Chapter 10 Section 1 Intermolecular Forces the forces between molecules or between ions and molecules in the liquid or solid state Stronger Intermolecular forces cause higher melting points and boiling points (takes more energy to separate out the molecules) General Rule more charge, more intermolecular forces Strongest ion/ion interaction Not considered an intermolecular force it's an ionic bond! Next ion/dipole interactions Dipoles (polar molecules) have separated partial charge which is attracted to an ion with opposite charge Ions in solution! Dissolved ions dissociate (come apart) in water Back to Coulomb's Law! ions have full charges, dipoles partial charges The polar water will orient itself around ions to neutralize charge
Enthalpy of hydration the energy associated with hydration of ions Water stabilizes ions charges this is an exothermic process Na+ (g) + n H 2 O [Na(H 2 O) n ] + (aq) n usually = 6 H rxn = 404 kj/mol Enthalpy of Hydration in kj/mol Covalent bonds 100 400 kj/mol Ionic Bonds700 1100 kj mole Enthalpy of Hydration increases with charge and decreases with size (CL!!!!) Enthalpy of Hydration explains why ionic compounds break apart into ions in water It takes a large amount of energy to break ionic compounds into their ions (lattice energy) but surrounding the ions by water is very exothermic!
Second strongest IMF (Intermolecular force) is dipole/dipole two partial charges is weaker force according to Coulomb's Law than a full charge and a partial charge Remember! The partial negative is on the more electronegative element! The more polar a compound is, the stronger the IMF are Higher the boiling point! Dipole dipole forces are only about 1% as strong as an ionic or covalent bond!
London Dispersion Forces The electron cloud around an atom or molecule can be distorted, the more electrons the more polarizable the molecule is if the electrons are not evenly distributed are an atom it can create an instantaneous dipole Once one molecule has an instantaneous dipole, it creates a dipole in neighboring molecules These dispersion forces are found in all molecular substances can range from very weak to quite strong general rule: the higher the molar mass, the great the polarizability of the molecule (more mass = more electrons!) Compounds with many lone pairs tend to be more polarizable (think halogens) Compound Boiling Point N 2 196 O 2 183 I 2 185 http://www.bozemanscience.com/ap chem 016 london dispersion forces
Overall Ranking of IMFs 1. Ion/Ion (Not really an IMF a Bond!) 2. Ion/Dipole (usually ions dissolved in H 2 O) 3. Dipole/Dipole (2 polar compounds) 4. Dipole/induced dipole (Polar with nonpolar) 5. induced dipole/induced dipole (Generally referred to as London Dispersion Forces LDF, between two nonpolar compounds) Remember all molecules have dispersion forces, may have other forces also (dipole/diple etc) For example, methanol (CH 3 OH) which is polar has dipole/dipole forces and dispersion forces (LDF)
The hydrogen bonding not a true bond! an IMF between a Hydrogen that is bonded to an electronegative atom (N, O, F) and a lone pair on a N, O, or F a dipole/dipole interaction! N H, O H, and F H bonds are very polar H has a larger partial positive charge when bonded to N, O, or F H is attracted to the lone pair on another N, O, or F which are negatively charged! Water has two hydrogens and two lone pairs the perfect ratio Causes the unusually high boiling point Draw Hydrogen bonds between molecules H H C O H H Remember! There are two conditions for a Hydrogen Bond 1. A hydrogen on a N, O, or F 2. A second N, O, or F with lone pairs Draw between NH 3 and H 2 O (can you do it in more than one way?)
Hydrogen bonds can occur between two different molecules The H has to be bonded to a N, O, or F It forms a "bond" with a lone pair on a N, O, F http://www.bozemanscience.com/ap chem 017 dipole forces
Homework P. 487 #14, 33 39, 41, 116, 131