1. 2004 Duchesne, Derek 08-07-2006 15:43

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1. 2004 Duchesne, Derek 08-07-2006 15:43

------1. 2004 Duchesne, Derek 08-07-2006 15:43 This was a tough question. My students get thrown anytime they see element X. Does anyone know of any good resources for determining element X throughout the entire AP curriculum? I need an element X question for each topic.

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2. 2004 (c) - I was confused but I must READ! Briley, Emmely 08-09-2006 12:05 I got the sign wrong because I did not READ PRECISELY!I used the formula Ecell = Eox + Ered. When I looked at the Table of Reduction Potentials, I saw that the Sn2+ reduction is above the Cr3+, so the Sn2+ must be reduced. I have to write the Cr3+ reaction as an oxidation, so I switch its sign to +0.74 V. My equation becomes +0.60 V = -0.14 V + 0.74 V. The QUESTION, however, asked for the "standard reduction potential" of that electrode. Does that seem like an evil AP trick to anyone else? Agh! Now, I will have to have mercy on my students.

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3. 2004 part f St Clair, Robert 08-11-2006 18:02 I didn't have a problem with the question but I feel that students would just because they would want to use their calculators to get a final answer rather than simply put the answer in an incomplete form and not give a numerical answer. I suppose I will need to have students practice this.

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4. 2005 Duchesne, Derek 08-07-2006 15:40 WHat a great question! I felt the difficulty level was truly appropriate. The college board left out the mass of metal plated question which I hate.

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5. 2005 (d) (i),(ii) Question? Briley, Emmely 08-09-2006 12:14 Why is the I2 reaction the oxidation when it has a higher reduction potential than the H2O reaction? Is is simply that I2 is not in the solution in the beginning?

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6. 2005 (d) (i),(ii) Question? Roamos Quevedo, Lourdes 08-13-2006 12:57 i remember taking the SSAT for chemistry when i was getting my credential. And i reviewed all major topics except for electrochem thinking they wouldn't even touch it. And they asked a similar question on the exam. From the deepest reaches of my brain, the information came flying out and I remembered to reverse the equations and change the signs. Otherwise I don't think I would have passed. I hope to it gets embedded in my students that way.

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7. 2006 Duchesne, Derek 08-07-2006 15:47 I'm not crazy about this question. It seems less logic oriented and more hiding behind equations.

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8. Curious as to how you teach Eo Cell Raines, Butler 08-10-2006 11:06 After you teach Hess' law and state functions and algebraic manipulation of thermo eqns, I find that students get really confused when you begin flipping Echem eqns and not changing signs. If the convention was kept the same as state functions then there might not be this confusion. But in most texts the algebra is already done for you with the stipulation of leaving the signs of EoCell alone. I checked with to old profs today and the both agreed it was by convention only and their was no physical reason to treat them any different that any other thermydyamic function. I was curious if someone had an explantion or how you went about explaining this difference.

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9. Electrochem problems Mccabe, Ann 08-12-2006 10:56 It seems like there is a huge difference in the difficulty of questions from year to year (at least for me). The 2004 question I found pretty straight forward once I got all my ox/redox/anode/cathode parts correct. The 2005 8d was really tough. It says NaI and you almost overlook the fact that they say it is in solution. That, of course, is critical to the problem. Then the 2006 question is straight forward again. It seems like you can get by with knowing the equations and basic half-potential problems but I do not want to short change my students.

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10. Electrochem questions Roamos Quevedo, Lourdes 08-13-2006 13:17 Because electrochem is not a part of our state standards, we don't focus on it. So I haven't taught it in quite a few years. But I'm thinking, if I'm going to prepare my regular and honors students for the AP class, maybe I should teach electrochem at the end of their school year after the state tests are given and have it lead into the AP class's introduction.

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11. How do we use these in class Engstrom, Harold 08-09-2006 18:30 First, I'll admit it was clever to handle the questions differently this time. I did struggle a bit but found all of these questions (form B helped confuse me at first) and navigated through the AP site. I have now saved and printed all the free response questions 2002 through 2006 and the available scoring guides. Using one question at a time (or part of a question) seems the way to go. This seems like a managable task. I think my students will be given sample questions at the start of applicable units. They can then learn the skills they need and know when and how they will be asked to apply the information.

