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M_Gabriela
Post  Post subject: chemistry problem  |  Posted: Thu Mar 02, 2017 4:35 pm
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Hello, I have this problem

H30+ + 2 NO2- + 2 I- --> I2 + 2 NO + 6 H2O

The problem asks for the order of reaction with respect to I2, the product. How do I do that?

Then it asks the reaction rate if formation rate of NO is xx M/s...

I have more data... For example for the first question I need to find the concentration of H30+ and I have the reaction rate, the k, and the concentration of NO2 and I-. And the expression of the reaction rate so I only have to clear H30+.

But then it asks me to find a new reaction rate if NO formation rate is xx M/s... I'm lost....

Thankss in advance


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PhDemon
Post  Post subject: Re: chemistry problem  |  Posted: Thu Mar 02, 2017 5:36 pm

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Could you post the whole question with the data (maybe as an image)? It might be clearer to me then how you go about solving it.
Asking for a reaction order WRT to a product seems a little odd...

Also your equation as written doesn't look correct, the charges don't balance...

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M_Gabriela
Post  Post subject: Re: chemistry problem  |  Posted: Thu Mar 02, 2017 5:57 pm
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It's in spanish :D.

Yes, the equation is wrong.

4 H30+ + 2 NO2- + 2 I- --> I2 + 2 NO + 6 H2O

The reaction rate is

v= k x [NO2 -] x [I -] x [H3O+]^2

1) Value of H3O+ if... and they give me all the data to clear H30+ from the equation above.

2) order of reaction with respect to I2.

3)Value of reaction rate when formation rate of NO is xxxxx M/s.


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PhDemon
Post  Post subject: Re: chemistry problem  |  Posted: Thu Mar 02, 2017 6:05 pm

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OK, I guess you've done part one (the easy bit :p)...

As for part two I2 does not appear in the rate equation and has no effect on the rate so the order with respect to it must be zero (although I still think it strange asking about a product in this way, I've never seen that before unless it's an equilibrium).

Part 3, "v" in your equation is equal to $ \displaystyle \frac{1}{2} \frac{d[NO_2]}{dt}$ and this is the reaction rate, so the rate of reaction is half of your xxxx M/s

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M_Gabriela
Post  Post subject: Re: chemistry problem  |  Posted: Thu Mar 02, 2017 6:25 pm
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PhDemon wrote:
OK, I guess you've done part one (the easy bit :p)...

As for part two I2 does not appear in the rate equation and has no effect on the rate so the order with respect to it must be zero (although I still think it strange asking about a product in this way, I've never seen that before unless it's an equilibrium).

Part 3, "v" in your equation is equal to $ \displaystyle \frac{1}{2} \frac{d[NO_2]}{dt}$ and this is the reaction rate, so the rate of reaction is half of your xxxx M/s


Thanksss. A few questions

About part 3, so you're saying that the difference between reaction rate and formation rate is only the 1/2???

Cause I thought that formation rate was $ \displaystyle \frac{1}{2} \frac{d[NO_2]}{dt}$. So I would have to multiply by 2 if I want to clear the $ \displaystyle {d[NO_2]}/{dt}$

I feel like I'm writing something really stupid....

About part 2. I only read about order of reaction respect to reactants, not products, that's why I was lost.


again thank you!


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PhDemon
Post  Post subject: Re: chemistry problem  |  Posted: Thu Mar 02, 2017 6:29 pm

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The rate of reaction (v in your equation) can be expressed in terms of any of the reactants or products as: $ \displaystyle \frac{1}{n} \frac{d[X]}{dt}$ where n is the stoichiometric coefficient in front of X... This should give the same value for the rate regardless of which reactant or product you use.

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Last edited by PhDemon on Thu Mar 02, 2017 6:34 pm, edited 3 times in total.


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M_Gabriela
Post  Post subject: Re: chemistry problem  |  Posted: Thu Mar 02, 2017 6:31 pm
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And what is this:

v= k x [NO2 -] x [I -] x [H3O+]^2

???


