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