1999
ANNOTATION (Question B)
- Note that they have the same molar, but their solute particle concentrations are different
(determine that through the subscript of each atom type).
- Freezing point is dependent on solute concentrations. Since MgCl2 has the highest i factor, that
will be our answer.
ANNOTATION (Question C)
- Again, look at solute concentration to determine vapor pressure.
- for highest vapor pressure, there must be least solute conc. C2H5OH fits this criteria, so it shall be our
answer.
GUIDELINES
c.
2000 NONE
2001 (A)
ANNOTATION
a) Since their masses are similar, it cannot be one factor to explain. If you actually read the
instructions, it tells you the answer: LOOK AT INTERMOLECULAR FORCES. And so we shall.
NH3 has hydrogen bonding between molecules. CH4 has London dispersion (basic). Hydrogen
bonding is obviously stronger, so stronger bonds mean harder to break which means higher bp.
b) Both of those have London dispersion. SO which one is stronger? C6H14 is larger and therefore
more chances to be polar (polar bonds means presence of dipole moments means stronger bond)
c) What is special about Si? It is a covalent NETWORK. That must mean something. And it does! It
has strong covalent bonds between molecules. Besides, Cl2 only has London dispersion (weakest).
Si has stronger bonds which are harder to break therefore higher bp.
GUIDELINE
2002 (A) NONE
2002 (B) NONE
2003 (A) QUESTION 8: Only do 8B. OMIT 8A.
ANNOTATIONS
6b) i) When they dissolve, salt and sugar becomes IMPURITIES and bond to the water molecules. So it’s
harder for water to break free into gas alone. Thus higher bp. That’s one way of thinking. Another (the
one AP wants) is that bp depends on vapor pressure above the solution to that above pure water. Salt &
sugar are solutes that do not vaporize with water. Their vapor pressure above is lower (harder for water
to vaporize) and therefore higher boiling point.
6d) There is water vapor in the air. When they touch the surface of the beaker that is colder, they pass
their heat to the beaker and loses heat and CONDENSES ON THE SURFACE of the beaker wall. You
must state about EQULIBRIUM vapor pressure for water at lower temp. is lower than pressure that
water vapor exerts in the room.
8b) i) Propane only have London dispersion. Propanone has London dispersion and dipole-dipole
(stronger). They have different intermolecular forces strengths, so they must have different H(vap).
ii) 1-propanol has London dispersion and hydrogen bonding. Propanone only has London dispersion and
dipole dipole. Again, different IMF, different H(vap).
GUIDELINES
8.
2003 (B) NONE
2004 (A)
CHOOSE ONLY 7a AND 7d
ANNOTATION:
7a) First, look at polarity. Both are nonpolar. Second, look at IMFs. Ok, both only have London
dispersion forces. So you can’t apply my chapter to answer. MUHAHAHAHAH. (but you can use my
chapter to get half points!) I2 is larger so more able to be polar than F2, so dispersion forces are stronger
in I2 (Now you get to use my section and DERIVE the second half of your points!)—yes, they have
dispersion forces, but which is stronger is the question.
7d) NH3 has hydrogen bonding. Phosphine only has dipole dipole or dispersion. Water the solvent has
hydrogen bonding. Since it is the same as NH3 in terms of IMF, these pairs are more compatible. NH3
molecules can hydrogen bond with H2O, while Phosphine disadvantage on this side.
GUIDELINES
2004 (B) NONE
2005 (A)
Question 7.
ANNOTATION:
7a) i) NH3 has H. So its London dispersion (basic one) and hydrogen bonding. SIMPLE. NH3 has
London dispersion. It’s not nonpolar so it has dipole dipole too. EASY.
ii) NH3 has hydrogen bonding, the strongest. So it’s harder to break the bonds so higher bp. EASY.
GUIDELINES
2005 (B)
Question 8.
ANNOTATION:
8d) Propane: dispersion (default)
Methanoic acid: Hydrogen bonding and dispersion.
e) Methanoic has stronger IMF (hydrogen bonding), needs more energy to break bonds, higher bp.
GUIDELINES