a level chemistry rates of reactions question

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mdbdidjdksnd
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#1
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https://i.imgur.com/PKVY4ZV.jpg
for this question, I plotted the points on the graph and the shape resembled a curve. but in the question, it says the rate depends only on the concentration of I- so H+ must be zero order? but then how is the rate changing as the concentration of H+ decreasing?
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Redmed_
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Yes you are right with respect to H+ the reaction is zero order. So the rate expression can be stated as : r = k [I-]2 .

In my opinion when we observe the table given we can make out that even after 1200 sec that is 20 min the change of concentration of H+ ions in the aqueous medium is very minimal or negligible ( i.e. it changes from 0.5 mol/dm3 to 0.12 mol/dm3, so the net change of concentration of the H+ ions in the reaction medium is 0.38 mol/dm3).

Since we know the basic definition of rate of reaction is change of concentration of reactants/time taken, here the denominator is large but the numerator is relatively lesser. Hence we can conclude that the rate of the reaction with respect to the [H+] ions alone is negligible or minimal that we need not possibly consider it.
Last edited by Redmed_; 1 month ago
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mdbdidjdksnd
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(Original post by Redmed_)
Yes you are right with respect to H+ the reaction is zero order. So the rate expression can be stated as : r = k [I-]2 .

In my opinion when we observe the table given we can make out that even after 1200 sec that is 20 min the change of concentration of H+ ions in the aqueous medium is very minimal or negligible ( i.e. it changes from 0.5 mol/dm3 to 0.12 mol/dm3, so the net change of concentration of the H+ ions in the reaction medium is 0.38 mol/dm3).

Since we know the basic definition of rate of reaction is change of concentration of reactants/time taken, here the denominator is large but the numerator is relatively lesser. Hence we can conclude that the rate of the reaction with respect to the [H+] ions alone is negligible or minimal that we need not possibly consider it.
Ohh okay, so the rate changes by such a small amount that you consider it to have zero order?
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Redmed_
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(Original post by mdbdidjdksnd)
Ohh okay, so the rate changes by such a small amount that you consider it to have zero order?
Yea it adds very little significance, meaning the change in concentration of H with respect to time ( the basic definition of rate that is concentration/time ) is so less that its contribution to the rate of the reaction as a whole is neglected.

So we say that the rate of the reaction with respect to H is negligible or rate of reaction with respect to H is of zero order.

But the rate of reaction wrt I- is 2 and since in the rate expression we only have I- we say finally the rate of the reaction in total is 2 or of second order.

For each reactant we can use the phrase the rate of the reaction with respect to (reactant) is of order (number)
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tony_dolby
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You can rest assured that this question caused a lot of problems for students. Personally, I think it's one of the worst questions that AQA have devised in the last twenty years or so! Aside from the crude experimental method, the reaction kinetics are complex anyway. They have told you in 01.3 that the order with respect to hydrogen ions is zero. Fair enough. But in 01.5 they've given you a graph in which the hydrogen ion concentration is decreasing and it appears to be something other than zero order.

Many students were expecting it to be zero order, because the question told them it should be earlier. Around 40% of students therefore tried to draw a straight line through these points. I don't believe for a minute that the majority of them cannot draw a best fit curve. However the examiners seemed to think it was the fault of the students: "students must learn that a best-fit line must be the best-fit to the points as plotted, so may be either straight or curved depending on the relationship between the plotted points; at A-level, students are expected to make this decision for themselves. Just under 40% of students did not gain this mark."

Anyway, back to the question. As the previous poster has pointed out, the method is complicated. I don't really understand it myself. What the examiners seem to have wanted to you to believe is that the reaction is always zero order in H+. Therefore, because [H+] is not involved in determining the rate, we can use it as a tool to see how fast the reaction is going.

So the second graph is not about studying how changing H+ affects the rate, it's about using H+ to find out what the rate is!
Last edited by tony_dolby; 1 month ago
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