Exp 17 - graphs

[robert zellmer]

Just a reminder to look at my files for instructions about what to
do for exp 17, especially the graphs.  You do different things if
you are graphing by hand versus doing things in Excel.

If using Excel you need to put the data for ALL 4 solns on
the zero, first and second-order graphs.

For the zero-order graph you should use an exponential or
parabola fit to get a smooth curve.

For the first and second-order graphs (graphs 3 and 4) you
should use LINEAR fits for both graphs.  Do NOT go back
and fit either to something other than linear.  You are looking
to see which graph has lines that are more closely parallel.
Generally speaking if you have good data you will see a distinct
difference between the two graphs.  Also, which ever one produces
lines that are more closely parallel those lines will fit the points 
better.

Sometimes one of the lines on the graphs will cross the others.
In that case the data for that soln is probably "bad" and you will
notice the slope isn't as similar to the other three slopes.
In this case you should report all four rate constants on the
report sheet but might consider not including the "bad" rate
constant in your average.  It depends on how different it is
from the other three.

Don't forget to discuss how you chose the order based on
graphs 3 and 4.  You should also discuss WHY the lines should
be parallel.

If the slope of your Beer's Law plot is a lot lower than 200
or a lot greater than 250 you may have done something wrong.
Also, if your zero-order graph shows all 4 lines pretty much
emanating from the same point (initial conc) or crossing early
on (at early times) you've likely made a mistake with your
initial concentrations.   The initial Cr^3+ in each test tube after
mixing is NOT what it was on the reagent bottle in lab.

Make sure each graph take up the whole page and your data
points occupy pretty much the whole graph area.  Look at
my Excel example for graph 2 (zero-order graph).

Remember, I sent out a couple of other e-mails about exp 17
already and I have the following help files,

*Laboratory* 
<http://chemistry.osu.edu/%7Erzellmer/chem1250/lab/lab_1250.htm> (see 
stuff dealing with exp 17)

*Pre and Post-lab Help for Exp 17 - (Answers to Students' Questions)* 
<http://chemistry.osu.edu/%7Erzellmer/chem1250/faq/exp17_help.txt>

*Handout for Exp. 17 - Report Tips (Data Analysis)* 
<http://chemistry.osu.edu/%7Erzellmer/chem1250/lab/exp17_tips.pdf>

*Exp 17 - Using MS Excel* 
<http://chemistry.osu.edu/%7Erzellmer/chem1250/lab/exp17/exp17.htm>


Dr. Zellmer
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