Post Lab Survey and HELP!

Below is the link for the after lab survey.  It will ask you to give the name of the lab that we did.  The official title is "How Can Color be Used to Determine the Mass Percent of Copper in Brass"  Thanks for doing the survey.  Also, if you want to earn even more extra credit, I need at least five students to complete a quick post lab assessment.  It is only one queston and involves using a Beer's Law plot to determine the concentration (molarity) of a copper solution and then using the copper ion concentration in a stoichiometry calculation.  I will give extra credit on top of the extra credit you are receiving to finish the post lab assessment.  Have a good summer if I do not see you again until next fall.

Mr. Brueckner

http://www.zoomerang.com/Survey/WEB22FN77H2M5P

AP Chemistry Test Information for Monday, May 7th

• The AP Chemistry test begins at 8:00 a.m. on Monday, May 7th, 2012.  The test will be given in the auxiliary gym.  Be at school at 7:30 a.m.  There was some discussion in class on Friday as to what type of writing utensil could be used.  I cannot find that only pens can be used, but I do not doubt it.  Bring both.  I will have a bunch of pencils and ball point pens in my room, so you can come and borrow both from me.  Also, if you need to borrow a calculator, please come and see me.  Make sure that your batteries are good before going into the exam.  
• Please go the the Events Calendar.  All VODCast solutions for the Day 7 review packet are posted under the dates Friday, May 4th and Saturday, May 5th.
• As you review for the test, make sure that you review all of the free response problems from last week and all of the problems from the Friday morning review sessions.  Also, going over the free response problems done in class from the last two units (Thermochemistry and Electrochemistry) would be a good idea.  Everyone should have both copies of the multiple choice practice exams given.  In both packets, a partial solutions guide to 1999 has been supplied and a full solutions guide to 2002 has been supplied.  Make sure that you go through both of those tests and cover the material that you missed when taking the practice tests.
• Another good form of review is to go back and watch the old VODCasts given throughout the year.  Just page back through the months of the Events Calendar.  A total review of the gas laws can be found in November.  Also in November are the VODCasts covering the independent study in organic chemistry.  Naming of compounds, writing chemical formulas, and simple stoichiometry VODCasts are made available on the first page of the AP Chemistry webpage.  Do not work all (or any) of the problems, but if you just watch, it is a good way to refresh your memory on the concepts.
• You can also page back through the entries of the blog to look at solutions to problems that were given as a way to review past concepts.
• The format of the test is first the multiple choice section then the free response section. You will have 90 minutes to answer 75 multiple choice questions. You will only be given a periodic table for the multiple choice exam. You will need to remember basic equations and do simple calculations without the aid of a calculator. You will have a list of equations and constants for the free response section of the test. You will have two parts to the free response section. Part (A) is the part of the free response test that calculators are allowed. You will have 55 minutes for part (A). Part (B) does not allow calculators and is when you answer conceptual questions. You will have 40 minutes to finish part (B). The multiple choice part of the test is worth 50% of your overall grade and the free response is the other 50%.
• On the second part of the free response test, the reactions section of the test will be given.  Do not spend more than 10 minutes (if even that - I would shoot for 5 to 7 minutes) on that section.  Remember, you get at least one point for writing correct reactants, so do that at least.  Write only the net ionic equation.  Know the BIG FIVE solubility rules.  Anything that does not fit, assume it will be a precipitate will form.
• In all of the VODCast solutions dealing with equilibrium, I showed I.C.E tables.  If determining the equilibrium concentrations is not dependent upon it, in the interest of time, do not show them.  Do show the equation, with values, used to determine the unknown value. 
• Remember, you are not only studying for the AP Chemistry exam, but also for the final for the AP Chemistry class, which will count for 20% of your semester grade. The test will given over a total of three days. On Tuesday, May 8th, you will take the multiple choice section of the final. On the AP multiple choice test, you get roughly 50 seconds per question. The multiple choice test for the final will be designed with 46 multiple choice questions to be completed in a 55 minute time frame. Wednesday, May 9th, you will be taking the 40 minute free response section (conceptual questions, equations supplied, but no calculator). On Thursday, May 10th, we will be tie-dyeing. On Friday, May 11th, the calculator portion of the free response part of the test will be given. You normally have 55 minutes for this part of the exam, but will only be given 50 minutes. I will remove parts of the original questions to make it fair terms of time. On Monday, May 14th, I am going to reserve that day for any make-up exams that need to be completed. 
• On Monday, May 14th, if you have completed all exams, you will be released from class or you can be part of the process of participating in the trial lab for the new AP Chemistry curriculum.  You will receive extra credit for participating in the trial lab event.  You will be required to do the lab and submit a graded lab report to receive the extra credit.  Your lab reports will be sent to the national AP offices to be critiqued for the effectiveness of the lab.  Taking part in a trial like this is special and something that you could include on a resume for college.  I will definitely include it in a letter of recommendation if I write one for you.  
• Remember, I will curve the final and then add the extra credit that you have earned by coming to the early morning sessions and the practice exams. After studying for the real AP Chemistry exam, you should not need to day any more studying for the class final. I will make sure to choose questions that address material that we have covered extensively in class and during the review process.
• For the class final, you will have an equilibrium problem for the first part of the calculator portion of the free response section. I will also guarantee you a reaction rate, thermodynamics (entropy, enthalpy, Gibb's free energy) and electrochemistry question in the either of the two free response sections of the test. I will also guarantee you a question covering chemical reactions like the two given in class in the last week of review.
• Feel free to call or text me on my cell phone if you have any questions as you study. You should all have my number. If not, call someone in class that may have it or email me.  I will check my e-mails frequently over the weekend.

