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Original post by shonaT
In one half life it will fall to half the original, 2 half lives it will fall to a quarter of the original, an 8th in three half lives and a 16th in 4 half lives.
BTW the half life of paracetamol is 1-4hrs.
BTW the half life of paracetamol is 1-4hrs.
That is not the correct answer? The book gets 1520 seconds.
Original post by shonaT
That's because 4x380s =1520. It takes 4 half lives for a substance to fall to a 16th of the original amount. They are basing it on a half life of 380s. I was just pointing out that the actual half life is 1-4hrs.
Bingo! I got it thanks!
One other question:- in rate determening step, why is the first reaction mechanism the slowest and the second step is fastest? Is it because in the first step an intermediate has to be made and takes long for it to be produced?
The rate determining step is always the slowest. The overall reaction cannot be faster than this. The first step is not always the slowest, it is possible for the rate determining step to come at a later point in the reaction. However the intermediate formed is often chemically unstable making the second step faster a lot of the time.
Original post by shonaT
The rate determining step is always the slowest. The overall reaction cannot be faster than this. The first step is not always the slowest, it is possible for the rate determining step to come at a later point in the reaction. However the intermediate formed is often chemically unstable making the second step faster a lot of the time.
oh but in the first step the intermediate is stable and so reacts more slowly? But when you talk about stability do you mean the electron determines the stability?
Original post by shonaT
Usually the electron yes. Which reaction specifically are you referring to.
Here's an example of when the first step isn't the rate limiting one:
Here's an example of when the first step isn't the rate limiting one:
NO + NO -> N
2O
2 (fast)
N
2N
O
2 + H
2 -> N
2O + H
2O (slow)
N
2N
O + H
2 --> N
2 + H
2O (fast).
oh right, for A levels, we have to take into account that the first reaction step is the slowest and second step is the fastest.
Original post by shonaT
Oh bother!! I'll try that again.
NO+NO ->N2O2 (fast)
N2O2 +H2 ->N2O +H2O (slow)
N2O+H2->N2->H2O (fast)
NO+NO ->N2O2 (fast)
N2O2 +H2 ->N2O +H2O (slow)
N2O+H2->N2->H2O (fast)
oh right, i never saw an example with the first step being fast? I mean in the exam how would we know whether which step is slow or fast? thanks you have been so helpful,truly appreciate it
Original post by shonaT
The slowest step is generally the step that requires the most energy. So have a look at what's happening in each step of a reaction you are given.
oh right, so the rule is more reactant the more energy and hence fastest step?
Original post by shonaT
It's more what is happening rather than how much reactant. eg the more covalent bonds that have to be broken in one step the more energy is required per mole of reactant.
oh so we look at the number of moles!
Original post by shonaT
No what is happening eg are covalent bonds being broken or are they being formed. breaking bonds requires energy forming them releases energy.
But in the exam, how would I know which one has more covalent bond, do i draw them out?
Original post by otrivine
oh right, i never saw an example with the first step being fast? I mean in the exam how would we know whether which step is slow or fast? thanks you have been so helpful,truly appreciate it
You mean in the F325 exam? Yeah i think because there was usually only two steps in the reaction mechanisms it was just a coincedence that the first step always ended up being called the slowest, but i once found a question with 3 steps where the middle step was the slowest
but they usually tell you don't they? Quick Reply
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