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12. My solutions for 2006 Briley, Emmely 08-09-2006 12:28 (a)(i) 3Sn2+ + 2Al --> 3Sn + 2Al3+ (ii) -0.14 V + 1.66 V = 1.52 V (iii) DGo = -nFEo = -6 mol e-(96500 C/mol e-)1.52 J/C = -8.80x10^5 J (iv) [took me a minute to figure out this was stoichiometry] (+0.08 mol Al3+) (3 mol Sn2+/2 mol Al3+)=0.12 mol Sn2+ used. => (Assume 1 L) 1.0 M - 0.12 M = 0.88 M Sn2+ (b) Ecell = 1.52V - 0.0592/6 log[(1.00M)^2/(0.010M)3] = 1.46V Please, let me know if you find any mistakes!

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13. Re: 2004 Briley, Emmely 08-09-2006 11:40 I think in this case that a way to help them find element X is to teach them to use the Table of Reduction Potentials and give them practice using standard cell potentials to identify unknowns. It seems like a nice work sheet could be made of questions like it.

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14. Re: 2004 Huebner, Verena 08-09-2006 18:29 I agree, this would confuse my students as well...

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15. Re: 2004 Engstrom, Harold 08-09-2006 18:33 If you make one I would be more than happy to steal it.

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16. Re: 2004 Balan, Irina 08-09-2006 21:21 Now that you mentioned this I remembered during one test last year - this is the Chemistry section from Integrated Coordinated Science for 9th graders though-several students when having to solve an "element X" type of problem came to me and said something like "You probably made a mistake because there is no element X in the periodic table, did you mean xenon?!" So yes, it can be confusing. Irina

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17. Re: 2004 Harley, David 08-10-2006 08:00 I think you all are on the right path when you suggest that this would be confusing - but maybe if we begin writing more problems with "element X" - it would seem to raise the comfort level of the students. David

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18. Re: 2004 Mccabe, Ann 08-12-2006 10:41 This element X thing also comes up in basic questions about families of elements. If they look at the details (3 electrons), it narrows down the possibilities to only three metals.

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19. Re: 2004 (c) - I was confused but I must READ! Engstrom, Harold 08-09-2006 18:50 I think all teachers should be required to always be enrolled in a class as a student to help keep us humble.

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20. Re: 2004 part f Groves, Paul 08-12-2006 07:37 Students need to be taught that the AP exam is not always looking for answers. If people are interested in whether a student understands what the Nernst equation is and whether a student knows how to use it, the answer is pretty much secondary. This question might be a good time to bring this idea home... a "teachable moment" as they say. --Paul

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21. Re: 2005 (d) (i),(ii) Question? Groves, Paul 08-11-2006 10:30 In the solution you have Na+, I-, and H2O. For oxidation, the Na+ is not to be considered... Na+ will not LOSE more electrons. I- may lose electrons and become I2. H2O may also lose electrons to become O2 + H+ The two possible oxidation half-reactions are given in the chart (backwards). 2 I- --> I2(s) + 2e- is the choice because it has the smaller REDUCTION potential (0.53 vs. 1.23) which means it would have the greater OXIDATION potential (the same number with a changed sign). In other words, this chart as written is useful in picking out which REDUCTION is more likely to occur. If we re-wrote the chart by reversing each equation and changing the sign on each potential, it would be useful in picking out which OXIDATION is more likely to occur. Without re-writing the chart, we look at the reverse reactions and look for the smaller potential. --Paul

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22. Re: 2005 (d) (i),(ii) Question? St Clair, Robert 08-11-2006 17:36 Thanks for the explaination. I had the same question and was waiting for an explaination.

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23. Re: 2006 Groves, Paul 08-07-2006 18:22 However, these are equations that are generally part of the curriculum. --Paul

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24. Re: 2006 Engstrom, Harold 08-09-2006 18:43 Fair enough answer. Philosophically though, do you ever feel like you're losing touch with chemistry and education just to battle the AP test. I sense some of the excitement being beaten to death with "the test". Maybe it is only because it is summer and I've been in my classroom for 7 hours.

