AP Chemistry
Thoughts from an Experienced Teacher
Dale Jensen
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10/7/2011
Welcome to installment 3. In the past two installments I gave my thoughts on teaching an AP Chemistry course, picking an approach that works best for you, and the Gas Laws.
This time I will address Kinetics. I must admit that this is my favorite topic to teach. Students like the labs and the beauty of the mathematics behind the Integrated and Differential approaches. My problem is that I sometimes find myself spending too many class periods doing extra labs and problems.
As mentioned in installment 2, I would like to address the AP Chemistry Lab notebook before looking at Kinetics.
As a consultant for the College Board, I received the 2011-2012 Handbook for AP Chemistry. On page 5, there is new wording under the Topic Outline section.
“Because chemistry professors at some institutions ask to see a record of the laboratory work done by an AP student before making a decision about granting credit, placement or both, in the chemistry program students should keep a laboratory notebook that includes reports of their laboratory work in such a fashion that the reports can be readily reviewed.”
This is the first time that the College Board has addressed the notebook and I believe it is very clear. If your students want to receive credit for the lab work they did in your course, require a very structured report for each lab. Students may then present their notebook to the Chemistry Department for evaluation.
The following will link you to my required format:
When I first started to use lab notebooks, I questioned my decision. It saw this huge stack of books coming in and I was intimidated. Actually, the notebooks have some great benefits:
- The quadruled composition book that I suggest may be used for my first year and my AP course. It is close, but for most students, the composition book has enough pages for both years.
- Students can see my past comments to learn their past mistakes.
- You can show the student’s growth (or lack of) to parents and counselors.
- The quadruled paper allows for easy construction of tables and graphs.
- The day labs are due, you will not have to go to the gym to do your upper body workout.
Kinetics …
DAY 1 …
After I finish reviewing the In Class exam for the Gas Laws, I begin Kinetics with a lab.
You will need about 30 minutes to introduce and perform the lab. I like to time the lab so that the students will collect the data and when they go home, they have 3 graphs to construct.
I am attaching a copy of my Integrated Rate Law lab. It may look similar to a radioactive decay lab that many do in first year chemistry, but it has a twist that makes it appropriate for Kinetics.
Students work in pairs and before they may begin they must roll a die. This is used to determine the time when the students need to stop and count their sample. This time becomes their half-life. After the students collect their data, they go home and construct their 3 graphs. I do not tell them what they are looking for or what the graphs mean. The next day I discuss the graphs and the calculations
One last note: The lab requires the students to ‘properly’ dispose of the chemical waste. If the lab is done at the beginning of the period, students will use valuable class time to draw graphs that they can do at home and (most importantly) candy will make some students become hyperactive. If done at the end of the class, the students will go to their next class with a little extra energy.
How to Utilize High School Chemistry for A.P. Achievement The recommendations are the minimum that your students should do for each of these topics. Of course I highly recommend that students use the interactive component of the DVD to test what they have learned. 13.1 Intoduction to Chemical Kinetics and Reaction Rates 13.3 Instantaneous Rate 13.5 Reaction Rate and Concentration Video |
DAY 2 …
I start class by grading the 3 required graphs that the students constructed. I give them credit for 3 graphs drawn correctly. This is a quick look by me. This helps keep the students on task.
Spreadsheets are my favorite tool for writing and correcting lab reports. If I thought my wife would say yes, I would like to name our next dog EXCEL. When my Integrated Rate Law lab is opened, you will see three tabs at the bottom. The Student’s version of the lab, the Instructor’s version with the calculations, and the 3 required graphs.
I have my students use the Instructor’s spreadsheet AFTER they have completed their calculations based on their data. If they agree with the spreadsheet, they print a copy of the Instructor’s spreadsheet and place it loosely in their lab notebook. I now do not need to check the calculations and I can focus my attention at other parts of the lab report. This has been a great time saver.
The 3 graphs that the students should have constructed should look like the following examples:
The underlying premise of the discussion that is to follow is that AP Chemistry students should have seen in their math courses and be familiar with the application of the equation of a straight line.
y = mx + b
I begin with looking at the graphs that the students generated. Unless they read ahead, they come in with 3 graphs, but are unsure of they mean.
Questioning Strategy:
Question: ‘Is the graph of [A] vs. time a straight line?’
Answer: ‘No.’
Question: ‘If it was a linear relationship, could we write an equation that represents the relationship between [A] and time?
Answer: Usually, it takes very little prompting to get ‘y = mx + b.’
The students and I then generate the specific equation
[A] = -kt + [A_{o}]
The ZERO Order relationship.
Question: ‘Is the graph of ln[A] vs. time a straight line?’
Answer: ‘Yes.’
Question: ‘Can we write an equation that represents the relationship between ln[A] and time?
Answer: ‘y = mx + b.’
Repeating the same questioning reinforces the student’s understanding.
The students and I then generate the specific equation
ln[A] = -kt + ln[A_{o}]
The FIRST Order relationship.
Question: ‘Is the graph of 1/[A] vs. time a straight line?’
Answer: ‘No.’
Question: ‘If it was a linear relationship, could we write an equation that represents the relationship between 1/[A] and time?
Answer: ‘y = mx + b’
The students and I then generate the specific equation
1/[A] = kt + 1/[A_{o}]
The SECOND Order relationship.
