Download Intermolecular Forces

Intermolecular Forces
The strength of the intermolecular forces determines the physical
properties of the substance:
- weaker than covalent bonds, but can be stronger than ionic bonds
- the temperature at which a liquid boils/solid melts reflects the kinetic
energy needed to overcome the attractive intermolecular forces
- intermolecular bonds are broken when a molecular compound melts and
boils
1. Hydrogen bonding
-water is a highly polar molecule: large electronegativity difference and shape
(bent)
boiling points
-since oxygen has the higher electronegativity, the electrons spend more time around oxygen
than they do around hydrogen
- the dipoles created by O-H in water are attracted to opposite charges creating
hydrogen bonds (1, right side): the strongest of all intermolecular forces
- as a consequence, water is capable of dissociating some
ionic compounds
2) van der Waals forces
-weak forces of attraction between molecules
a) dipole–dipole force (DDF)
-occurs between polar molecules, such as hydrogen chloride, HCl
-the slightly positive end of one hydrogen chloride molecule is
attracted to the slightly negative end of a neighbouring hydrogen
chloride molecule
b) London dispersion force (LDF)
- an intermolecular force of attraction that forms between atoms of
neighbouring molecules as a result of a temporary imbalance in the
position of the atoms’ electrons
-forms between all molecules, polar and nonpolar
- the side of the atoms with more electrons develops a temporary negative charge, and the
side with fewer electrons develops a temporary positive charge; if same happens to
neighbouring molecule they attract each other
- since electrons move quickly, the dipole lasts for only a fraction of a second!
physical change: involves intermolecular bonds and therefore changes of state (s, l, g)
chemical change: involves intramolecular bonds and leads to formation of new products!
TYPES OF CHEMICAL REACTIONS
1. Synthesis Reactions (combination/addition reaction)
-the combination of two or more simple substances to form a more complex substance
A+B
-->
AB
ex. hydrogen gas and oxygen gas react to produce water
2. Decomposition Reactions
-the breakdown of large, more complex molecules or ionic compounds into smaller and
simpler entities
AB --> A + B
ex. hydrogen peroxide (H2O2 )decomposes to produce oxygen gas and water
3. Single Displacement Reactions
-reaction of an element and a compound to produce a new compound
AB + C
-->
AC + B
ex. solid zinc in lead (II) nitrate solution will give you solid lead and aqueous zinc nitrate
4. Double Displacement Reactions
- a chemical reaction in which two compounds in aqueous solution react to form two new
compounds
AB + CD
-->
AD + CB
ex. aqueous silver nitrate + aqueous calcium chloride produce solid silver chromate + aqueous calcium
nitrate