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Chem 102 â€“ Lecture 4 â€“ Monday, January 15, 2018

Zero order reactions

[A] = - kt + [A]0

â€¢ Donâ€™t need to memorize it

â€¢ The trend is usually a linear line that goes down

Half-life

â€¢ The half-life of a reaction is the time required for a reactant so that half its original

concentration is reacted

t1/2 = [A]0 / 2k

â€¢ Half-life of zero is dependent on the initial concentration

First order reactions

â€¢ aA ïƒ products

â€¢ ln[A] = -kt + ln[A]0

â€¢ ln[A] = natural log of concentration of A at time t

â€¢ ln[A]0 = natural log of initial concentration of A

â€¢ Another form of first order integrated rate law

â€¢ [A] = [A]0 e-kt

â€¢ Half-life for first order reaction is constant

â€¢ What can we say about the half-life of first-order reactions?

o Half-life of A is always 24 minutes

o For first-order reactions, half-life is independent of initial concentrations

â€¢ From the half-life of a first-order reaction, the rate constant, k, can be determined

Homework question

â€¢ Do 1, then substitute that into 2 and 3

Second order reactions

â€¢ Note: you do not need to memorize formulas, but you need to know which formula is

used for what purpose

â€¢ 1/[A] versus time, t

â€¢ Reciprocal of the [A], then the reciprocal of the initial concentration

â€¢ First half-life is short, second one is longer, third one is even longer

â€¢ Half-life is dependent (inversely proportional) on initial concentration

Summary

â€¢ First order: k=[A]

â€¢ Second order: k=[A]2

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