## Question about half-life regarding hw 15.35

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

Lizzie Zhang 2C
Posts: 31
Joined: Fri Sep 29, 2017 7:05 am

### Question about half-life regarding hw 15.35

For hw 15.35, we need to use the integrated rate law to calculate how long does it take for the reactant concentration to fall to one sixteenth of its original value. Why we cannot just use half-life times 4 to get the answer but instead we have to do the whole rate law?

Full question: The half-life for the second-order reaction of a substance A is 50.5 s when [A]0=0.84 mol L-1. Calculate the time needed for the concentration of A to decrease to (a) one- sixteenth of its original value

Why don't we just let 50.5s time 4?

Emily Oren 3C
Posts: 50
Joined: Fri Sep 29, 2017 7:07 am

### Re: Question about half-life regarding hw 15.35

I think you can't just do the half-life times four because the half-life is 50.5 s when [A]0=0.84 mol/L, but the half-life changes as the concentration changes, so later in the reaction it is a different value.