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25. Re: 2006 Groves, Paul 08-11-2006 09:52 It is certainly a fine line that I tread with my students. I like having the AP exam as a goal and therefore a focus of the class, but my rule is always to learn enough chemistry to do well on the test... not just teach toward the test. --Paul

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26. Re: 2006 Mccabe, Ann 08-12-2006 10:43 But the equations are given on the reference sheets. Some chemistry is just math and equations. It doesn't make it easy to teach but what can you do?

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27. Re: Curious as to how you teach Eo Cell Groves, Paul 08-11-2006 10:18 I teach students to find the two half-reactions involved and then find the DIFFERENCE in their reduction potentials. To me, it makes sense that the difference in the pull for electrons by two substances leads to a voltage. I ignore the fact that many books and videos tell students to change the sign and add rather than simply subtracting. My analogy is having my classroom of students and measuring how well they are at Tug-of-War. We could all tug against everyone which would involve something like 30 x 29 x 28 x 27... or we could pick one person and all tug against that person... maybe me. I'm sure that half of my students are stronger than me and I hope I am stronger than some of my students. We would come up with numbers (some positive and some negative)... maybe the distance the loser was pulled over the tug-of-war line. If they win, it is a + number... if I win, it is a - number. If the two students were to tug against each other, we would expect the difference in their tug #'s to be the result. For example, Jerry is -3 (I beat him and pulled him 3 feet over the line) while Madeline is +5 (she beat me and pulled me five feet in her direction). When Jerry and Madeline tug, we might expect Madeline to win and pull Jerry 8 feet over the line (what a woman!) The score is 5 - (-3) = +8. In my analogy, I play the part of the Hydrogen cell which happens to be near the half-way mark in a large list of half-cell potentials. I don't think there is anything magical about that cell. Half the chemicals known have a greater pull for electrons and half have a lesser pull for electrons. --Paul

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28. Re: Curious as to how you teach Eo Cell Roamos Quevedo, Lourdes 08-13-2006 13:11 I tink it is these little analogies that you use in you class that make it so much more interesting. You should add a week six dedicated to analogies of various topics.

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29. Re: Curious as to how you teach Eo Cell Groves, Paul 08-13-2006 14:03 Thanks for the suggestion. I guess I think in analogies. I only think of them when I try to explain something. --Paul

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30. Re: Electrochem problems Groves, Paul 08-12-2006 11:34 With practice and experience, students should be ready for varied levels of questions. The important part is that they UNDERSTAND what the equations do for them and what is going on chemically in the various electrochemical situations. --Paul

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31. Re: Electrochem questions Groves, Paul 08-13-2006 14:04 The electrochemistry ideas are also a great place to pull redox into the student's everyday experiences. How do batteries work? --Paul

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32. Re: How do we use these in class Harley, David 08-10-2006 08:02 That sounds like a great way to go - then they can begin with seeing what will be expected. David

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33. Re: How do we use these in class Koch, Brennan 08-10-2006 09:54 I am planning on implementing a tool that I stole from the AP Central site. I was reading through the sample syllabi and found one teacher that had students answer a free respnse question on the overhead. You could introduce the question at the beginning of the unit and then select someone to go over it at the end of the chapter.

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34. Re: My solutions for 2006 Huebner, Verena 08-09-2006 18:31 I got the same answers...

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35. Re: My solutions for 2006 Balan, Irina 08-09-2006 21:16 Emely, I got the same answers so I think we're on the right path here. Irina

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36. Re: My solutions for 2006 St Clair, Robert 08-11-2006 09:26 I also got the same answers.

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37. Re: My solutions for 2006 Groves, Paul 08-11-2006 11:16 I agree with your answers, too. Here are my answers with annotations. 2006 Form B #2 (.gif) [Page 1A> | Page 2A>] --Paul

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38. Re: Redox questions Groves, Paul 08-11-2006 10:39 Students should know that silver nitrate solid dissolves easily because they know their solubility rules. See my answer above for choosing half-reactions. --Paul

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39. Re: Some thoughts on problems and time Bronk, Tina 08-11-2006 05:44 I was thinking about doing a type of relay or race to solve a problem. Each person would have to do their own problem and the first to finish would win a point for their team or individual. The point could be extra credit or whatever you pick. You could rotate and take turns so the brightest kid doesn't always win. The race could be a warm up activity that you could do daily or an every Friday kind of event. There are lots of possibilities.

