds O HH -- ++ ++

Hydrogen bonding is found in covalent substances where the molecules have O H bonds and N H bonds for example

sugars glucose

amino acids an amino acid

proteins

DNA 3 billion

haemoglobin

DNA