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

Joined: Tue May 03, 2016 2:25 pm
Posts: 211
Location: Bs As, Argentina

 Hello, I have this problemH30+ + 2 NO2- + 2 I- --> I2 + 2 NO + 6 H2OThe 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
PhDemon
 Post subject: Re: chemistry problem  |  Posted: Thu Mar 02, 2017 5:36 pm

Joined: Wed Jun 26, 2013 10:44 am
Posts: 428
Location: Newcastle-upon-Tyne

 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...
M_Gabriela
 Post subject: Re: chemistry problem  |  Posted: Thu Mar 02, 2017 5:57 pm

Joined: Tue May 03, 2016 2:25 pm
Posts: 211
Location: Bs As, Argentina

 It's in spanish .Yes, the equation is wrong.4 H30+ + 2 NO2- + 2 I- --> I2 + 2 NO + 6 H2OThe reaction rate isv= k x [NO2 -] x [I -] x [H3O+]^21) 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.
PhDemon
 Post subject: Re: chemistry problem  |  Posted: Thu Mar 02, 2017 6:05 pm

Joined: Wed Jun 26, 2013 10:44 am
Posts: 428
Location: Newcastle-upon-Tyne

 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
M_Gabriela
 Post subject: Re: chemistry problem  |  Posted: Thu Mar 02, 2017 6:25 pm

Joined: Tue May 03, 2016 2:25 pm
Posts: 211
Location: Bs As, Argentina

 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/sThanksss. A few questionsAbout 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!
PhDemon
 Post subject: Re: chemistry problem  |  Posted: Thu Mar 02, 2017 6:29 pm

Joined: Wed Jun 26, 2013 10:44 am
Posts: 428
Location: Newcastle-upon-Tyne

 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. Last edited by PhDemon on Thu Mar 02, 2017 6:34 pm, edited 3 times in total.
M_Gabriela
 Post subject: Re: chemistry problem  |  Posted: Thu Mar 02, 2017 6:31 pm

Joined: Tue May 03, 2016 2:25 pm
Posts: 211
Location: Bs As, Argentina

 And what is this:v= k x [NO2 -] x [I -] x [H3O+]^2???
PhDemon
 Post subject: Re: chemistry problem  |  Posted: Thu Mar 02, 2017 6:33 pm

Joined: Wed Jun 26, 2013 10:44 am
Posts: 428
Location: Newcastle-upon-Tyne

 That is the rate law. It tells you how the rate depends on the concentration of each reactant.
M_Gabriela
 Post subject: Re: chemistry problem  |  Posted: Thu Mar 02, 2017 6:44 pm

Joined: Tue May 03, 2016 2:25 pm
Posts: 211
Location: Bs As, Argentina

 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.$\displaystyle \frac{1}{2} \frac{d[NO_2]}{dt}$ = vif v= xxxx M/s, then$\displaystyle \frac{1}{2} \frac{d[NO_2]}{dt}$ = xxxx M/sso why divide it?
PhDemon
 Post subject: Re: chemistry problem  |  Posted: Thu Mar 02, 2017 6:58 pm

Joined: Wed Jun 26, 2013 10:44 am
Posts: 428
Location: Newcastle-upon-Tyne

 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! ) Last edited by PhDemon on Thu Mar 02, 2017 7:06 pm, edited 2 times in total.
PhDemon
 Post subject: Re: chemistry problem  |  Posted: Thu Mar 02, 2017 7:05 pm

Joined: Wed Jun 26, 2013 10:44 am
Posts: 428
Location: Newcastle-upon-Tyne

 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
M_Gabriela
 Post subject: Re: chemistry problem  |  Posted: Thu Mar 02, 2017 7:38 pm

Joined: Tue May 03, 2016 2:25 pm
Posts: 211
Location: Bs As, Argentina

 Ok. I think I understood. Thank you ph!!!
PhDemon
 Post subject: Re: chemistry problem  |  Posted: Thu Mar 02, 2017 7:42 pm

Joined: Wed Jun 26, 2013 10:44 am
Posts: 428
Location: Newcastle-upon-Tyne

 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 )
M_Gabriela
 Post subject: Re: chemistry problem  |  Posted: Thu Mar 02, 2017 7:45 pm

Joined: Tue May 03, 2016 2:25 pm
Posts: 211
Location: Bs As, Argentina

 The data given is the rate without dividing it by 2. Thats what you are saying. It's confusing...
PhDemon
 Post subject: Re: chemistry problem  |  Posted: Thu Mar 02, 2017 7:48 pm

Joined: Wed Jun 26, 2013 10:44 am
Posts: 428
Location: Newcastle-upon-Tyne

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

Joined: Tue May 03, 2016 2:25 pm
Posts: 211
Location: Bs As, Argentina

 Yesssss. I finally understoodddd
PhDemon
 Post subject: Re: chemistry problem  |  Posted: Thu Mar 02, 2017 8:28 pm

Joined: Wed Jun 26, 2013 10:44 am
Posts: 428
Location: Newcastle-upon-Tyne

 Good! Another happy customer
M_Gabriela
 Post subject: Re: chemistry problem  |  Posted: Thu Mar 02, 2017 9:54 pm

Joined: Tue May 03, 2016 2:25 pm
Posts: 211
Location: Bs As, Argentina

 I had to explain this to a student Supposedly i should know this,hehehehheheh
PhDemon
 Post subject: Re: chemistry problem  |  Posted: Thu Mar 02, 2017 10:24 pm

Joined: Wed Jun 26, 2013 10:44 am
Posts: 428
Location: Newcastle-upon-Tyne

 If you have a chemistry degree, yes you should!
M_Gabriela
 Post subject: Re: chemistry problem  |  Posted: Thu Mar 02, 2017 11:13 pm

Joined: Tue May 03, 2016 2:25 pm
Posts: 211
Location: Bs As, Argentina

 I don't. I have a degree in biology. But it has been a while since i practiced it....
PhDemon
 Post subject: Re: chemistry problem  |  Posted: Fri Mar 03, 2017 12:42 am

Joined: Wed Jun 26, 2013 10:44 am
Posts: 428
Location: Newcastle-upon-Tyne

 That explains a lot! (I'm joking... at work the chemists and physicists like to tease the biologists We get together to take the piss out of the sociology and psychology departments though )
M_Gabriela
 Post subject: Re: chemistry problem  |  Posted: Fri Mar 03, 2017 9:21 am

Joined: Tue May 03, 2016 2:25 pm
Posts: 211
Location: Bs As, Argentina

 We do make fun of psychologists....
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