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40. Re: Some thoughts on problems and time Harley, David 08-11-2006 09:02 I like it - thanks for the idea. David

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41. Re: Some thoughts on problems and time St Clair, Robert 08-11-2006 09:42 The relay idea is great. I will adapt that for use in my classes

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42. Re: Some thoughts on problems and time Groves, Paul 08-11-2006 10:06 When we first encounter AP problems, we take as much time as we need on them. Later, as the test gets closer, I only allow limited time. I say... get as many points as you can in ____ minutes. Again, they don't need to answer everything to pass. If they CAN answer everything, it may pay to spend more time on this problem than on another problem. Again, practice helps students answer questions more quickly. --Paul

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43. Re: Some thoughts on problems and time Asbury, Felicia 08-11-2006 15:30 I like the relay/race idea, but I have a few concerns. Students may develop a bad habit of rushing and not checking their work or reading questions thoroughly. There's a lot of pressure to finish first. Instead of awarding points to the first one done, maybe incorporate a timed competition. For example, they all get 10 minutes to work on a problem. When time is up, grade their answers assigning points for showing their work, equations, etc. The student(s) with the most points get a prize, extra credit, etc.

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44. Re: Struggles Raines, Butler 08-10-2006 11:01 the big monster with electro chem and redox is making the kids understand that they have to balance mass and electrons. I try to hit on it all year long. I do simple demos like silver plating on copper to get them used to thinking about electrons moving. when i get to setting up cells i draw a big picture on the board of electron moving around and where they are going etc etc. It is a difficult concept any which way you look at it. Building actual cells help a lot so I just pull out the voltmeter and go for it. Plus its fun wet chemistry. I still have a hard time explaining it and I am/was an electrochemist. It didn't click to me until I took Dr. Charles Martin's class at UF. Then it made some sense (some).

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45. Re: Struggles Bronk, Tina 08-11-2006 05:33 Our school doesn't touch on the subject at all for general chemistry. I'm going to spend a lot of time reviewing this year.

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46. Re: The 3 problems we had to analyze this week: Briley, Emmely 08-09-2006 11:46 I do my best to teach my students that a volt is a joule/coulomb, so they can just cross cancel. It also helps me explain what a volt is - the amount of energy ("pushing or pulling")per charge. An easy way to remember that logs of fractions are negative is to remember the formula for pH since pH = - log [H+], and [H+] is usually some small fraction.

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47. Re: The 3 problems we had to analyze this week: Engstrom, Harold 08-09-2006 18:45 I agree about familiarizing students with the reference pages. I let them use copies of those pages during the year. I also do the same with regular chemistry and reference pages from the STAR tests,

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48. Re: The 3 problems we had to analyze this week: Engstrom, Harold 08-09-2006 18:48 How do different schools handle prerequisites? Wouldn't it be nice if the kids took physics and knew what Joules are? Our counselors keep trying to eliminate fundamental coursework and put everyone in all AP.

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49. Re: The 3 problems we had to analyze this week: Groves, Paul 08-11-2006 09:57 Prerequisites are pretty much a thing of the past in our AP classes. Philosophically, we are trying to be more inclusive because you don't really have to pass the AP exam to get something out of the experience. On the other hand, I think everyone is looking at the number of students who take AP as a measure of the success of the school. I try to teach whoever comes through my door, but I also discuss with my students whether AP chemistry is where they should be. After exams and after difficult assignments, I bring up the subject. I don't generally get anyone dropping the class after the first week or two. --Paul

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50. Re: The 3 problems we had to analyze this week: Engstrom, Harold 08-11-2006 13:17 Thank you. Last year was my first with AP chem and I tried to be "the nice guy". The students took this as a chance to negotiate for less work. That was a mistake. I think I should offer as much as they can handle and let them do their best. Perhaps that is how you are running this class.

