[CHM 112] CHM 112

[Andrew Pounds]

On 07/13/2015 08:23 PM, wrote:
> Dear Dr. Pounds.
>
> 	I have a few questions on the labs that are due tomorrow. On the “Kinetics: Temperature Effect” lab, I was wondering if the change in concentration of the MnO4 would just be the stock solution molarity, which is 0.0050 M, since the MnO4 is used up in the overall reaction. Therefore to find the rate would I just multiply the 0.0050 by one half and divide by the overall reaction time to find the rate?

Yes.   Think about it you took a stock solution and mixed it with the 
same volume of another compound.  This effectively halved the 
concentration.  The rate was then this concentration divided by the 
time.   The lab report form is a little confusing -- because the column 
for the rate includes a header that includes the one half. Just make 
sure you (1) divide the stock concentration by two and then (2) divide 
that by the time to get the rate.
>   On the “Kinetics: Concentration Effect” lab I am having problems trying to find the initial concentrations. I know I have to use M1V1=M2V2, would the V1 be the amount we use in the reaction or 1 liter since the molarity is defined as moles of solute over liters of solvent.
Its the amount you use for each reaction.  For example, in the first 
reaction the conentration of I- would be (0.30 M)(2 ml)/(150 ml)=0.004 
M.  This assumes I only used 1 ml of starch, so my final volume is 150 ml.
>   After finding the rates and the order of the reactions would I then make two separate rate laws, RATE=k[I-]^m and RATE=k[H2O2]^n to solve for the bottom portion of the lab report where it asks for the “Effect Trials”? Thank you for your time.
>
>
>
>
Nope -- it is one rate law $RATE = k [I^-]^m [H_2O_2]^n$.  The two plots 
you create let you determine the orders.  Once you have the orders, you 
can use your eight rates and eight different initial concentrations to 
solve for $k$. You will get eight different values of $k$ -- you take 
the average of all these and put that in the last blank.

-- 
Andrew J. Pounds, Ph.D.  (pounds at theochem.mercer.edu)
Professor of Chemistry and Computer Science
Mercer University,  Macon, GA 31207   (478) 301-5627

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