Thursday, April 27, 2006
AP daily corrections
AP: make sure that you check this blog daily as I will use the "power of the blog" to live blog the common errors that I see among your homeworks. This way, you can alll learn from each other's errors and BE AWARE of these errors as you take the real AP exam in 13 days.
#1 AP HW errors:
1- many of you misread the question on which solution would have the highest H2O vapor pressure; you should have reasoned that the colligative property of "vapor pressure depression" is solely dependent on the number of dissolved particles per unit volume of solution. Since ethanol does not ionize in water, its solution has the lowest concentration of dissolved particles (compared to the other salts or weak acid) and therefore had the LEAST vapor pressure lowering effect i.e. the highest Pvap of H2O. Many of you answered in terms of which substance had the weakest intermolecular attractions (ignoring the fact that these were SOLUTIONS, NOT pure substances) and thus reasoned that ethanol had the weakest intermolecular attractions and the highest vapor pressure at a given temp, WHICH IS TRUE BUT IRRELEVANT. THE MOST IMPORTANT RULE OF ALL ON THE AP EXAM IS TO ANSWER EXACTLY AND ONLY WHAT IS ASKED FOR!
2- many of you STILL do not know the logic of molecular geometry EVEN THOUGH ALL of the molecular geometry GRAPHICS have been on the class webpage since DECEMBER. Look at them; draw them out; KNOW THEM.
SeF4 has 34 valence electrons of which 32 are used on the F octets so the Se has ONE lone pair. That is the classic AX4E general formula which is NOT trigonal bipyramidal (AX5 ONLY!!!) but rather a SEE-SAW or SAW- HORSE shape. Trig Bipyramidal is the ELECTRON GEOMETRY of that molecule.
DO NOT CONFUSE ELECTRON GEOMETRY WITH MOLECULAR GEOMETRY- THEY WILL BE DIFFERENT WHENEVER THERE IS/ARE LONE PAIR(s).
3. One important note on semantics: I worry that even AP readers may incorrectly interpret the phrase " delta G increases". Some of them may think that you mean that delta G becomes more negative from that statement (and THEY would be wrong, mathematically).
Instead, to take no chances, always write either delta G BECOMES MORE NEGATIVE or delta G BECOMES MORE POSITIVE depending on your calculation.
Also, avoid using the phrases "more spontaneous" or "less spontaneous" because a given reaction either is spontaneous or non-spontaneous; instead, use "more product favored" or "more reactant favored".
4. Remember, the rate of a reaction depends on the fraction of reactant particles that meets or exceeds the activation energy barrier. Activation energy of a given reaction HAS NOTHING AT ALL to do with delta H, delta S, or delta G of that reaction. Therefore even the most spontaneous reaction in the universe (large negative delta G) may occur ridiculously SLOWLY (very large energy of activation) at , say, room temperature.
5. When a photon is emitted or absorbed by an atom, state the LAW OF CONSERVATION OF ENERGY FIRST - AP readers LOVE that law: "energy cannot be created or destroyed"; that is why the energy of the photon emitted must equal the difference in energy between the energy levels of the electron transition.
#1 AP HW errors:
1- many of you misread the question on which solution would have the highest H2O vapor pressure; you should have reasoned that the colligative property of "vapor pressure depression" is solely dependent on the number of dissolved particles per unit volume of solution. Since ethanol does not ionize in water, its solution has the lowest concentration of dissolved particles (compared to the other salts or weak acid) and therefore had the LEAST vapor pressure lowering effect i.e. the highest Pvap of H2O. Many of you answered in terms of which substance had the weakest intermolecular attractions (ignoring the fact that these were SOLUTIONS, NOT pure substances) and thus reasoned that ethanol had the weakest intermolecular attractions and the highest vapor pressure at a given temp, WHICH IS TRUE BUT IRRELEVANT. THE MOST IMPORTANT RULE OF ALL ON THE AP EXAM IS TO ANSWER EXACTLY AND ONLY WHAT IS ASKED FOR!
2- many of you STILL do not know the logic of molecular geometry EVEN THOUGH ALL of the molecular geometry GRAPHICS have been on the class webpage since DECEMBER. Look at them; draw them out; KNOW THEM.
SeF4 has 34 valence electrons of which 32 are used on the F octets so the Se has ONE lone pair. That is the classic AX4E general formula which is NOT trigonal bipyramidal (AX5 ONLY!!!) but rather a SEE-SAW or SAW- HORSE shape. Trig Bipyramidal is the ELECTRON GEOMETRY of that molecule.
DO NOT CONFUSE ELECTRON GEOMETRY WITH MOLECULAR GEOMETRY- THEY WILL BE DIFFERENT WHENEVER THERE IS/ARE LONE PAIR(s).
3. One important note on semantics: I worry that even AP readers may incorrectly interpret the phrase " delta G increases". Some of them may think that you mean that delta G becomes more negative from that statement (and THEY would be wrong, mathematically).
Instead, to take no chances, always write either delta G BECOMES MORE NEGATIVE or delta G BECOMES MORE POSITIVE depending on your calculation.
Also, avoid using the phrases "more spontaneous" or "less spontaneous" because a given reaction either is spontaneous or non-spontaneous; instead, use "more product favored" or "more reactant favored".
4. Remember, the rate of a reaction depends on the fraction of reactant particles that meets or exceeds the activation energy barrier. Activation energy of a given reaction HAS NOTHING AT ALL to do with delta H, delta S, or delta G of that reaction. Therefore even the most spontaneous reaction in the universe (large negative delta G) may occur ridiculously SLOWLY (very large energy of activation) at , say, room temperature.
5. When a photon is emitted or absorbed by an atom, state the LAW OF CONSERVATION OF ENERGY FIRST - AP readers LOVE that law: "energy cannot be created or destroyed"; that is why the energy of the photon emitted must equal the difference in energy between the energy levels of the electron transition.
# posted by C @ 10:36 AM 
