September 27, 2023, 11:52:15 PM
Forum Rules: Read This Before Posting

### Topic: Which reaction order  (Read 5527 times)

0 Members and 1 Guest are viewing this topic.

#### Plumbum

• Full Member
• Posts: 133
• Mole Snacks: +2/-0
• Gender:
##### Which reaction order
« on: November 09, 2015, 08:46:22 AM »
Hey guys

I need to determine which reaction order I have here.

This is a curve showing the decomposition of a molecule. I have no idea how i can determine the reactin order with this graph. I remember that i have to linearise the graph, but I dont know how in this case?? I hope somebody can help me out here, it is realy urgent.

thank u
There might be some language misunderstandings - I'm from Germany

#### Corribus

• Chemist
• Sr. Member
• Posts: 3422
• Mole Snacks: +515/-23
• Gender:
• A lover of spectroscopy and chocolate.
##### Re: Which reaction order
« Reply #1 on: November 09, 2015, 10:43:54 AM »
Each reaction order has a different linearized integrated rate law.

Check out the table on the wikipedia article, maybe about halfway down, titled "Summary for reaction orders 0, 1, 2, and n". There's an entry called "Linear Plot to determine k". This should help you figure it out.

https://en.wikipedia.org/wiki/Rate_equation
What men are poets who can speak of Jupiter if he were like a man, but if he is an immense spinning sphere of methane and ammonia must be silent?  - Richard P. Feynman

#### Plumbum

• Full Member
• Posts: 133
• Mole Snacks: +2/-0
• Gender:
##### Re: Which reaction order
« Reply #2 on: November 09, 2015, 11:18:30 AM »
So in this Case I need to finde out k first ?
My problem is I don't have a concentration. I have the optical density and the time....
There might be some language misunderstandings - I'm from Germany

#### Corribus

• Chemist
• Sr. Member
• Posts: 3422
• Mole Snacks: +515/-23
• Gender:
• A lover of spectroscopy and chocolate.
##### Re: Which reaction order
« Reply #3 on: November 09, 2015, 11:26:09 AM »
Optical density is a surrogate for concentration, assuming you are in the linear range (Beer Law) and assuming the product has no absorption at the detection wavelength.
You find k by performing the fit to the appropriately linearized data.
What men are poets who can speak of Jupiter if he were like a man, but if he is an immense spinning sphere of methane and ammonia must be silent?  - Richard P. Feynman

#### Plumbum

• Full Member
• Posts: 133
• Mole Snacks: +2/-0
• Gender:
##### Re: Which reaction order
« Reply #4 on: November 09, 2015, 11:32:28 AM »
So my reaction cant be 0th because fo 0 it needs to be linear right?

I need to plot lnOD, 1/OD to find out whether 1st or 2nd?
There might be some language misunderstandings - I'm from Germany

#### Corribus

• Chemist
• Sr. Member
• Posts: 3422
• Mole Snacks: +515/-23
• Gender:
• A lover of spectroscopy and chocolate.
##### Re: Which reaction order
« Reply #5 on: November 09, 2015, 11:52:40 AM »
Sounds like a good place to start.
What men are poets who can speak of Jupiter if he were like a man, but if he is an immense spinning sphere of methane and ammonia must be silent?  - Richard P. Feynman

#### sjb

• Global Moderator
• Sr. Member
• Posts: 3650
• Mole Snacks: +222/-42
• Gender:
##### Re: Which reaction order
« Reply #6 on: November 09, 2015, 11:58:21 AM »
So my reaction cant be 0th because fo 0 it needs to be linear right?

I need to plot lnOD, 1/OD to find out whether 1st or 2nd?

You say that, but there is a lot of noise on that signal. Could it be linear?

#### Plumbum

• Full Member
• Posts: 133
• Mole Snacks: +2/-0
• Gender:
##### Re: Which reaction order
« Reply #7 on: November 09, 2015, 12:02:56 PM »
So my reaction cant be 0th because fo 0 it needs to be linear right?

I need to plot lnOD, 1/OD to find out whether 1st or 2nd?

