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20030919, 14:21 #1
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Finding the rate and Graph Question (Excel 2002)
I have a problem with Excel. I have data in a table for pH and Temperature study.
I did an experiment dealing with absoption (OD) and time using pH and temp as conditions. The temperatures and pH changes. We are working with an enzyme and how the pH and temp affects it.
I can make the Excel graph  my problem is that of finding the rate. I have one example that was done (Problem 1 Sheet). I will send it to you.
Rate = change in absorption
Change in time
I wondered if you could help with the problem. I will attach the file with three sheets. Hopefully you see what my problem (s) are.
Is this possible for each of those experiments to be put in a graph and show rate.

20030919, 15:56 #2
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Re: Finding the rate and Graph Question (Excel 2002)
The problem is not really an excel problem. It is a kinetics problem. Though I do see some excel issues I will mention along with other things.
1) These are first order kinetics and curve is exponential. The rate changes over time. Usually one speaks of a rateconstant or a halflife, not a RATE. Could you give more details about what you need?
Your charts are "screwy" since your xrange includes the "RATE" in cell A13. Eliminate this and your charts will plot correctly.
Also at pH<10 the decay is so rapid that in <0.25 sec you have removed all you are going to (or your initial values have lots of errors and they do not change at all.) You need quicker methods to see and get the decay for these.
Instead of using absorbance, you might want to use some type of concentration. Does that absorbance min for each pH reflect a concentration of "0". If not the kinetics get tough to do since the equations must be solved more indirectly.
2) In problem you do not plot the initial since you have a text value instead of the number 0 for the initial time. These plots are similar to 1, change is either too rapid or initial value has lots of variation.The 100

20030921, 20:06 #3
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Re: Finding the rate and Graph Question (Excel 2002)
Absorbance vs time should be plotted as four line graphs on one single graph
Constants
pH  2/5/7/10 or
Temperature  5 /22/ 37/ 100
This graph can be used to determine the time period during which the rate is maximized
Therefore using this TIME segment to calculate the reaction rate
Calculate the reaction rate as change in absorbance per unit of time
The Experiment was working with salivary amalase... pH or temperature affecting the enzyme.... using starch and iodine... and reading absorbance and time in the experiment  spec 20 and 70 used
The initial velovity is most directly affected by limiting factors such as concentration, pH and temperature. In this experiment it is difficult to capture the initial velocity because it may be over within the first few minutes. Therefore, when deciding which portion of absorbance vs. time graph to use for calculating reaction rate....I wanted to choose the earliest period when the slope is STEEP and CONSTANT.

20030921, 21:20 #4
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Re: Finding the rate and Graph Question (Excel 2002)
Most scientific data should NOT be plotted using a line graph, but should use the scatter chart. The Line chart uses CATEGORIES for an XAxis, the scatter is the ONLY excel chart which uses NUMBERS fro Both axes.
If you want the INITIAL rate (from 0 to 0.25 min) then just use the formula, the change in Y value vs Change in X, in Problem1!B14 enter:
<pre>=(B5B4)/($A$5$A$4)</pre>
and copy it from C14:F14.
You must state somewhere that this is the INITIAL rate, since the rate is proportional to amount of the material (that is why it is exponential, and the RATE of absorbance change per time will change over time.
The rate is STEEPEST at the beginning, but in the 1st 3 pHs it is MORE constant at the END! In your first 3 charts it is plainly evident that rate of change AFTER 0.25 min is ZERO!
In the first 15 secs it looks like astratight line since you have only 2 points (2 points make a line), but if you had more (or a higher pH like pH=10) you can see how the rate changes over time. This chart, also will reach a "steady state" after some period.
As I said earlier, one usually uses RATE constant or halflife to indicate a rate, since it is constantly changing.
Steve
Steve

20030922, 08:19 #5
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Re: Finding the rate and Graph Question (Excel 2002)
Well done, Steve, on a clear and succinct refresher course on first order kinetics.
I was once a chemistry teacher!

20030922, 08:43 #6
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Re: Finding the rate and Graph Question (Excel 2002)
I was never a chemistry teacher (a Teacher's aid in Grad school, watching over the labs, proctoring exams, doing review sessions, etc), but I did take lots of chemistry!
Steve