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51. Re: The 3 problems we had to analyze this week: Roamos Quevedo, Lourdes 08-13-2006 12:52 i too give them the refernce sheets to use so that they are familiar with the equations and information that does not need to be memorized. And then I have to continue to remind them. After a quiz or test, the students will say, "I couldn't solve the problem because I didn't memorize the equation." And I'll ask them id they looked at theiir reference sheet. And of course, they say "reference sheet?"

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52. Re: Tips for 2005? Huebner, Verena 08-09-2006 18:32 I think my students would be confused by this as well...it took me a while to figure it out! I too would like hints on how to chose the proper equations.

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53. Re: Tips for 2005? Groves, Paul 08-11-2006 10:37 This chart is easy to use as a way to pick out which REDUCTION is more likely to occur. The solution has Na+, I-, and H2O. I- is not likely to GAIN electrons, so we must consider reactions with Na+ and H2O. The bottom two equations fit the bill. The larger the voltage, the more likely the reduction will occur. H2O will change into hydrogen gas and hydroxide ions before sodium ions become sodium metal. A quick demo will convince students that sodium metal will not be stable in water. To determine the OXIDATION that will take place, we consider the H2O and the I-. Since this is a chart of reductions, we need to re-write the chart (in our heads) and reverse the sign of each voltage. Equations 1 and 2 (in reverse) show the two possible OXIDATIONS. The larger OXIDATION POTENTIAL would be iodine's (-0.53 vs. -1.23). Moral: The chart (as written) can be used to determine which REDUCTION is most likely. The chart (reversing the reactions and changing the signs) can be used to determine which OXIDATION is most likely. It certainly takes a little practice, but is rathere easy when it clicks. --Paul

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54. Re: Tips for 2005? Asbury, Felicia 08-11-2006 15:10 Thanks for the explanation. It really helped! Felicia

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55. Redox questions Huebner, Verena 08-09-2006 18:39 I have to admit, that having only touched on electrochemistry very briefly in my first year courses, this is not an area of strength for me. Overall, the problems did seem pretty straight forward. In the 2005 quesion -The fact that AgNO3 is an ionic compound and dissolves spontaneously in water is obvious to me... but I'm not sure if it would be so to my students. I guess it's something they should put in their "bag of tricks" to remember. I also felt that student would have difficulty chosing the correct half-reactions in the second half of the 2005 question.

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56. Some thoughts on problems and time Harley, David 08-10-2006 08:07 I like these sorts of questions for the student that is a killer math kid - and I think they do too. I can see that the time restraint is going to need to be something taught, because 1) they will either spend too much time on it, or 2) not budget enough time to check their work. We have been working these at our leisure -how do you help with the understanding of time limits, short of placing time limits on every test/quiz you give? Ideas? David

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57. Struggles Koch, Brennan 08-10-2006 09:57 I too barely touched on electro last year in my general class. So it has been a long time since I worked through these. I did, however, limp my way through them. I realise that I will definitely need to do a better job of introducing the concepts to my general chem kids. I think I made it as far as LEO GER last year!

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58. The 3 problems we had to analyze this week: Balan, Irina 08-08-2006 08:32 Even though there has been a while since I last taught electrochemistry or oxidation-reduction reactions (these are not part of the CA Chem standards) I found the problems easy to solve – especially since the formulas and constants were provided. I do see some challenging parts for the students: the measurement units – especially for the 2006 a iii) question – Coulombs x Volts = Joules, then for the 2004 f ii) question the justification for the increase in E cell without doing the math involved (I guess students can might be tempted to still do the math rather than justify their answer and that means partial credit). The 2005 question was extremely easy so I would not expect students having a hard time solving it – except for … maybe the last question. Lastly – I’m just curious as to what was the scoring rubric for the 2006 question, which wasn’t hard at all. Like I said before – since I am new at this – I’d like to know if the list of formula and constants is changing from one year to another – but in any case I think it is extremely important to familiarize the students with the reference pages so they waste as little time as possible looking for information and also so they know what info is available for them when they take the test.

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59. Tips for 2005? Asbury, Felicia 08-09-2006 14:29 I remember my students coming back a little confused on this question. They freaked out when they saw 4 equations to choose from because we usually only do problems with 2 equations. Any tips or suggestions to help students choose the correct equations? Felicia

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