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### 15.27 vs 15.35

Posted: **Wed Mar 07, 2018 11:25 pm**

by **Justin Chu 1G**

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?

### Re: 15.27 vs 15.35

Posted: **Thu Mar 08, 2018 12:48 pm**

by **Manvir2K**

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.

### Re: 15.27 vs 15.35

Posted: **Thu Mar 08, 2018 3:58 pm**

by **Gwyneth Huynh 1J**

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.

### Re: 15.27 vs 15.35

Posted: **Thu Mar 08, 2018 4:20 pm**

by **Jennie Fox 1D**

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.