Re: Chem 3610/3611/3620/3621

Moderator: Ron Robertson

Re: Chem 3610/3611/3620/3621

Postby Ron Robertson » Tue Nov 15, 2016 11:09 am

Lots of problems with the HW you handed in on the method of initial rates (Self test 21.4)

1. In general, make sure you make a table of the numbers you are going to graph. Make sure you do this for the HW for Wednesday as well.
2. On this assignment there was confusion about graphing using log or ln. You can use either but several of you mixed when you were trying to use the y- intercept.
3. For this assignment you had to graph using the entire number with common units in order to get the correct y-intercept. For the slope you can use any units because with a ln/ln plot (or log/log) the units cancel out on the slope. (Think back to the heat of vaporization lab.) For example, on the first point you must graph the ln or log of 5.0E-3 moles/l and the ln or log of 3.6E-7 mole/l-s.
4. Don't use one data point with the slope that you found and solve for the specific rate constant. This does not give you the most accurate value. You want to get the best value from all the data for the specific rate constant. Make sure you give the proper units of "k".
5. Make sure you explain using an equation why you are taking "e" to some power or "10" to some power to get "k".

Make sure you have 3 graphs with the associated R^2 values for Prob. 21.1 to prove the order and calculate the rate constant from the slope. Then use the determined rate law to get the mass of cyanate left after 300. min. Make sure you graph the amount of cyanate and not the amount of urea at each instant of time.

We have a lot of graphical analysis as well as regular equations and derivations these last few days together. Finish strong - no regrets!
Ron Robertson
 
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Re: Chem 3610/3611/3620/3621

Postby Ron Robertson » Wed Nov 16, 2016 5:49 pm

Help Session on Wed. is in E306 (our classroom)
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Re: Chem 3610/3611/3620/3621

Postby Ron Robertson » Thu Nov 17, 2016 2:30 pm

All lab campers

I have set up an assignment on Turnitin.com for the Solid/Liquid Phase Diagram lab. The due date for both Tues and Thursday labs will be next Tuesday at 11:59 pm. This will assist me in making sure that the members of a lab group are not sharing more than data.
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Re: Chem 3610/3611/3620/3621

Postby Ron Robertson » Fri Nov 18, 2016 12:03 pm

Comments on the grading of Prob 21.1

1. Make sure you have data tables of all numbers to be graphed
2. Conceptually the balanced equation gives a conservation of mass between reactants and products instead of moles. (Think H2 + O2 --> 2H2O) This is why you should have taken the number of grams of urea and subtracted from the initial number of grams of cyanate to get the number of grams remaining and then convert to moles of cyanate. Subtracting moles does work on this problem because you have 1 mole on both sides. Be careful!
3. Be careful in Excel when using the slope from the trendline. The number of sf may not be adequate. You can reformat the trendline to be a certain number of decimal places or in scientific notation. This is not a problem in Logger Pro.
4. When solving for the mass of cyanate left you need to find the concentration using the 2nd order equation. Since there is 1 L of solution the molarity is numerically equivalent to the number of moles. You need to explain or show this and then show the conversion to grams.
5. Units are so important. Show them!

These same ideas are important on the graphical problems for today. It is always safer to graph the entire number and not a proportional number. Look at the number and see if it is multiplied by some factor or if the unit is in mmoles instead of moles for example. You need to have common units on the axes.

The material we cover in the final 3 days of class is the most important material of the entire semester for pre-professional health students. The concept of steady state is used all the time in pharmacology - sometimes incorrectly. You need to understand how to use it.

Finish strong. No regrets!
Ron Robertson
 
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Re: Chem 3610/3611/3620/3621

Postby Ron Robertson » Mon Nov 21, 2016 8:27 am

Comments on grading of 21.3 and 21.12

1. We're getting better. . .
2. Make sure you have a table of data with units
3. You Excel folks need to reformat your trendline to get the proper number of sf
4. Look to see if there is a multiplier for the units of a number. Graph the entire number. Also look to see if there are unit changes like mmoles or the like and graph common units on both axes. This is the safest approach.
5. Don't confuse rate law graphical analysis with activation energy analysis. 21.12 was not about a first order analysis. It did have first order rate constants but putting that into the title indicates you don't know what you're doing.