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PhDemon
Post  Post subject: Re: chemistry problem  |  Posted: Thu Mar 02, 2017 6:33 pm

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That is the rate law. It tells you how the rate depends on the concentration of each reactant.

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M_Gabriela
Post  Post subject: Re: chemistry problem  |  Posted: Thu Mar 02, 2017 6:44 pm
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Ok... I don't understand why I have to divide by 2. I understand where the 1/2 comes from but I don't understand why, to calculate the reaction rate, I have to do that.

$ \displaystyle \frac{1}{2} \frac{d[NO_2]}{dt}$ = v

if

v= xxxx M/s, then

$ \displaystyle \frac{1}{2} \frac{d[NO_2]}{dt}$ = xxxx M/s


so why divide it?


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PhDemon
Post  Post subject: Re: chemistry problem  |  Posted: Thu Mar 02, 2017 6:58 pm

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You are given the rate of formation of NO, not the "rate of the reaction". The reaction rate is defined in my above post, you need to use that 1/n bit. If you didn't you would get a different answer for the rate depending on which reactant ir product you chose to measure the concentration change of...

Edit: the NO2 in my equations above should be NO, does that clear up your confusion? (It's been a long day! ;))

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Last edited by PhDemon on Thu Mar 02, 2017 7:06 pm, edited 2 times in total.


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PhDemon
Post  Post subject: Re: chemistry problem  |  Posted: Thu Mar 02, 2017 7:05 pm

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I'm obviously not explaining it very well (sorry about that). Does the "Formal definition" section of this link do a better job?

https://en.m.wikipedia.org/wiki/Reaction_rate

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M_Gabriela
Post  Post subject: Re: chemistry problem  |  Posted: Thu Mar 02, 2017 7:38 pm
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Ok. I think I understood. Thank you ph!!!


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PhDemon
Post  Post subject: Re: chemistry problem  |  Posted: Thu Mar 02, 2017 7:42 pm

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No problem, despite using it a lot myself (it became second nature) I last taught this in 2003 to a first year undergrad class (probably why my explanation wasn't great :lol:)

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M_Gabriela
Post  Post subject: Re: chemistry problem  |  Posted: Thu Mar 02, 2017 7:45 pm
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The data given is the rate without dividing it by 2. Thats what you are saying. It's confusing...


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PhDemon
Post  Post subject: Re: chemistry problem  |  Posted: Thu Mar 02, 2017 7:48 pm

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You seem to be confusing rate of formation of NO with the rate of reaction, they are not the same (they would be the same if there was not a 2 in front of NO in your equation). Read how rate of reaction is defined in the link I gave, I think it mentions where this 1/n term comes from.

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M_Gabriela
Post  Post subject: Re: chemistry problem  |  Posted: Thu Mar 02, 2017 8:13 pm
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Yesssss. I finally understoodddd


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PhDemon
Post  Post subject: Re: chemistry problem  |  Posted: Thu Mar 02, 2017 8:28 pm

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Good! Another happy customer ;)

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M_Gabriela
Post  Post subject: Re: chemistry problem  |  Posted: Thu Mar 02, 2017 9:54 pm
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I had to explain this to a student :lol:
Supposedly i should know this,hehehehheheh


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PhDemon
Post  Post subject: Re: chemistry problem  |  Posted: Thu Mar 02, 2017 10:24 pm

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If you have a chemistry degree, yes you should! :lol:

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M_Gabriela
Post  Post subject: Re: chemistry problem  |  Posted: Thu Mar 02, 2017 11:13 pm
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I don't. I have a degree in biology. But it has been a while since i practiced it....


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PhDemon
Post  Post subject: Re: chemistry problem  |  Posted: Fri Mar 03, 2017 12:42 am

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That explains a lot! ;)

(I'm joking... at work the chemists and physicists like to tease the biologists :lol: We get together to take the piss out of the sociology and psychology departments though ;))

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M_Gabriela
Post  Post subject: Re: chemistry problem  |  Posted: Fri Mar 03, 2017 9:21 am
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:o :o

:lol:

We do make fun of psychologists....


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