exp 17 questions, sample calculations and sig. fig. for slopes, graphs. etc.
robert zellmer
zellmer.1 at osu.edu
Tue Jun 21 17:17:26 EDT 2016
I always get lots of questions about exp 17. I have may have already
addressed them in a previous e-mail or my help links but here they are
one more time.
1) In my on-line example I used *different conc. *than what you used in
the
actual exp. *Do NOT use the conc. I have in my example. Use yours.*
2) Your initial concs. for each solution are different. You need to do a
dilution calc (M2*V2=M1*V1) to get the initial Cr^3+ conc. for each
solution at the moment of mixing (with the EDTA soln.). The
initial Cr^3+
conc. and the conc. of the CrEDTA- product in the boiled solns are
related since all the Cr^3+ is converted to the product in the
boiled solns.
The Cr^3+ is the limiting reactant so how are they related?
3) If you are using Excel and have your data tables set up so they look
like
those on the report sheets (headings, units, etc.) you do not have
to recopy
all the data to the report sheets. You can turn in your Excel
sheets. However,
you still have to leave the blank report sheets in the lab
report. Make sure
you have the correct number of sig.fig. and units in your tables
(and graphs).
4) Look at the following link for a summary of the calculations and
equations
you'll be using,
*Handout for Exp. 17 - Report Tips (Data Analysis)*
<http://chemistry.osu.edu/%7Erzellmer/chem1220/lab/exp17_tips.pdf>
For the sample calculations, see the rubric and item # 12 in the
following link,
*Exp 17 - Help for Exp 17*
<http://chemistry.osu.edu/%7Erzellmer/chem1220/faq/exp17_help.txt>
5) You need to report the correct number of s.f. for the rate constants
To set the number of s.f. for the slopes in your graph you
need to right click on the equations on the graph. Then choose
"format
equation", then choose "number" and then set it to enough decimal
places so
the slope has the correct number of s.f. For most of you
measuring the time
to the minutes using the wall clock, the slope should have 3 s.f.
6) For Part C, item #6 is asking for the Average value of your rate
constant
AND the avg. deviation. See the link in the "Laboratory" link
which discusses
the treatment of numerical data (Appendix F in your manual),
*Treatment of Numerical Data (Error Analysis, sig. fig., graphing)*
<http://chemistry.osu.edu/%7Erzellmer/chem1220/lab/App_D_122_lab_manual.pdf>
7) I've also received questions about the graphs.
If the slope of your Beer's Law plot is a lot lower than 200 or a
greater than 240 you may have done something wrong. Even if
the slope is outside of this range give it a try and see what the
zero-order graph looks like. This graph should have 5 data points
(4 solutions and the origin (0,0)) and be forced through the origin
(an option when you do the trend line). You will know if you didn't
do this if your eqn. has an intercept (it should be y = m*x).
If using Excel, you need to put the data for ALL 4 solns on the zero,
first and second-order graphs.
Take a look at my help files and the Exp 17 Excel example. Make
sure your graphs take up the whole page (one per page) and your
data points occupy pretty much the whole graph area. Take a good
look at the picture I have for graph 2 (zero-order). It will show
you an
example of what a good graph should look like,
Graph 2
<http://chemistry.osu.edu/%7Erzellmer/chem1220/lab/exp17/exp17_web_graph2_ex.pdf>
- Examples of a good graph
These examples are for the zero-order plot. This is actual
data and graphs from a previous year that I cleaned up.
You can use an exponential or parabola for the zero-order plot.
(which ever seems to give a better fit, usually exponential).
Do *NOT *use a *linear *fit for the zero-order plot.
Note two lines cross toward the end. They shouldn't cross. There
was something wrong with the data toward the end of the run
for one of the solutions. When this happens you will notice in
graphs 3 or 4 (which ever produces the most parallel lines) the
slope for one of the lines which cross isn't as similar to the slopes
for the other lines (not as closely parallel). 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. You should discuss this
in the Discussion section of the report.
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
calculations for the initial concentrations for Cr^3+. Your initial
concentrations for each solution should be different. Also, your
data points for each should all be starting at pretty much the same
time (time = 0) and be pretty much lined up vertically as time
proceeds (see graph 2).
For both graphs 3 and 4 fit the data to LINEAR trend lines. 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 graph has
lines which are more closely parallel will often produce a better fit
to the points.
See the following link for examples of what you might see,
Graphs 3 & 4
<http://chemistry.osu.edu/%7Erzellmer/chem1220/lab/exp17/exp17_web_graphs_3_4_exs.pdf>
- Exs of what Graphs 3 & 4 might look like
For graph 3 (1st-order) the label for the axes will likely be at the
top of the graph. You can move it to the bottom. You actually
have to right-click on the Y-axis, choose "Format Axis" and then
choose "Value (X) axis crosses at:" or "Horizontal axis crosses:"
and set this to the minimum value on the Y-axis. It should move
to the bottom of the graph.
Set all the graphs to be printed in *LANDSCAPE *mode (normally the
default for Excel). This gives better looking graphs.
Don't forget to discuss how you chose the order based on graphs 3
and 4.
Also, discuss *WHY *one of the graphs should have lines which are
parallel.
Compare the rates for the four solutions and do they make sense.
Don't
forget your rate constants and the rate law. Look at the "Points
to Consider".
Please *remember*, *my Excel example *is just that, an *example
*of what
to do and how to do it. *It is not a complete example.* I did
*NOT* use
the *same initial concentrations for the Cr^3+ as you are using*.
It was
also produced a long time ago when most students were still graphing
this by hand. The example was done based on the manual at the time
when we used Excel to do it the same as someone doing it by hand.
I've changed some of the instructions in my Excel example to
explain what to do if doing it by hand or using Excel.
8) Finally, remember to use the template provided on Carmen for the report.
I hope this helps.
Dr. Zellmer
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