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<big><font size="2"><big>I'm getting a number of questions
concerning exp 18 and sig. fig.,<br>
particularly about those for your times.<br>
<br>
I know a lot of you use your phones (stopwatch feature) to
time things<br>
(which actually caused some problems for exp 17). I know
these<br>
stopwatches give times to fractions of a second. However, do
you really<br>
believe you were that accurate in deciding exactly when the
color disappeared?<br>
I think you are lucky if your times are accurate to ± 1 sec.
Most likely, your<br>
overall accuracy is to the units place (e.g. if you recorded
15 secs for the<br>
75 deg C run you are likely only accurate to the units place).<br>
<br>
Remember from a previous e-mail you can't use times like
h:mm:ss.<br>
You can use minutes or seconds for your times, it doesn't
matter. Using one<br>
or the other simply shifts your line up or down but doesn't
change the slope.<br>
Even so, the number of s.f. will be different for the various
times. If you were<br>
timing in seconds or convert to seconds most of you will have
about 2 s.f. for<br>
time for the first 2 or 3 time and temp readings. Then the
rest will generally<br>
have 3 s.f., maybe 4 for the last reading. In the last reading
you probably got<br>
something between 15-20 minutes. This would be 900-1200
seconds. If you<br>
had recorded this in seconds you would still be accurate only
to about + or - 1<br>
second (units place in seconds). That means your times will
have a different<br>
number of s.f. and ln(t) will have different numbers of
decimal places. Remember,<br>
the number of decimal places in the ln(t) will be the same as
the number of s.f. in<br>
the time.<br>
<br>
When you graph, since your ln(t) will generally have 2-4
decimal places (mostly 3)<br>
just us 3 decimal places on the ln(t) axis. You will have to
format the axis by<br>
right-clicking the axis and then choose "format axis" and
choose "number" with<br>
3 decimal places.<br>
<br>
The s.f. for you temperature values (in kelvin of course) will
depend on<br>
how accurately you recorded the temperatures. You should have
recorded<br>
the temps to 1 decimal place in lab but some of you may have
recorded your<br>
temps to only the units place. Use whatever s.f. you got in
the lab while doing<br>
the exp. You can't simply add another s.f. to the
temperatures to get the correct<br>
number of s.f. Your temps in kelvin will wind up with either
3 or 4 s.f. (depending<br>
on whether you recorded them to 1 decimal place). This is the
addition/subtraction<br>
rule. You should have seen this in exp 14.<br>
<br>
Since your ln(t) and temp will generally have 3 or 4 s.f. you
can safely<br>
report your slope and Ea to 3 s.f. Even though some of the
time values have<br>
only 2 s.f. you can gain one from the graph for the slope.
Don't worry about<br>
the s.f. on the graph for the slope if it has more than needed
as long as you<br>
report it on the report sheet to the correct s.f. If the
slope on the graph doesn't<br>
have at least the number of s.f. you need you can format it so
it will. Right click<br>
on the equation for the line and format the equation (as you
would for the numbers<br>
in the table to give the correct s.f.). </big></font></big><big><font
size="2"><big><big><font size="2"><big>Make sure you have enough
s.f. for the slope<br>
from the graph in order to get the correct number of
s.f. for Ea.<br>
</big></font></big> <br>
Make sure your graph conforms to the instructions I've been
giving all semester.<br>
Some of the graphs for exps 14, 15 and 17 are not even close
to what they<br>
should have been (more than one to a page, not taking up
pretty much the whole<br>
page, incorrect s.f., missing units, non-descriptive titles
and labels, etc.)<br>
<br>
As stated in the rubric your sample calculations should
include a sample calculation<br>
for the slope of the line. If you use Excel you still need to
show a sample calc. for<br>
the slope from your line using points from the line (not your
data points). However,<br>
use the Excel slope to calc. Ea. <br>
<br>
The 3rd Point to Consider in the manual has an error. It
states the to discuss the time<br>
it took the reaction to proceed at each temperature measured.
It then states "relate<br>
this to k, the reaction rate," It should state "<b>relate
this to k, the rate constant,<br>
and the</b><b> </b><b>reaction rate</b>".<br>
<br>
Dr. Zellmer</big></font></big>
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