## Half life of second order reactions

$\frac{d[R]}{dt}=-k[R]^{2}; \frac{1}{[R]}=kt + \frac{1}{[R]_{0}}; t_{\frac{1}{2}}=\frac{1}{k[R]_{0}}$

Ethan-Van To Dis2L
Posts: 50
Joined: Fri Sep 29, 2017 7:05 am

### Half life of second order reactions

In the book, it says that we don't use the equation for a half-life for a second order reaction, but we still used it in one the exercises. Can someone explain why, it says "not used"

Emma Li 2C
Posts: 51
Joined: Fri Sep 29, 2017 7:07 am
Been upvoted: 1 time

### Re: Half life of second order reactions

I was a little confused too seeing that in the table, but I just thought it meant that the book didn't use the half life equation for a second order reaction in one of their examples, or that they just didn't derive the equation for it like they did for the half life of a first order reaction.

Gevork 2E
Posts: 32
Joined: Fri Sep 29, 2017 7:05 am
Been upvoted: 1 time

### Re: Half life of second order reactions

Hello,

I remember reading this as well and I am pretty much going to ignore what it says. We have the equations and understand how a second order reaction occurs so I wouldn't stress about what the book says. Maybe it had to do with the publisher or authors not including them? Hard to tell for the reasoning.

Jason Liu 1C
Posts: 52
Joined: Fri Sep 29, 2017 7:04 am

### Re: Half life of second order reactions

I would still study the half life of second order reactions and understand how they work because it was on the homework and was on my test as well. I think it'd be a good idea to study it in case it's on the final.

Hailey Johnson
Posts: 31
Joined: Wed Nov 16, 2016 3:04 am

### Re: Half life of second order reactions

Definitely ignore that in the book. It confused me too, but on the third test the equation for the half-life of a second order reaction was on the formula sheet, so we are expected to know how to use it.

DamianW
Posts: 35
Joined: Fri Sep 29, 2017 7:06 am

### Re: Half life of second order reactions

Do any of you know if lavelle has ever asked us to derive a half life formula on a previous test ?

miznaakbar
Posts: 55
Joined: Thu Jul 13, 2017 3:00 am
Been upvoted: 4 times

### Re: Half life of second order reactions

I'm not sure if they've asked to derive a half life formula before, but it would probably be best to know how to considering we spent time deriving formulas in the notes. Deriving the half life formula is also fairly simple when given the integrated rate law because you just plug in (1/2)[A] initial as the final [A] and rearrange the equation so it equals t. Hope this helps!

Meredith Steinberg 2E
Posts: 50
Joined: Thu Jul 13, 2017 3:00 am

### Re: Half life of second order reactions

I wouldn't worry too much about deriving the half life of a second order process. If we ever needed to use the equation, we would need 2 of the 3 unknowns: k, initial concentration, or the half life.

Emily Mei 1B
Posts: 50
Joined: Fri Sep 29, 2017 7:04 am

### Re: Half life of second order reactions

Either way, I'm pretty sure the 2nd order half life equation is on the constants and equations sheets on our tests.