Wednesday, May 03, 2006
AP HW corrections
1. I still can't get two people in a row to simply plug the numbers into a Hess Law equation; will you please write out the equation and then explicitly substitute the numbers and then PROPERLY follow PEMDAS (when you remove the parentheses from something that is being subtracted, you MUST change the sign of EACH term that was in the parentheses).
2. This isn't a correction, just an important note:
the % ionization of a weak acid is usually pretty small: about 5 to 10 percent in most common solutions. However, you can SUPPRESS ionization of the WEAK acid EVEN FURTHER by adding a STRONG ACID to the weak acid solution. You can see this effect via Le Chatelier:
HA --> H+ + A -
by adding a strong acid, the concentration of H+ in solution increases which causes a stress on the PRODUCT side of the weak acid ionization equilibrium; the system relieves that stress by shifting towards the reactants. Kinetically, the reverse rate has to increase due to the increased collision frequency between the H+ and the A- due to the increased concentration of H+.
3. You CANNOT claim that a molecule is polar simply because it has very polar BONDS within the molecule. If you think that you can, just think of CO2 to see that YOU can have very polar bonds WITHIN a molecule yet, the dipoles cancel due to the CO2 linear molecular geometry that causes the equal C-O dipoles to be oppositely oriented; thus, the dipoles cancel causing CO2 to be a non-polar molecule.
Any molecule that has polar bonds will be polar ONLY IF the dipoles formed by the polar bonds DO NOT CANCEL i.e. ONLY IF the DIPOLES ARE NOT be symmetrically distributed.
Therefore, you MUST MUST MUST draw the molecule AND EXPLCITLY refer to its MOLECULAR GEOMETRY/SHAPE in your answer....i.e. CH2F2 is polar BECAUSE the H and F atoms are TETRAHEDRALLY ORIENTED about the carbon; if the H and F atoms were in a SQUARE PLANE (with the F's at opposite corners) then that molecule would actually be nonpolar because the two C-F dipoles would cancel because they would be equal and oppositely oriented.
4. I just graded THREE TESTS IN A ROW in which the Lewis structure of NO3 - had SIX VALENCE ELECTRONS AROUND THE CENTRAL ATOM!!!
If you have used up all of the valence electrons on the TERMINAL atoms, you must then use lone pairs from the terminal atoms to form DOUBLE OR TRIPLE BONDS UNTIL THE CENTRAL ATOM HAS AN OCTET!!! (except for B and Be and odd-electron moecules/ions!)
2. This isn't a correction, just an important note:
the % ionization of a weak acid is usually pretty small: about 5 to 10 percent in most common solutions. However, you can SUPPRESS ionization of the WEAK acid EVEN FURTHER by adding a STRONG ACID to the weak acid solution. You can see this effect via Le Chatelier:
HA --> H+ + A -
by adding a strong acid, the concentration of H+ in solution increases which causes a stress on the PRODUCT side of the weak acid ionization equilibrium; the system relieves that stress by shifting towards the reactants. Kinetically, the reverse rate has to increase due to the increased collision frequency between the H+ and the A- due to the increased concentration of H+.
3. You CANNOT claim that a molecule is polar simply because it has very polar BONDS within the molecule. If you think that you can, just think of CO2 to see that YOU can have very polar bonds WITHIN a molecule yet, the dipoles cancel due to the CO2 linear molecular geometry that causes the equal C-O dipoles to be oppositely oriented; thus, the dipoles cancel causing CO2 to be a non-polar molecule.
Any molecule that has polar bonds will be polar ONLY IF the dipoles formed by the polar bonds DO NOT CANCEL i.e. ONLY IF the DIPOLES ARE NOT be symmetrically distributed.
Therefore, you MUST MUST MUST draw the molecule AND EXPLCITLY refer to its MOLECULAR GEOMETRY/SHAPE in your answer....i.e. CH2F2 is polar BECAUSE the H and F atoms are TETRAHEDRALLY ORIENTED about the carbon; if the H and F atoms were in a SQUARE PLANE (with the F's at opposite corners) then that molecule would actually be nonpolar because the two C-F dipoles would cancel because they would be equal and oppositely oriented.
4. I just graded THREE TESTS IN A ROW in which the Lewis structure of NO3 - had SIX VALENCE ELECTRONS AROUND THE CENTRAL ATOM!!!
If you have used up all of the valence electrons on the TERMINAL atoms, you must then use lone pairs from the terminal atoms to form DOUBLE OR TRIPLE BONDS UNTIL THE CENTRAL ATOM HAS AN OCTET!!! (except for B and Be and odd-electron moecules/ions!)
# posted by C @ 9:29 AM 
