exp 17 questions, sample calculations and sig. fig. for slopes, graphs. etc.
robert zellmer
zellmer.1 at osu.edu
Sun Mar 31 01:07:26 EDT 2019
We did a new exp 17 last week. Here's a few things which might help.
1) My on-line example**is for the old exp. You can still look at it to
see what a zero-order plot should look like. If you scroll down
to the bottom you'll find what you might find for the first and second
order plots might look like. See item #6 below for more explanation
and links.
2) Your initial concs. for each solution are different. You need to do a
dilution calc (M2*V2=M1*V1) to get the initial Dye conc., [Dye]o,
for each solution at the moment of mixing. The M1 is the conc. on
the bottle. The V1 are the volumes you were told to use in lab, they
may be different than what was in the manual since we made some
changes as we went through the week and you didn't have to have
volumes which exactly matched those in the manual for the Dye and
the water. Use your recorded volumes from lab. The V2 should be
the total volume of Dye, H2O and bleach at the moment of mixing.
This should be close to 40 mL.
3) The new manual gives an idea of what the report sheet should look like
and what the tables should look like. Use Excel to make your data
tables and the report sheet. The tables in the manual pages won't
work well in that you can't program equations into them. Make sure
you have the proper headings, units, s.f., etc. Import your tables
from
Excel into the report. Do NOT scan tables or graphs and include a
scan.
My suggestion is to use a different worksheet in Excel for the data for
each solution for Part A data. That means you'll have 4 worksheets in
Excel, one for each solution and each table would be on a single page
in the report. I'm referring to the table at the bottom of page 2
of the
report sheet in the manual pages on Carmen for Part A time, absorbance
and conc. data.
Make a worksheet for solution 1 (i.e. get everything set up
correctly in
this table, labels, units, s.f., equations) and then copy the
worksheet for
soln 1 to three other worksheets for solutions 2, 3 and 4. Then
all you
have to do is change your times and absorbance values in the tables for
solutions 2, 3 and 4. If you do this before making your graphs the
s.f.
set in your tables should transfer over to the graphs. The graphs
also need
to have the correct s.f. and units.
Since you'll have multiple s.f. for the Abs values (ranging from 1
to 3,
mostly 2) just use 2 s.f. for the y-axis (conc. related data) you
derive from
the Abs using the Beer's law eqn. Go ahead and use 3 s.f. for the
slopes
and rate constants.
If you have any negative Abs readings you can't use that data.
4) Sample calculations:
[Dye]o and [Bleach]o (at the moment of mixing)
Beer’s law constant (slope calc.)
[Dye]t
ln [Dye]t
1 / [Dye]t
Rate constant from any slope (graph 3 or 4)
Average of the rate constants
Average deviation for rate constants
5) You are suppose to report the Average value of the rate constants and
the average deviation of the rate constants. See the link at the
"Laboratory"
link which discusses the treatment of numerical data (Appendix F in
your
manual),
*Treatment of Numerical Data (Error Analysis, sig. fig., graphing)*
<https://www.asc.ohio-state.edu/zellmer.1/chem1250/lab/App_F_1250_lab_manual.pdf>
6) Graphs
A) Beer's law graph
This graph should have 6 data points (5 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.
The slope of this graph should be over 20,000.
B) Zero-order graph
I have an example of a good zero-order graph for the old exp 17.
Your graph won't look exactly the same but should be similar.
Zero-Order graph
<https://www.asc.ohio-state.edu/zellmer.1/chem1250/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. A zero order graph should eventually
approach zero so the lines for all 4 solutions will likely converge
toward the end of the lines. If they cross well before near the end
that's a problem. When this happens you will notice in graphs
3 or 4 for the 1st and 2nd order graphs (which ever produces the
most parallel lines) the slope for the line which crosses the
others 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.
C) 1st and 2nd order graphs
For the first and second-order graphs (graphs 3 and 4) 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
straight line fit to the points.
See the following link for examples using data from the old exp 17
of what
you might see,
Graphs 3 & 4
<https://www.asc.ohio-state.edu/zellmer.1/chem1250/lab/exp17/exp17_web_graphs_3_4_exs.pdf>
- Exs of what Graphs 3 & 4 might look like
What you should see is one of these two graphs will give parallel
lines and
the other won't. So if the first-order graph has parallel straight
lines the 2nd
order graph will have lines which clearly aren't parallel and the
data clearly
doesn't fit a linear trend line. If the 2nd-order graph has
parallel straight lines
the 1st-order graph will have lines which clearly aren't parallel
and the data
clearly doesn't fit a linear trend line.
D) Misc.
For graph 3 (1st-order) the label for the x-axis 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.
7) What if one of your solutions gives "bad" data? How will you know?
Lets say for your zero-order graph one of the lines crosses the other
lines well before 4 minutes. What should you do? Do your zero-order
graph twice, once with all four solutions and once with only the three
good ones. Then do the first and second-order graphs with only the
three good ones.
8) 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 and it's for
the older
exp 17.
*
I hope this helps.
Dr. Zellmer
**
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