As of right now I don't intend on having a group help session tomorrow (Tuesday) after lab. I will answer any and all questions anyone has of course but I figure most of you want to get home for Thanksgiving and I will be putting together another load of stuff to take home. We will have a group help session next Tuesday at some time. Right now there is no lab scheduled so anytime Tuesday would be OK. Let's see if any groups need makeup time to redo the kinetics lab and then we will schedule next Monday during class the help session time.

Our problem set HW for next Monday requires you to use your curve-fitting skills to get rates of a reaction by obtaining the 1st derivative of the curvefit. Also you will use your propagated error skills to find the best value of "k". I recommend once again Logger Pro as the best (and certainly quickest) option to carry out these tasks.

Next Monday we will finish the steady state derivation by Briggs and Haldane of the Michaelis Menton mechanism for enzyme analysis. There will be one more problem set due on Wednesday when we take Exam 4.

Finish strong - no regrets!
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Re: Chem 3610/3611/3620/3621

Postby Ron Robertson » Mon Nov 21, 2016 11:35 am

Update on the Activation Energy Problem for today (Monday)

To find the error in the slope that you need to propagate the error for the activation energy, either use:

1. Logger Pro in curve fit mode to y=mx + b (the error in the slope is auto given on the graph where it is not in linear fit mode)

2. Excel with the data analysis tool pak for linear regression which gives the error in the slope (or find the formula function for the standard error of the slope)

This problem is very similar to the propagated error on our lab for Heat of Vaporization.
Ron Robertson
 
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Re: Chem 3610/3611/3620/3621

Postby Holly Jones » Wed Nov 23, 2016 4:13 pm

Hey everyone, you may have seen the flyers around, we have a group project (data analysis) in Chem 4840 so it would be super helpful if you would take our survey and let others know about it too. We need a large sample size! The ad for it is also on facebook, you can find it on the chemistry department page. Here is the link if you just want to go to it from here. https://t.co/iYVUnnnMhd
I would really appreciate it! -Holly Jones, Rhett Milam, William Hutcherson
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Re: Chem 3610/3611/3620/3621

Postby Holly Jones » Sun Nov 27, 2016 6:11 pm

Hi Dr. Robertson, on the kinetic problem worksheet, what is meant in #2 by "what would be the official "rate" of the reaction at 300 s and 3000 s?"
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Re: Chem 3610/3611/3620/3621

Postby Ron Robertson » Sun Nov 27, 2016 9:47 pm

Holly,

Hope you had a good Thanksgiving. It is time to get back to work though.

This question goes back to the first day on kinetics when I defined the official "rate" of a reaction in terms of the rate of disappearance of reactants and the rate of appearance of products. Look at the first homework problems and the first set of notes to see how the official rate is defined.
Ron Robertson
 
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Re: Chem 3610/3611/3620/3621

Postby Ron Robertson » Mon Nov 28, 2016 11:38 am

Comments on HW problems 21.37 and handout Activation Energy Problem

1. Units, units, units! You must understand and use them properly.

2. You made numerous errors with the graphical analysis of the Arrhenius equation and finding the slope (with proper units) and setting it equal to -Ea/R. In general you need a data table for the numbers you are going to graph.

3. Most of you seem to have forgotten how to do propagated error from the 1st of the semester in lab. I hope you did review this process for the HW that is due today. You will find a previous posting (just a couple back) on the Board about getting the error in the slope.

4. Attention to detail!

For some of you this may be the most important academic week of your life. I truly hope that you spent some meaningful time studying this break as well as getting a little rest and time with family. I know I did.

Finish strong - no regrets!
Ron Robertson
 
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