exp 3 (DRL) questions, sample calculations and sig. fig. for slopes, graphs. etc.
robert zellmer
zellmer.1 at osu.edu
Sun Sep 17 22:22:33 EDT 2017
I always get lots of questions about exp 3 (DRL) . I have may have already
addressed them in a previous e-mail or my help links but here they are
one more time. Dr. Moga sent in e-mail with some things about this
exp. with my old links, some of which have been updated. There's not
a lot a difference in the info in the old and new the links.
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.
* My example shows 4 solutions. In the previous version of this exp
(exp 17,CrY) 4 solutions were used rather than the 3 used in DRL. The
volumes Cr^3+ solution used and thus the initial concentrations
are also
different.
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.),
[Cr^3+]o. The
initial Cr^3+ conc., [Cr^3+]o, and the conc. of the CrEDTA-
product in
the boiled solns, [CrEDTA-]f , are related. The reaction is 1 Cr^3+
reacting with 1 H2EDTA^2- to give 1 CrEDTA^-. Thus, ALL the Cr^3+
at the moment of mixing is converted to the product, CrEDTA^-, in the
boiled solns since the H2EDTA^2- is in vast excess and the Cr^3+
is the
limiting reactant. So how are [Cr^3+]o and [CrEDTA^-]f related?
3) You should use Excel (or something similar) and have your data
tables set
up so they look like those in the data tables in the report
template (headings,
units, etc.). It's pretty easy to copy and past headings from the
tables in the
template to Excel. If you do this you do not have to recopy all
the data to the
report data tables. You can import your Excel sheets into the
report template.
Then delete the blank data tables.
Make up a page with Parts A, C and D data as shown in the data
table in the
report template.
My suggestion is you use a different worksheet in Excel for the
data for each
solution (part B, 2nd table). Transfer each to the report template.
Make sure you have the correct number of sig.fig. and units in
your tables.
Do this for solution 1 (i.e. get everything set up correctly in
this table) and
then copy the worksheet for soln 1 to another two worksheets for
solutions
2 and 3. Then all you have to do is change your times, absorbance
values
and the initial Cr^3+ in the tables for solutions 2 and 3. If you
do this before
making your graphs the s.f. set in your tables should transfer
over to the graphs.
4) Look at the following link for a summary of the calculations and
equations
you'll be using,
*Handout for Exp. 3 (DRL) - Report Tips (Data Analysis)*
<https://cbc-wb01x.chemistry.ohio-state.edu/%7Erzellmer/chem1220/lab/exp3_DRL_tips.pdf>
For the sample calculations, see the rubric and item # 12 in the
following link,
***Exp 3 (DRL) - Help for Exp 3*
<https://cbc-wb01x.chemistry.ohio-state.edu/%7Erzellmer/chem1220/faq/exp3_DRL_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 D, item #4 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 D 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
greater than 240 you may have done something wrong. However,
even if the slope is outside of this range give it a try and see
what the
zero-order graph looks like, even though you don't need this graph for
the report (although there is a question in the report dealing
with this).
This graph should have 4 data points (3 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).
You need to put the data for ALL 3 solns on the first and
second-order graphs.
Take a look at my help files and the Exp 3 Excel example. It may
refer
to exp 17 at times since that was the old exp number. 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 the
old graph 2 (zero-order). It will show you an example of what a
good graph
should look like,
Zero-Order graph
<http://chemistry.osu.edu/%7Erzellmer/chem1220/lab/exp17/exp17_web_graph2_ex.pdf>
- Examples of a good graph (do *NOT *include in the report)
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. *Do **NOT*
*include this graph in the report. *I'm suggesting you do a rough
graph in Excel to see what the data looks like. It can tell you
if there might be a calculation error or if your data has problems
(see
below). Plus, it will give you an idea of how to prepare the 1st and
2nd order plots which you will have in the report and be graded.
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 2 or 3 (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 rate constants in the report but might consider not
including the "bad" rate constant in your average. It depends on
how different it is from the other two. You should discuss this
in the Discussion section of the report.
Also, if your zero-order graph shows all 3 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+. The initial
Cr^3+ in each test tube after mixing is NOT what it was on the
reagent bottle in lab. Your initial concentrations for each solution
after mixing should be different. Also, your data points for each
should all be starting at pretty much the same time (time = 0, or
slightly
after time = 0) and be pretty much lined up vertically as time
proceeds
(see the zero-order graph). Graphs 2 (1st order) and 3 (2nd
order) will
also show odd behavior in that neither will have parallel lines but
instead
seem to emanate from essentially the same point.
For the first and second-order graphs (graphs 2 and 3) you should use
LINEAR fits (trend lines) 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
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 2 & 3
<http://chemistry.osu.edu/%7Erzellmer/chem1220/lab/exp17/exp17_web_graphs_3_4_exs.pdf>
- Exs of what Graphs 2 & 3 might look like
For graph 2 (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. Set margins to
zero.
What if one of your solutions gives "bad" data? How will you know?
Lets say for your first-order graph (or zero-order if you do it) two
solutions have lines which seem to be parallel but then one of the
lines
seems to be far from parallel, maybe even crossing the other lines.
What should you do? Do your first-order graph twice, once with all
three solutions and once with only the two good ones. Then do the
second-order graph with only the two good ones.
Remember, I sent out a couple of other e-mails about exp 3
already and I have the following help files,
*Laboratory*
<http://chemistry.osu.edu/%7Erzellmer/chem1220/lab/lab_1220.htm> (see
stuff dealing with exp 3)
*Pre and Post-lab Help for Exp 3 - (Answers to Students' Questions)*
<http://chemistry.osu.edu/%7Erzellmer/chem1220/faq/exp3_DRL_help.txt>
*Handout for Exp. 3 - Report Tips (Data Analysis)*
<http://chemistry.osu.edu/%7Erzellmer/chem1220/lab/exp3_DRL_tips.pdf>
*Exp 3 - Using MS Excel*
<http://chemistry.osu.edu/%7Erzellmer/chem1220/lab/exp17/exp17.htm>
9) Don't forget to discuss how you chose the order based on graphs 2 and 3.
Also, discuss *WHY *one of the graphs should have lines which are
parallel.
Compare the rates for the three 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.
10) Finally, remember to use the template provided on Carmen for the
report.
I hope this helps.
Dr. Zellmer
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