[Kate]

Hi. I need help solving a problem concerning chemical kinetics. Here it goes:

Consider the reaction N₂ (g) + 3H₂ (l) -> 2NH₃ (g). If the velocity of formation of NH3 is 1.15 mol/dm³, determine the velocity for the reaction in mol/dm³ in terms of

a) concentration of H₂  (solution is - 1.72)
b) concentration of N₂  (solution is - 0.58)

[Kate]

Ok, I have another question:

For the reaction 2 N₂O₅ (g) -> 4 NO₂ (g) + O₂ (g) the following data was obtained:


I need to confirm that the reaction above is a first order reaction. How do I do that ? I know that for 1st order reactions,
v = k [A] and ln [A] = ln [A]₀ - kt. But what do I do with this ?

[sjb]

Hi. I need help solving a problem concerning chemical kinetics. Here it goes:

Consider the reaction N₂ (g) + 3H₂ (l) -> 2NH₃ (g). If the velocity of formation of NH3 is 1.15 mol/dm³, determine the velocity for the reaction in mol/dm³ in terms of

a) concentration of H₂  (solution is - 1.72)
b) concentration of N₂  (solution is - 0.58)

If one molecule of NH₃ is created, how much N₂ (for instance), have you used up?

[sjb]

Ok, I have another question:

For the reaction 2 N₂O₅ (g) -> 4 NO₂ (g) + O₂ (g) the following data was obtained:

Time /s	[N2O5]
0	0.00215
4000	0.00188
8000	0.00164
12000	0.00143
16000	0.00125

I need to confirm that the reaction above is a first order reaction. How do I do that ? I know that for 1st order reactions,
v = k [A] and ln [A] = ln [A]₀ - kt. But what do I do with this ?

Does the form ln [A] = ln [A]₀ - kt remind you of anything else? Can you plot, for instance a straight line using these data points?

[Kate]

If one molecule of NH₃ is created, how much N₂ (for instance), have you used up?

I have used up 1/2 molecule of N₂. So to determine the velocity of for the reaction in mol/dm³ in terms of concentration of N₂, I divide 1.15 by 2, right ?

Does the form ln [A] = ln [A]₀ - kt remind you of anything else? Can you plot, for instance a straight line using these data points?

Yes, I can plot a straight line using those data points. But I can do that for a zero and 2nd order reactions too. What's the difference ?

[cliverlong]

Ok, I have another question:

For the reaction 2 N₂O₅ (g) -> 4 NO₂ (g) + O₂ (g) the following data was obtained:


I need to confirm that the reaction above is a first order reaction. How do I do that ? I know that for 1st order reactions,
v = k [A] and ln [A] = ln [A]₀ - kt. But what do I do with this ?

How about these alternate approaches
1.
Rate = change in concentration / time take for change

So for 5 measurements of concentration you will determine 4 rates.

Plot rate on vertical axis against concentration on horizontal axis

2.
Plot  concentration on vertical axis against time on horizontal axis

Take four tangents along the curve. This will give chnage is concentraiton at a particular time i.e. the rate. Note for which concentration this rate occurs.

Again plot rate on vertical axis against concentration on horizontal axis.


Interpretation

Do you know what type of lines or curves to expect when plotting rate against conc. for zero / first / second order reactions?

Clive

[sjb]

If one molecule of NH₃ is created, how much N₂ (for instance), have you used up?

I have used up 1/2 molecule of N₂. So to determine the velocity of for the reaction in mol/dm³ in terms of concentration of N₂, I divide 1.15 by 2, right ?

Don't forget velocity has a direction too, but yes, that's the essence of the answer.

Does the form ln [A] = ln [A]₀ - kt remind you of anything else? Can you plot, for instance a straight line using these data points?

Yes, I can plot a straight line using those data points. But I can do that for a zero and 2nd order reactions too. What's the difference ?

