### First Order Reaction Equations

Posted:

**Tue Feb 27, 2018 12:29 am**How do you know when to use

or when to use ?

Are there specific instances when we use each?

or when to use ?

Are there specific instances when we use each?

Created by Dr. Laurence Lavelle

https://lavelle.chem.ucla.edu/forum/

https://lavelle.chem.ucla.edu/forum/viewtopic.php?f=148&t=28612

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Posted: **Tue Feb 27, 2018 12:29 am**

How do you know when to use

or when to use ?

Are there specific instances when we use each?

or when to use ?

Are there specific instances when we use each?

Posted: **Tue Feb 27, 2018 9:05 am**

I believe that both those equations are equivalent. I'm not sure where you got the equation with ln( [A]o / [A]t ) and a positive +kt, but they seem to be the same thing. Use them to find the k reaction constant value or t time it takes to get to a certain concentration from a given initial concentration. (all for a first order reaction only)

Posted: **Tue Feb 27, 2018 9:28 am**

They are the same equation. The only difference is the way you write the equation that they are derived from. In this case for ln [A]t/[A]o = -kt, it is derived from ln[A]t - ln[A]o = -kt

While the other equation comes from ln[A]o - ln[A]t = kt, which can be rewritten as -ln[A]t +ln[A]o =kt, if you compare this to the above equation, you just multiplied both sides by -1, therefore they are pretty much the same thing and should give you the same answer.

While the other equation comes from ln[A]o - ln[A]t = kt, which can be rewritten as -ln[A]t +ln[A]o =kt, if you compare this to the above equation, you just multiplied both sides by -1, therefore they are pretty much the same thing and should give you the same answer.

Posted: **Tue Feb 27, 2018 10:24 pm**

It's the same thing, you should get the same answer.