<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
</head>
<body text="#000000" bgcolor="#FFFFFF">
I always get lots of questions about exp 3 (DRL) . 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. Dr. Moga sent in e-mail with some things about this<br>
exp. with my old links, some of which have been updated. There's
not<br>
a lot a difference in the info in the old and new the links.<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.<br>
<br>
</b> My example shows 4 solutions. In the previous version of
this exp<br>
(exp 17,CrY) 4 solutions were used rather than the 3 used in
DRL. The<br>
volumes Cr^3+ solution used and thus the initial concentrations
are also<br>
different.<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.),
[Cr^3+]o. The<br>
initial Cr^3+ conc., [Cr^3+]o, and the conc. of the CrEDTA-
product in<br>
the boiled solns, [CrEDTA-]f , are related. The reaction is 1
Cr^3+<br>
reacting with 1 H2EDTA^2- to give 1 CrEDTA^-. Thus, ALL the
Cr^3+<br>
at the moment of mixing is converted to the product, CrEDTA^-,
in the<br>
boiled solns since the H2EDTA^2- is in vast excess and the
Cr^3+ is the<br>
limiting reactant. So how are [Cr^3+]o and [CrEDTA^-]f
related?<br>
<br>
3) You should use Excel (or something similar) and have your data
tables set<br>
up so they look like those in the data tables in the report
template (headings,<br>
units, etc.). It's pretty easy to copy and past headings from
the tables in the<br>
template to Excel. If you do this you do not have to recopy
all the data to the<br>
report data tables. You can import your Excel sheets into the
report template.<br>
Then delete the blank data tables.<br>
<br>
Make up a page with Parts A, C and D data as shown in the data
table in the<br>
report template.<br>
<br>
My suggestion is you use a different worksheet in Excel for the
data for each<br>
solution (part B, 2nd table). Transfer each to the report
template.<br>
<br>
Make sure you have the correct number of sig.fig. and units in
your tables.<br>
Do this for solution 1 (i.e. get everything set up correctly in
this table) and<br>
then copy the worksheet for soln 1 to another two worksheets
for solutions<br>
2 and 3. Then all you have to do is change your times,
absorbance values<br>
and the initial Cr^3+ in the tables for solutions 2 and 3. If
you do this before<br>
making your graphs the s.f. set in your tables should transfer
over to the 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="https://cbc-wb01x.chemistry.ohio-state.edu/%7Erzellmer/chem1220/lab/exp3_DRL_tips.pdf"><b>Handout
for Exp. 3 (DRL) - Report Tips (Data Analysis)</b></a><br>
<br>
For the sample calculations, see the rubric and item # 12 in
the following link,<br>
<br>
<b> </b><a
href="https://cbc-wb01x.chemistry.ohio-state.edu/%7Erzellmer/chem1220/faq/exp3_DRL_help.txt"
moz-do-not-send="true"><b>Exp 3 (DRL) - Help for Exp 3</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 need
to<br>
right click on the equations on the graph. Then choose "format
equation",<br>
then choose "number" and then set it to enough decimal places
so the<br>
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 D, item #4 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 D 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<br>
greater than 240 you may have done something wrong. However, <br>
even if the slope is outside of this range give it a try and
see what the<br>
zero-order graph looks like, even though you don't need this
graph for<br>
the report (although there is a question in the report dealing
with this).<br>
This graph should have 4 data points (3 solutions and the
origin (0,0))<br>
and be forced through the origin (an option when you do the
trend line).<br>
You will know if you didn't do this if your eqn. has an
intercept (it<br>
should be y = m*x).<br>
<br>
You need to put the data for ALL 3 solns on the first and
second-order graphs. <br>
<br>
Take a look at my help files and the Exp 3 Excel example. It
may refer<br>
to exp 17 at times since that was the old exp number. Make
sure your graphs<br>
take up the whole page (one per page) and your data points
