half life

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half life

Postby skyeblee2F » Sat Mar 16, 2019 2:21 pm

Why is the half-life for first order reactions (t1/2=(ln2/k) independent of the initial reactant concentration [A]0, while the half-life of zero order and second order reactions are dependent? ([A]0 is included in both of their t1/2 formulas)

Luc Lorain 1L
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Re: half life

Postby Luc Lorain 1L » Sat Mar 16, 2019 2:58 pm

The value of the half life is ultimately dependent on the order of the reaction. For first order equations, d[A]= -kdt, in which the reactant is conspicuously absent.

In reality, we just have to accept that it just be like that sometimes.
(Sorry fam, I just need those fat Chemistry community points)

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Re: half life

Postby Chem_Mod » Sat Mar 16, 2019 3:13 pm

The half-life equations are derived using the integrated rate law formulas by setting the final concentration equal to half the initial concentration and solving for time (t). When deriving the half-life equation for the 1st order reactions, the initial concentration terms will cancel out. However, they will not cancel out when deriving the half-life equations for the 0th and 2nd order reactions.

Hope this helps! :)

Jack Hewitt 2H
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Re: half life

Postby Jack Hewitt 2H » Sat Mar 16, 2019 6:16 pm

Because when the first order half-life equation is derived the initial concentration terms will cancel out.

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