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# Half life Watch

1. Paracetomol has a biological half life of 380s. How long will it take for the level of paracetomol in the body to fall to one-sixteenth of its original value?
2. 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.
3. Awwww, I thought this was gonna be a thread about Gordon Freeman
4. (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.
That is not the correct answer? The book gets 1520 seconds.
5. 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.
6. (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?
7. 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.
8. (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?
9. 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:

NO + NO -> N
2
O
2
(fast)

N
2
O
2
+ H
2
-> N
2
O + H
2
O (slow)

N
2
O + H
2
--> N
2
+ H
2
O (fast).

10. (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:

NO + NO -> N
2
O
2
(fast)

N
2
O
2
+ H
2
-> N
2
O + H
2
O (slow)

N
2
O + H
2
--> N
2
+ H
2
O (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.
11. Oh bother!! I'll try that again.

NO+NO ->N2O2 (fast)

N2O2 +H2 ->N2O +H2O (slow)

N2O+H2->N2->H2O (fast)
12. Sorry I'm a post grad so completely lost track of what you know at which point.
13. (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)
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
14. 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.
15. (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?
16. 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.
17. (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!
18. No what is happening eg are covalent bonds being broken or are they being formed. breaking bonds requires energy forming them releases energy.
19. (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?
20. (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?

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Updated: April 2, 2013
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