half-life


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Jane Burgan 1C
Posts: 73
Joined: Fri Sep 28, 2018 12:15 am

half-life

Postby Jane Burgan 1C » Sun Mar 10, 2019 10:32 pm

For problems where the textbook gives us the half-life of a second order reaction of a substance and it asks us to calculate the time needed for the concentration of the substance to decrease by 1/16, why can't we just multiply the half-life by 4 to get the time needed?

Sierra Cheslick 2B
Posts: 61
Joined: Fri Sep 28, 2018 12:27 am

Re: half-life

Postby Sierra Cheslick 2B » Sun Mar 10, 2019 11:21 pm

The speed of the reaction depends on how much reactant is left, so it is not directly proportional to a quarter of the half-life.

Sarah Zhao 4C
Posts: 64
Joined: Fri Sep 28, 2018 12:28 am

Re: half-life

Postby Sarah Zhao 4C » Sun Mar 10, 2019 11:23 pm

You can't multiply it by four but you can square it by four!
Break down the math:
1/2 x 4 = 2
vs
(1/2)^4 = 1/16

Shivangi_2J
Posts: 65
Joined: Fri Sep 28, 2018 12:15 am

Re: half-life

Postby Shivangi_2J » Wed Mar 13, 2019 6:55 pm

Sarah Zhao 4C wrote:You can't multiply it by four but you can square it by four!
Break down the math:
1/2 x 4 = 2
vs
(1/2)^4 = 1/16


This doesn't give you the correct answer :( I think it's because, as someone said above, the half-life of a second-order reaction depends on how much reactant is left

Ethan Breaux 2F
Posts: 63
Joined: Sat Sep 29, 2018 12:16 am

Re: half-life

Postby Ethan Breaux 2F » Thu Mar 14, 2019 1:30 am

you have to get the formula for t from the second rate equation which causes you to get t = (1/[A]t - 1/[A]o)/k and in this case you can replace [A]t with (1/16) x [A]o causing the equation to be t = (16/[A]o - 1/[A]o)/k and you just plug stuff in :)


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