Actually, having just done the plots myself in Excel, I see where you're coming from (I think). Can someone confirm that the plots for 0th, 1st, and 2nd order reactions here are all more or less a straight line (within experimental error). Perhaps the problem here is that the reaction is too slow? I'm not sure you can confirm the reaction order from the data you've got. Hmmm.

Basically, what I was getting towards talking about - what's the equation of a straight line?

[Kate]

Interpretation

Do you know what type of lines or curves to expect when plotting rate against conc. for zero / first / second order reactions?

Clive

I don't know.

Actually, having just done the plots myself in Excel, I see where you're coming from (I think). Can someone confirm that the plots for 0th, 1st, and 2nd order reactions here are all more or less a straight line (within experimental error). Perhaps the problem here is that the reaction is too slow? I'm not sure you can confirm the reaction order from the data you've got. Hmmm.

Basically, what I was getting towards talking about - what's the equation of a straight line?

y = mx + b

[sjb]

OK, compare y = mx + b to ln [A] = ln [A]0 - kt. What can you plot, and what can you tell from your graph?

[Kate]

I can plot ln [A] (would be y) and t ( would be x) and m is -k. I knew this already but if, instead of using ln [A] = ln [A]0 - kt, I use 1/[A] = 1/[A]₀ + kt (2nd order reactions) I also get a straight line. So my question was, using the data above, how can I know the order of reaction ?

[cliverlong]

Ok, I have another question:

For the reaction 2 N₂O₅ (g) -> 4 NO₂ (g) + O₂ (g) the following data was obtained:


I need to confirm that the reaction above is a first order reaction. How do I do that ? I know that for 1st order reactions,
v = k [A] and ln [A] = ln [A]₀ - kt. But what do I do with this ?

How about these alternate approaches
1.
Rate = change in concentration / time take for change

So for 5 measurements of concentration you will determine 4 rates.

Plot rate on vertical axis against concentration on horizontal axis

2.
Plot  concentration on vertical axis against time on horizontal axis

Take four tangents along the curve. This will give chnage is concentraiton at a particular time i.e. the rate. Note for which concentration this rate occurs.

Again plot rate on vertical axis against concentration on horizontal axis.


Interpretation

Do you know what type of lines or curves to expect when plotting rate against conc. for zero / first / second order reactions?

Clive

Graph paper says first order (see attached scan of hand-drawn graphs)

Of course this is very approximate.

What I would want to do is feed the data into a program that can produce an approximate polynomial (or probably better, an exponential) curve from the data.
Then extrapolate beyond the data values and determine two or three half-lives where concentration has halved (the data doesn't allow one half-life to be calculated but it will be greater than 16,000 seconds for this reaction from the data given). If the three half-lives are fairly similar this will be more evidence of first-order reaction

Clive

[sjb]

Thanks Clive,

That's essentially what I had in oocalc, with r² values for the three plots in the region of 0.98-1.00, so pretty much a straight line within experimental error, so though the first order plot had the best fit, it was still very close. Hence my comment that "the plots for 0th, 1st, and 2nd order reactions here are all more or less a straight line (within experimental error). Perhaps the problem here is that the reaction is too slow? I'm not sure you can confirm the reaction order from the data you've got". Perhaps we need to present all three and show that they are all fairly straight - even a third order plot isn't too shabby in that regard, and perhaps conclude that we *can't* be sure it's first order!

[cliverlong]

Thanks Clive,

That's essentially what I had in oocalc, with r² values for the three plots in the region of 0.98-1.00, so pretty much a straight line within experimental error, so though the first order plot had the best fit, it was still very close. Hence my comment that "the plots for 0th, 1st, and 2nd order reactions here are all more or less a straight line (within experimental error). Perhaps the problem here is that the reaction is too slow? I'm not sure you can confirm the reaction order from the data you've got". Perhaps we need to present all three and show that they are all fairly straight - even a third order plot isn't too shabby in that regard, and perhaps conclude that we *can't* be sure it's first order!

Good points - I agree. I didn't think of the issue about it being a slow reaction as th probable coause of this problem in analysing order - makes sense when one looks at the graphs.

Clive