Intermolecular Forces
 Intramolecular force - forces that act within a molecule (ie.
Ionic, covalent and metallic bonding).
 Intermolecular force - forces that act between stable molecules.
Differences in physical properties are the direct result of
intermolecular forces.
Types of Intermolecular forces
1. Dipole-Dipole Force
 Attractions between opposite charges
in polar molecules
2. Ion-Dipole Force
 Force of attraction between an ion and a polar molecule
 Ie. NaCl dissolves in water because of these forces
3. Hydrogen Bonding
 A strong type of dipole-dipole attraction that occurs in one of
these types of bonds:
 H-O
 H-N
 H-F
Intermolecular Forces and Solubility
 “Like dissolves Like”
 For example polar substances will dissolve in other polar
substances and nonpolar in other nonpolar substances.
 But Why?
Solubility of Polar Covalent Substances in Water
 The partial charges in a polar substance are attracted to the
opposite partial charge in water’s polar molecules.
 As a result water molecules surround the polar molecules, causing
them to dissolve.
 Example methanol (CH3OH)
 Solubility therefore is the result
of a special dipole-dipole
attraction called hydrogen
bonding.
Solubility of Ionic Compounds in Water
 When a soluble ionic compound
dissolves in water the attraction
between the two ions is broken
(ionized) and the two ions are
surrounded by molecules of water.
 Solubility therefore is the result of
ion-dipole attraction.
 Some ionic compounds are not soluble
in water. This is the result of the
very strong attractions of their ions. Ie. Silver chloride
 When the electronegativity difference of a binary compound is
small (ie. not ionic or polar), it is likely to be soluble.
Factors Affecting Solubility
1. Molecule size
 Small molecules are more soluble than large molecules because it
is easier for solvents to surround these particles.
 Many larger molecules are also less polar overall therefore
intermolecular forces such as hydrogen bonding have less effect.
 Example: Methanol (CH3OH) vs. Ethanol (CH3CH2OH)
2. Temperature
 When solids dissolve in a liquid energy is required to break the
bonds. Increased temperatures supply this energy. Therefore
solubility increases in solids as temperature increases.
 Intermolecular forces between liquids are not as strong;
therefore solubility of liquids is not greatly affected by
temperature.
 Gases already move very quickly. Therefore if the temperature
of a solution of a gas dissolved in liquid increases, the gas will
gain energy and be less soluble.
3. Pressure and solubility
 Pressure mainly affects the solubility of gases.
 Direct relationship – as pressure increases, solubility increases.
 Pressure acts to keep gases from escaping the solution.
Factors that affect solubility of ionic compounds:
Some ionic substances such as sodium chloride [NaCl] are soluble in water,
while other ionic substances, such as calcium phosphate [Ca3(PO4)2] are
insoluble.
 Ion charge – compounds of ions with large charges tend to be
insoluble because increasing the charge, increases the force that
holds the ions together.
 Example: Phosphates PO43- tend to be insoluble while salts of alkali
metals (ie. Na+1, K+ etc.) tend to be soluble.
 Ion size – compounds containing small ions tend to be less soluble than
compounds with larger ions of the same charge.
 Small ions bond more closely together and therefore the force of
attraction is much more difficult to break (ionize).
 Example: Fluoride ions are less soluble than chlorine ions.