Chemical Forums

Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: inspiration100 on April 02, 2008, 09:32:12 PM

Title: How can I measure the change in concentration?
Post by: inspiration100 on April 02, 2008, 09:32:12 PM: Hey Guys. I'm doing a clock reaction lab and I'm wanting to find a formula for the reaction by using the rate law equation. I've figured all the variables out except how to calculate the change in concentration (mol/L*S) for the rates. I'm stuck here. I can't find the orders until I can somehow figure out the rate. Can someone help me out with this? Basically I know the concentrations and I will know the rate constant as soon as someone can help me with this.
Title: Re: How can I measure the change in concentration?
Post by: Rabn on April 02, 2008, 10:07:04 PM: you need to give us more information. what are the reactants? what are the concentrations? you need to provide that information.
Title: Re: How can I measure the change in concentration?
Post by: inspiration100 on April 02, 2008, 10:29:49 PM: I am doing the persulfate reaction -
2I-(aq) + S2O82-(aq) → I2 (aq) + 2SO42-(aq)

I have not calculated the molarities however I'm just looking for a general approach how to find the initial rates.
Title: Re: How can I measure the change in concentration?
Post by: inspiration100 on April 03, 2008, 08:29:33 PM: I appear to be getting no help on this, and I really do need help (it would be much appreciated!!). I finished all the reactions today and have all the concentrations. This is where I am stuck. How do I calculate the initial rate of the reaction? As stated before, I am using the reaction
2I-(aq) + S2O82-(aq) → I2 (aq) + 2SO42-(aq).

I have the original molarity of all the substances before the reaction and I have calculated the molarity of the substances when mixed together.

I will upload a my lab notebook table here shortly. I'm just not getting anyplace right now.
Title: Re: How can I measure the change in concentration?
Post by: ARGOS++ on April 03, 2008, 08:43:35 PM: Dear Inspiration100;

I think there could be something for you under “Instructions”: "5.02 Kinetics of … (http://ptcl.chem.ox.ac.uk/~hmc/tlab/experiments/502.html)”!

Good Luck!
ARGOS⁺⁺
Title: Re: How can I measure the change in concentration?
Post by: inspiration100 on April 03, 2008, 08:50:13 PM: Thanks for your reply ARGOS. That's a great page and I have been to it, however it doesn't talk about the initial rates and how to calculate them. That is where I am lost.

Here is my lab notebook calculations.
(https://www.chemicalforums.com/proxy.php?request=http%3A%2F%2Fimg213.imageshack.us%2Fimg213%2F5569%2Fuploadimagerg4.th.jpg&hash=19fac453551511c0bf036e2d159e62b03e6306f7) (http://img213.imageshack.us/my.php?image=uploadimagerg4.jpg)
Title: Re: How can I measure the change in concentration?
Post by: ARGOS++ on April 03, 2008, 09:02:37 PM: Dear Inspiration100;

In this moment I don’t see where you run into troubles with the graphical Recipe under “Rate Equation”?
Are you able to show us your Graph?

Good Luck!
ARGOS⁺⁺
Title: Re: How can I measure the change in concentration?
Post by: inspiration100 on April 03, 2008, 09:09:28 PM: Hey ARGOS. I appreciate the help. I'm trying not doing this graphically (unless I need to). I'm just missing one step to solve for the orders in the rate equation.
Title: Re: How can I measure the change in concentration?
Post by: ARGOS++ on April 03, 2008, 09:48:56 PM: Dear Inspiration100;

I don’t believe hat you can apply the recipe of Question 4 of your link “directly” to the Clock Reaction.
Question 4 is only a hint how you may find out relations between concentrations changes and the influence of the Initial Rate (IR), but not its inital value self.

As you may see there: Doubling [B.] ends in Doubling IR; and triple the [A.] is (triple)² the IR.
But that’s only valid for the particularly given Reaction and Data in Question 4, and nowhere else!

Sorry!, ─ At least I believe you will have to do it graphically, expect you like a lot of statistics instead.

Good Luck!
ARGOS⁺⁺
Title: Re: How can I measure the change in concentration?
Post by: inspiration100 on April 05, 2008, 10:06:41 PM: Thanks. I figured out the rate law equation. Now I am curious how I can solve for the change in heat. Is there an equation I can use for that?
Title: Re: How can I measure the change in concentration?
Post by: Rabn on April 06, 2008, 04:18:55 AM: Clock reactions are just redox reactions if I remember right. With the rate law and the nernst equation you should be able to figure out delgtaG(reaction at Temp T) at the point when the reaction occurs by measuring the voltage and from there get the heat. Using the voltage when the reaction occurs will allow you to solve for the value of RT(etc...) in the nernst equation once you subtract the clalculated voltage at standard conditions from a redox table. From there you can get concentrations, they give you the value of Q right when the equilibrium reaches the point where it will react. THen use that data to solve for deltaG and from there you can find the change in entropy. The change in entropy mutliplied by the temp( in K ) will equal the change in enthalpy( which equals the change heat since there is no PV work, assuming no gases are produced).
Title: Re: How can I measure the change in concentration?
Post by: inspiration100 on April 06, 2008, 08:31:50 PM: Thanks for the information! That will help. I'm leaning towards trying to solve this using Arrhenius's equation but that will be of help.

I'm stuck on something else now :-[. I'm trying to create a sort of a "calculator" for the clock reaction time. I've sucessfully done this for the first reaction, but I'm stuck how to do this for the Na2S2O3 since this is in a different equation and thus a different k value.... BLAST!
Say, anyone wanna make $7 via paypal? If you got MSN messenger and know this clock reaction stuff backwards and forwards, I could really use your help/tutoring.