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Rate Constant Calculation

Hi guys :smile:

I'm having some real problems with this calculation for calculating the rate constant of a reaction we did in the lab last week.

(see the attachment for the information) Basically I have (dz/dt) as that is just the gradient which units are in mL/min. And I used:

d[I2]/dt = (c/20) x (dz/dt) to calculate d[I2]/dt, but the problem with this is that it gives the wrong units.

c = mol/L
20 = mL, but I converted into L
(dz/dt) = mL/min but I converted the mL into L

So its: ((mol/L)/L) x (L/min) which ends up giving you the units mol L/min

The next step is:

d[I2]/dt = -K1[acetone]
K1 = -(d[I2]/dt) / acetone

which means

(mol L min-1) / (mol L-1) which gives you L2/min-1

But the final unit has to be in just min-1 as its a first order reaction. So I'm guessing that d[I2]/dt = (c/20) x (dz/dt) is wrong? Any help would be greatly appreciated :biggrin:
Picture 19.png31.0KB
Reply 1
Avatar for cptbigt
cptbigt
OP
7
Either d[I2]/dt = (c/20) x (dz/dt) is wrong....

Or the 20 magically has units of L-1 for no reason :P
(c/20) x (dz/dt) = molL−1L×Lmin=molL−1min−1\frac{mol L^{-1}}{L} \times \frac{L}{min} = mol L^{-1} min^{-1}

should be okay now :smile:
Reply 3
Avatar for cptbigt
cptbigt
OP
7
EierVonSatan
(c/20) x (dz/dt) = molL−1L×Lmin=molL−1min−1\frac{mol L^{-1}}{L} \times \frac{L}{min} = mol L^{-1} min^{-1}

should be okay now :smile:


Ah of course!

Damn :P All these units give me a headache, thanks for saving me hours of pain :biggrin:
cptbigt
Ah of course!

Damn :P All these units give me a headache, thanks for saving me hours of pain :biggrin:


You're doing it properly though :yep:

A lot of students would just blindly follow that kind of 'fill in the blank' set up and not think anything of it. The pain is good for you :tongue:

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