Question: ‘For a first order reaction, can we write the Integrated Rate Law:’
ln[A] = -kt + ln[A_{o}]
‘What would this equation look like when half of the original reactant is used up?’
Answer: The students and I then generate the specific equation
ln[1] = -kt + ln[2]
-ln[2] = -kt
.693 = kt
t1/2 = .693/k
Students can calculate the half-life of their lab. They should see the relationship of their half-life and the initial roll of the die.
I finish the lab discussion with a series of Integrated Rate Law questions selected from the textbook to be done as homework.
Final thought before students leave – if temperature is held constant, k is a constant.
How to Utilize High School Chemistry for A.P. Achievement 13.6 Rate Law and Rate Constant |
I have been asked by participants of my summer workshops if I have a zero order and a second order. I have 2 labs that are almost ready to be revealed. They are formatted similar to the MM lab and use very common lab materials. When they are ready, I will post them in a future installment.
DAY 3 …
I begin the class by going over the assigned homework.
The first differential rate law lab that we do takes about 40 minutes to complete. The lab is my version of the Sulfur Clock Reaction.
Sulfur Clock Reaction: Photo-Lab
Sulfur Clock Reaction: Student Copy
Sulfur Clock Reaction: Instructor’s Spreadsheet
This lab is easy for the students to start and finish in half of a period (40 min). The key is drawing a light X and stopping when the X is no longer visible. It is a good lab and the students will get good results.
IT IS VERY IMPORTANT THAT YOU TELL YOUR STUDENTS TO WASH OUT THE WELL PLATE AS SOON AS POSSIBLE AFTER THE REACTION IS COMPLETE. FAILURE TO DO SO WILL RESULT IN A WELL PLATE THAT IS PERMANENTLY PITTED.
I do not show the students the slide that goes over the calculations of order. This is saved for the next class.
Homework: Determine the initial concentrations of the Na2S2O3 and HCl in each of the 6 trials. This is relatively easy for students to do and therefore other problems should be assigned.
How to Utilize High School Chemistry for A.P. Achievement 13.7 Rate Law Expressions and Rate Constant Calculations Involving a Single Reactant |
DAY 4 …
I begin the class by going over the assigned homework.
Discussion of the Sulfur Clock Reaction will take some time. For many students, this will be their first exposure to math that incorporates logarithms.
Before the period ends, I make every effort to show a differential rate law problem that may be seen on the AP Chemistry exam. The following is very important to remember …
The AP Chemistry Exam is not a test of the student’s math skills, but rather a test of the student’s knowledge of Chemistry.
The problems on the test use simple numbers that me answered mathematical or verbally. (i.e.; doubling the concentration of the reactant doubles the reaction rate. The order of the reactant is first-order.)
Assign differential problems from the text.
DAY 5 …
I begin the class by going over the assigned homework.
It is important to spend time on determining the rate constant and its units. Students will be asked to do this on the exam.
The last half of the period is used to prepare for Sally Vonderbrink’s Lab #12 - Study of the Kinetics of a Reaction
This is one of the more complicated labs that we do. Preparation is critical. The students have the following tasks:
- Clean a 96 well plate
- Label pipettes (one for each reactant)
- Label test tubes (one for each reactant)
- The student that will administer the drops must practice. (see lab)
- Secure all equipment until the next class period.
Homework: Students should do the initial concentration calculations for each of the 8 experiments.
Sally Vonderbrink’s Lab #12 - Study of the Kinetics of a Reaction: Photo-Lab
DAY 6 …
Sally Vonderbrink’s Lab #12 - Study of the Kinetics of a Reaction
This will take most of the period. I have not had good success with changing the temperature or adding a catalyst. Therefore, I skip these parts.
DAY 7 …
The postlab calculations will take time. Be prepared to explain the fundamentals of a clock reaction.
DAY 8...
The Arrhenius Equation and the Mechanisms are the last 2 topics.
From past AP Chemistry questions, the Arrhenius Equation and the determination of Ea are always associated with a graph. Students may be asked to …
- label the x- and y- axes of a graph used to determine the Ea.
- Sketch a line that represents the
- Given a graph , explain what the slope of the graph represents
Mechanisms …
For many years, the free response portion of the AP Chemistry test did not address mechanisms. But in 2008 and 2009 two great questions were on the AP Chemistry exam. I cannot post these questions in this format, but they may be found on AP Central.
For me, the most important statement that I make to my students is …
A mechanism may be possible if it meets the requirements of the stoichiometry and the rate law expression. Then only experimentation can determine if what is proposed is the true mechanism.
How to Utilize High School Chemistry for A.P. Achievement 13.20 The Arrhenius Equation |
FINAL NOTE:
As I mentioned earlier, I like teaching Kinetics. One of my new favorite labs is Kinetics: Differential and the Integrated Rate Laws, found in the Hostage-Fossett lab manual.
This lab, as all of the labs found in this publication, is extremely well written. Actually, it is a compilation of 4 labs.
Differential Rate Law: Catalyzed Decomposition of H2O2
Method 1: Heterogeneous Catalyst
Method 2: Homogeneous Catalyst
Integrated Rate Law: Kinetics of Radioactive Decay
Unfortunately time is the AP Chemistry teacher’s worst enemy. I find myself picking each year from all of the labs that I have discussed. I wish we had the time to do them all. There is always after the exam.
I hope these thoughts have been useful. I am now preparing my class for Equilibrium. This will also be the subject of my next installment.
Dale Jensen