exp 17 questions, sample calculations and sig. fig. for slopes, graphs. etc.

[robert zellmer]

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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