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# A difficult half-life question! :/ Watch

1. If, during a first order reaction, the concentration fell to 12.5% of its original value in 24 minutes, what is the half life of the reaction?
2. It's not really that difficult.
Think about it - there is a 12.5% decrease in 24 minutes so how long will it take for it to have a 50% decrease.

12.5% = 24
50% = ?

You need to do 50/12.5 and with that answer multiply it to 24
3. (Original post by Namita Gurung)
It's not really that difficult.
Think about it - there is a 12.5% decrease in 24 minutes so how long will it take for it to have a 50% decrease.

12.5% = 24
50% = ?

You need to do 50/12.5 and with that answer multiply it to 24
I got 96 min but the answer in my book is 8 min?!
4. You first have to find out how many half lifes there is overall. So you can keep halving from 100 until you get to 12.5. 100 - 50 - 25 - 12.5 which equates to 3 half lifes because it has been halved 3 times. So now you know there are 3 half lifes in 24 minutes so to find the length of one you do 24/3 which = 8 minutes and therefore one half life is 8 minutes. Hope that helped!
5. (Original post by Samii123)
You first have to find out how many half lifes there is overall. So you can keep halving from 100 until you get to 4. 100 - 50 - 25 - 12.5 which equates to 3 half lifes because it has been halved 3 times. So now you know there are 3 half lifes in 24 minutes so to find the length of one you do 24/3 which = 8 minutes and therefore one half life is 8 minutes. Hope that helped!
Oh i see, thank you!
Oh i see, thank you!
You're welcome
7. (Original post by Samii123)
You're welcome
I've got one more question please.

If the initial concentration in a first order reaction [would it be any different if it was a zero or a second order reaction??] with a half life of 12 minutes was 0.400 moldm-3, what would the concentration be after 48 minutes?
I've got one more question please.

If the initial concentration in a first order reaction [would it be any different if it was a zero or a second order reaction??] with a half life of 12 minutes was 0.400 moldm-3, what would the concentration be after 48 minutes?
With a half life of 12 minutes, in 48 minutes there would be 4 half lifes that have occurred as 48/12 = 4. So then you must half the concentration a total of 4 times. 0.400 - 0.200 - 0.100 - 0.05 - 0.025. So your final answer is 0.025moldm^-3.
Also first order reactions are when the rate of reaction is directly proportional to the change in one reactant and therefore half life can be calculated accurately. In a second order reaction the rate depends on two substances and therefore half life cannot be calculated as accurately and it becomes a more complex calculation.
9. (Original post by Samii123)
With a half life of 12 minutes, in 48 minutes there would be 4 half lifes that have occurred as 48/12 = 4. So then you must half the concentration a total of 4 times. 0.400 - 0.200 - 0.100 - 0.05 - 0.025. So your final answer is 0.025moldm^-3.
Also first order reactions are when the rate of reaction is directly proportional to the change in one reactant and therefore half life can be calculated accurately. In a second order reaction the rate depends on two substances and therefore half life cannot be calculated as accurately and it becomes a more complex calculation.
I see thank you so much!
So do you think we'll only be asked half-life questions for 'a first order' at A2?
I'm doing Edexcel.
I see thank you so much!
So do you think we'll only be asked half-life questions for 'a first order' at A2?
I'm doing Edexcel.
I'm not sure, you could have a look at the specification however.

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