Ksp Problems - Solutions

Below are the solutions to problems 3, 4, and 5 of the Ksp packet. Just like the weeks prior to spring break, we will be having a quiz on Friday, April 6th, 2012 on an old AP Chemistry test problem like the problems presented in this packet. Before the quiz on Friday, I will use the 1st 20 minutes of the period reviewing with you. Tomorrow, (Thursday, April 5th, 2012) you will be completing the Ksp / Titration Lab. The lab write-up will be due on Monday, April 9th, 2012 so you can spend tomorrow night preparing for the quiz.

On Tuesday, April 3rd, 2012, we worked problem #2 of the Ksp packet. We finished all but part D of the problem. The answer to D is not a numeric value, but rather a conceptual answer. We determined that strontium fluoride compound would begin to precipitate first because of having the lower Ksp value. The percentage of anion (fluoride ion) present when the other precipitate begins to form will be zero. This is because the stronger attraction that strontium ion has for fluoride ion over sulfate ion will cause only strontium flouride precipitate to be formed. Only after the last fluoride ion has been captured and the strontium ion concentration has increased will the precipitate strontium sulfate will be produced.

• Number 3, part B is an ICE problem with an initial concentration of one of the reactants. This will inhibit the right shift that would occur if no products were initially present. "x" is added to both products as a Change because the initial concentration of magnesium ion is zero.


• Number 3, part C gives initial conditions of both ions that can form a precipitate. Whenever initial conditions are given, you must determine the "Q" value to see if it exceeds the Ksp value and a precipitate will form. The molarities and and volumes of each solution are used to determine the millimoles of each ion and then divided by the total volume after the solutions have been poured together. The Q does not exceed Ksp, so no precipitate will form.


• Number 3, part D requires you to think about LeChatelier's Principle. Because the concentration of magnesium decreases after heat is added to the system (the temperature went up), a left shift towards the solid reactant had to occur. Since the equilibrium system must try and remove some of the heat added and the left shift occured, heat must be a product. This makes the dissolving of magnesium fluoride an exothermic reaction.

• 
  Number 4, part B (i) asks for the solubility of zinc hydroxide in a solution that already contains hydroxide. The fact that the solution already contains hydroxide ion is that a pH with a value greater than 7 was given. Finding the initial concentration of is shown in the solution to the problem. The presence of hydroxide initially inhibits the right shift of the zinc hydroxide dissolving. This reduces the amount of zinc hydroxide that can dissolve.


• Number 4, part B (ii) is a beast. The first thing that must be determined is if a precipitate will form, thus Q must be found. The millimoles of each ion are found and then divided by the total volume of solution once both solutions have been added together to determine the initial concentration of each ion. The Q ends up being larger than the Ksp, so a precipitate forms. The problem then turns into a limiting and excess reactant problem to determine which ion (zinc or hydroxide) is in excess. A BCE table is used. Know that equilibrium will be established again, so the millimoles of the excess hydroxide will be divided by the total volume of solution from adding the two solutions together. If you can do a problem like 4 B (ii), you ROCK!

• 
  The solution to part C in #5 is a conceptual question. The question states that solid still remains in the beaker. This means that the solution is still saturated with silver and bromide ions. Therefore, the concentration must be the same value that was calculated in part B because the system is still at equilibrium even though there is a larger volume of solution.


• The answer to part D seems like a silly answer (5.0 grams of AgBr in 37,000 liters of water), but it does make sense if you think of how hard it is to dissolve silver bromide with a fairly low Ksp value. The concentration of the 5.0 grams of AgBr in an unknown amount of water must be equal to the concentration determined in part B. After converting to mole of AgBr, I just solved for the unknown volume of solution.