You say that, but there is a lot of noise on that signal. Could it be linear?

its not suposed to be linear
There might be some language misunderstandings - I'm from Germany

#### Plumbum

• Full Member
• Posts: 133
• Mole Snacks: +2/-0
• Gender:
##### Re: Which reaction order
« Reply #8 on: November 09, 2015, 12:09:53 PM »
thats what i did right now...1/OD and ln(OD)
pretty confused with the results
There might be some language misunderstandings - I'm from Germany

#### Corribus

• Chemist
• Sr. Member
• Posts: 3422
• Mole Snacks: +515/-23
• Gender:
• A lover of spectroscopy and chocolate.
##### Re: Which reaction order
« Reply #9 on: November 09, 2015, 12:56:42 PM »
First, please get in the habit of labeling your plots, with units. It immensely helps to prevent misunderstandings.
The top one is clearly now a linear plot. The original one, though noisy, is clearly an exponential decay to my eyes. Not entirely sure what is going on in the second one but without plot labels I'm also not really sure what I'm looking at.

EDIT: My guess is that if the second one is 1/OD, then the spikes are artifacts caused by the noise in the original plot (some points accidentally come very close to zero, or even negative values, and taking the inverse of those points results in very large numbers). Typically what I find it useful to do in these cases is do a little data averaging before I process it (combine every 5 points into 1, say). Alternatively you can subject the original data to a simple exponential fit and then do the transform on that.
What men are poets who can speak of Jupiter if he were like a man, but if he is an immense spinning sphere of methane and ammonia must be silent?  - Richard P. Feynman

#### Corribus

• Chemist
• Sr. Member
• Posts: 3422
• Mole Snacks: +515/-23
• Gender:
• A lover of spectroscopy and chocolate.
##### Re: Which reaction order
« Reply #10 on: November 09, 2015, 01:05:22 PM »
By the way, is this for a lab class?

To reduce the experimental noise, I suggest either increasing the original concentration of your starting material, increasing the instrumental data integration time (or whatever the instrument calls it), or both. You can do the integration time adjustment manually by the data averaging I suggested (it's essentially the same thing), but it's handy to have the instrument do it for you.
What men are poets who can speak of Jupiter if he were like a man, but if he is an immense spinning sphere of methane and ammonia must be silent?  - Richard P. Feynman

#### Plumbum

• Full Member
• Posts: 133
• Mole Snacks: +2/-0
• Gender:
##### Re: Which reaction order
« Reply #11 on: November 09, 2015, 01:20:25 PM »
Thanks for you answers ! It is somethin like lab class, im not quite sure how things go in the USA or wherever you are frmo:) I can't change anything, i cant go back to the lab, i have to work with what ive got!

I was wondering if it could be order 0 ?  I mean 0th is supposed to be a decomposition, which my reaction is, and i thought it cant be 1st because it is not dependant to the concenrtation.

But I have no idea how I can plot order 0 ?
« Last Edit: November 09, 2015, 02:01:32 PM by Plumbum »
There might be some language misunderstandings - I'm from Germany

#### Corribus

• Chemist
• Sr. Member
• Posts: 3422
• Mole Snacks: +515/-23
• Gender:
• A lover of spectroscopy and chocolate.
##### Re: Which reaction order
« Reply #12 on: November 09, 2015, 02:07:00 PM »
Why do you think your top plot is strange? It looks perfectly fine to me.
Why do you think a decomposition reaction has to be zeroth order? There are many decomposition reactions that are not zeroth order.
What men are poets who can speak of Jupiter if he were like a man, but if he is an immense spinning sphere of methane and ammonia must be silent?  - Richard P. Feynman

#### Plumbum

• Full Member
• Posts: 133
• Mole Snacks: +2/-0
• Gender:
##### Re: Which reaction order
« Reply #13 on: November 09, 2015, 02:13:58 PM »
I was reading that 0 orders are ofte photochemical reactios, which fits perfectly into my reaction.

The first picture I added that looks kinda linear was ln(OD) so basically it could also be first reaction. And k ist also negativ which fits to grade 1.

Now how do I find out which one of them it is now xD?

There might be some language misunderstandings - I'm from Germany

#### Corribus

• Chemist
• Sr. Member
• Posts: 3422
• Mole Snacks: +515/-23
• Gender:
• A lover of spectroscopy and chocolate.
##### Re: Which reaction order
« Reply #14 on: November 09, 2015, 03:35:28 PM »
Alright, let's back up. What exactly is the nature of this experiment? Based on the data you have presented, it appears first order is the best fit, but so far it's just numbers. Let's get some chemical details.
What men are poets who can speak of Jupiter if he were like a man, but if he is an immense spinning sphere of methane and ammonia must be silent?  - Richard P. Feynman