The three states of matter, solid, liquid, and gas
Example:
What is the volume of 1.00 g water when it is completely changed into
gaseous form (not the visible "moisture" or "steam")?
(Here is the information you need: water density, 1.00 g/cm3;
gaseous water occupies 22.4 L at STP; molar mass; and compare a fixed amount
of water and its gaseous form)
1.00 g water is 1.00 g ÷ 18.0 g/mol
= 0.0556 mol
0.0556 mol water is changed into 0.0556 mol of gaseous water, which
is
0.055g mol × 22.4 L/mol = 1.25 L
Remember that 1.00 g water is 1.00 mL since the density of water (at 4 deg-C) is 1.00 g/mL.
Attractive forces between molecules: secondary forces
van der Waals forces: the weakest attractive force
Dipole-Dipole interactions: between molecules with permanent
dipole moments, like H2O, CO, and so on.
Bond polarity is caused by uneven distribution of electrons due to different electronegativities of different atoms.
London force: resulting from temporary dipole
Non-polar molecules can condense to liquid due to this force (but at much lower temperature),
e.g., CH4 condenses at –164 ºC, but H2CO at –21 ºC.
Hydrogen Bond: the kind of dipole interaction when H is involved, occurred when H is bounded to a very electronegative atom, like F, N, and O.
e.g., in proteins shown below
in DNA and in water and ice
Surface tension: caused by net attraction of molecules inward toward the interior of the liquid, that resists the expansion of the liquid surface and prevent a paper clip from sinking, but can be greatly decreased by detergents.
water<
>grease
Chemistry within us, 6.1 and 6.2
(6.1) The bile salt, which acts like "detergent", emulsifies lipid
in the intestine for the purpose of lipid digestion.
(6.2) A "surface-active agent" (surfactant) is synthesized in alveolar
cells which decreases water surface tension of better expansion of the
alveoli.
Formation of solution: "Like dissolves like."
e.g., salt in water, Na+ and Cl– ions are surrounded
by water molecules via dipole-dipole interaction
Vaporization: the "escape" of a molecule form a liquid
(Sublimation: escape from a solid, e.g., dry ice.)
A molecule must have enough kinetic energy in order to escape from a
liquid. The higher the temperature, the easier a molecule can escape, thus
the higher the vapor pressure of the liquid. Therefore,
Liquid + heat ® vaporization
into vapor
And, Vapor ®
liquid + heat
Influence of secondary forces on vapor pressure: The greater
the attractive forces between molecules, the smaller the vapor pressure.
A liquid boils when its vapor pressure reaches atmospheric pressure.
Thus, the higher the atmospheric pressure, the higher the boiling temperature
is like in the case of a pressured cooker. The opposite is like in the
case of cooking on the top of a high mountain.
Would be a joke if you say: "The boiling temperature of a liquid is the temperature when the liquid boils." Duh!! It does not define anything! Just like it is a joke by saying: "Liquid can vaporize because the temperature is high!" "One can make ice cube in a freezer because it is cold in the freezer."
Normal boiling point: the temperature at which boiling occurs under (an external pressure of) 1 atm.
Vapor pressure is influenced by H-bond (above right), size of the molecule (propane and higher b.p. than methane).
Relative humidity of water = (partial pressure/equilibrium vapor pressure) ´ 100%
example:
The relative humidity at 25 ºC is 95%, what
is the partial pressure of water under that condition? (The equilibrium
vapor pressure of water at 25 ºC is 23.8 torr.)
95% (=
0.95) = Vp/23.8
Vp =
____ torr
Effect of H-bonds on water
Read Section 6.9 about "attractive forces and the structure of solids": examples on amorphous; covalent crystals, etc.