second and zeroth order half life "not used"


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inlovewithchemistry
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Joined: Fri Sep 28, 2018 12:19 am

second and zeroth order half life "not used"

Postby inlovewithchemistry » Tue Mar 12, 2019 2:48 pm

the book states: "the half life increases as the reaction proceeds and reactant concentration decreases. Because of this variation, the half-life is not very useful for describing reactions with second-order kinetics"

can someone explain this in a different way? why is the second order half life not useful but first is?

why is the zeroth order half life not useful either?

Tuong Nguyen 2I
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Re: second and zeroth order half life "not used"

Postby Tuong Nguyen 2I » Tue Mar 12, 2019 3:56 pm

For a first order reaction, the half life is the same value regardless of where you are in the reaction. However, in second and zeroth order reactions, the half life value changes depending on the concentration, so it's more difficult to calculate and therefore not as useful.

josie 1C
Posts: 18
Joined: Fri Sep 28, 2018 12:19 am

Re: second and zeroth order half life "not used"

Postby josie 1C » Tue Mar 12, 2019 4:11 pm

the half life changes for 2nd and 0th order reactions based on the concentration of reactants, where the half life of 1st order reactions stays the same no matter the concentration of reactants at time t, since it depends only on the concentration at time =0. For the 2nd order reaction, 1/2 life increases as concentration decreases (rate=k[A]^2), and for 0th order reactions, 1/2 life decreases and concentration decreases (rate=k). it helps if you plot it out.


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