• Part E only asks you what is observed and not to determine the concentration of any ions after equilibrium is established again. This means that "Q" must be found using the conditions given. The millimoles of silver ion (from silver nitrate) and bromide ion (from sodium bromide) were found by muliplying the volume of each solution by the molarity of each solution and then dividing by the total volume (12.0 mL) after the two solutions are added together. In this problem, the Q exceeds Ksp, thus a precipitate is formed.


• I will do a demonstration on Friday, April 6th to illustrate part F. I will explain it more then.

Solutions to Buffer Problems

Below are the solutions to AP Buffer problems #3 through #5. I know that I said that I would only supply the solutions to #3 and #4, but I liked a couple of the aspects of #5. One aspect of #5 is part b when the Kb value is to be calculated, but no concentrations are given to do this. This can be accomplished by knowing the weak acid that the base came from. Kw can be divided by Ka to determine the Kb. Tomorrow's quiz is another old AP Chemistry free-response question dealing with acid-base equilibrium with a focus on buffers. There will also be a question on determination of acid strength. Remember, this can be determined by knowing the Ka value or the structure of the acid. The structure, to be a "stronger" weak acid is to either have more oxygens or a more electronegative element in the acid structure. Come see me before class if you have any further questions. Remember, there is a study session tomorrow at 6:30 a.m.

Solutions to Acid / Base Equilibrium Problems 4 through 6

Below are the solutions to the acid / base equilibrium problems #4 through #6. The solutions to #4 are included even though they were presented in class. Tomorrow, you will have a quiz over the types of problems presented in the packet of old AP Chemistry exam questions dealing with acid / base equilibrium. In the bulleted text below, I have attempted to explain some of the more difficult parts of the questions.

• #5 part (c) caused some confusion in class. The acid (hypochlorous, HOCl) is reacted with the strong base sodium hydroxide (NaOH). Whenever a weak acid or base is reacted with a strong base or acid, the strong base or acid reacts completely. The reaction is not shown at equilibrium, but rather with only a right arrow showing a complete shift to the right. All of the hydroxide is consumed and converted into water and the conjugate base OCl^-1. Notice that the spectator ion Na^+1 was not included in the net ionic equation. This balanced equation will be needed for problem (d) (ii).


• #5 part (d) (ii) is a very involved problem. The moles of hydroxide ion will react completely with the weak acid HOCl. All of the moles (or millimoles) of hydroxide will be converted into water and the conjugate base OCl^-1. That is the reason that the B.C.E. table is shown. The moles (or millimoles) left of the HOCl and OCl^-1 after the reaction is completed will be used to determine the initial concentrations of each to re-establish equilibrium. Each ending mole value (or millimole value) will be divided by the total volume of 0.0400 liters (for moles) or 40.0 mL (for millimoles). The initial concentrations of each will then be used to determine the hydronium ion concentration when equilibrium is re-established.


• #6 (a) (iv) shows a B.C.E. table to illustrate that all of the moles of hydroxide ion (equal to the moles of NaOH found in question 6 (a) (iii)) will convert into moles of the conjugate base benzoate (C6H5COO^-1). The intent of the question is to determine if you know that the moles of the strong base will completely react and convert to the conjugate base.


• #6 (a) (v) is a different problem in that the initial concentration of benzoic acid (C6H5COOH) is not given. This can be treated just like any other unknown variable in an I.C.E. table. Once the equilibrium concentratio of benzoic acid is determined, the volume given in the problem can be used to determine the moles of benzoic acid present. The initial and equilibrium concentrations are considered to be the same since the change amount is miniscule compared to the initial value.

LeChatlier's Worksheet Solutions

Below is the key for the LeChatlier's worksheet. You will have two questions that relate to this worksheet on tomorrow's exam. Remember, tomorrow's test is entirely free response (written). Please go over the calculations for the lab. Remember, if you end up with a negative value in your ICE table, you did something wrong!!

Practice Test for Reaction Rate Unit - Solutions

Below are scans of the practice quiz given out in class on Thursday. Unfortunately, I cannot upload the pdf of the practice exam like I said in an earlier post. If you did not get a copy of the practice exam, you will just have to use the images below. Also, an explanation to problem #19 is below. Also, the practice exam did not cover any nuclear material from the Chapter 21 work packet. There will be questions pertaining to nuclear chemistry on Monday's exam.

• Problem #19 could be solved with a calculation using the equation to find activation energy (Arrenius Equation - don't worry about the name). A simpler way (and the expected way) is to notice the trend of increase in temperature and rate constants. The increase in temperature of 20 Kelvin (273 K to 293 K) had a rate constant "k" increase of 0.046 to 0.081. A roughly 20 Kelvin increase in temperature (293 K to 310 K) should also cause another 0.04X increase in the rate constant. Therefore, 0.081 + 0.04 = 0.12.