## 15.27 vs 15.35

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

Justin Chu 1G
Posts: 52
Joined: Thu Jul 13, 2017 3:00 am
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### 15.27 vs 15.35

For the homework problems, I noticed that in 15.27 (first-order reaction), you could simply calculate how many half lives have occurred and multiply the number by the half-life (ex: 1/8 of the original concentration would equal (1/2)^3 so you could multiply the half life by 3). However, I noticed that in 15.35 (second-order reaction), I couldn't simply do the same thing. Is there a specific reason why?

Manvir2K
Posts: 32
Joined: Fri Sep 29, 2017 7:05 am

### Re: 15.27 vs 15.35

This method works for 27 because it is in first order, while 35 is in second order. I think calculating the half lives that have occurred and multiplying the number by the half-life only works for 1st order.

Gwyneth Huynh 1J
Posts: 30
Joined: Fri Sep 29, 2017 7:05 am

### Re: 15.27 vs 15.35

I agree! For first order reactions, the half life is independent of initial concentration. However, for second order reactions, the half life depends on initial concentration.

Jennie Fox 1D
Posts: 66
Joined: Sat Jul 22, 2017 3:01 am

### Re: 15.27 vs 15.35

This is because in second order reactions, half life depends on initial concentration, whereas half life in first order reactions does not depend on concentration.