occupy pretty<br>
much the whole graph area. Take a good look at the picture I
have for the<br>
old graph 2 (zero-order). It will show you an example of what a
good graph<br>
should look like, <br>
<br>
<a
href="http://chemistry.osu.edu/%7Erzellmer/chem1220/lab/exp17/exp17_web_graph2_ex.pdf">Zero-Order
graph</a> - Examples of a good graph (do <b>NOT </b>include in
the report)<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. <b>Do </b><b>NOT</b><br>
<b>include this graph in the report. </b>I'm suggesting you
do a rough<br>
graph in Excel to see what the data looks like. It can tell
you<br>
if there might be a calculation error or if your data has
problems (see<br>
below). Plus, it will give you an idea of how to prepare the
1st and<br>
2nd order plots which you will have in the report and be
graded.<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 2 or 3 (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 rate constants in the report but might consider not<br>
including the "bad" rate constant in your average. It depends
on<br>
how different it is from the other two. You should discuss
this<br>
in the Discussion section of the report.<br>
<br>
Also, if your zero-order graph shows all 3 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+. The
initial<br>
Cr^3+ in each test tube after mixing is NOT what it was on the<br>
reagent bottle in lab. Your initial concentrations for each
solution<br>
after mixing should be different. Also, your data points for
each<br>
should all be starting at pretty much the same time (time = 0,
or slightly<br>
after time = 0) and be pretty much lined up vertically as time
proceeds<br>
(see the zero-order graph). Graphs 2 (1st order) and 3 (2nd
order) will<br>
also show odd behavior in that neither will have parallel lines
but instead<br>
seem to emanate from essentially the same point.<br>
<br>
For the first and second-order graphs (graphs 2 and 3) you
should use<br>
LINEAR fits (trend lines) for both graphs. Do NOT go back and
fit either<br>
to something other than linear. You are looking to see which
graph has lines<br>
that are more closely parallel. Generally speaking, if you
have good data<br>
you will see a distinct difference between the two graphs.
Also, which ever<br>
graph has lines which are more closely parallel will often
produce a better<br>
fit 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
2 & 3</a> - Exs of what Graphs 2 & 3 might look like<br>
<br>
For graph 2 (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. Set
margins to<br>
zero.<br>
<br>
What if one of your solutions gives "bad" data? How will you
know?<br>
Lets say for your first-order graph (or zero-order if you do
it) two<br>
solutions have lines which seem to be parallel but then one of
the lines<br>
seems to be far from parallel, maybe even crossing the other
lines.<br>
What should you do? Do your first-order graph twice, once with
all<br>
three solutions and once with only the two good ones. Then do
the<br>
second-order graph with only the two good ones.<br>
<br>
Remember, I sent out a couple of other e-mails about exp 3<br>
already and I have the following help files,<br>
<br>
<a
href="http://chemistry.osu.edu/%7Erzellmer/chem1220/lab/lab_1220.htm"><b>Laboratory</b></a>
(see stuff dealing with exp 3)<br>
<br>
<a
href="http://chemistry.osu.edu/%7Erzellmer/chem1220/faq/exp3_DRL_help.txt"><b>Pre
and Post-lab Help for Exp 3 - (Answers to Students' Questions)</b></a><br>
<br>
<a
href="http://chemistry.osu.edu/%7Erzellmer/chem1220/lab/exp3_DRL_tips.pdf"><b>Handout
for Exp. 3 - Report Tips (Data Analysis)</b></a><br>
<br>
<a
href="http://chemistry.osu.edu/%7Erzellmer/chem1220/lab/exp17/exp17.htm"><b>Exp
3 - Using MS Excel</b></a><br>
<br>
9) Don't forget to discuss how you chose the order based on graphs
2 and 3.<br>
Also, discuss <b>WHY </b>one of the graphs should have lines
which are parallel.<br>
Compare the rates for the three 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>
10) Finally, remember to use the template provided on Carmen for
the report.<br>
<br>
I hope this helps.<br>
<br>
<br>
Dr. Zellmer
</body>
</html>