Question

Consider the reaction:
$3I^{-}\left(aq\right)+S_2{O}_8^{2-}\left(aq\right)\to I_3^{-}\left(aq\right)+2S{O}_4^{2-}\left(aq\right)$

At a particular time t,
$\frac{d\left[S{O}_4^{2-}\right]}{dt}=2.2\times {10}^{-2}M/s$
What will be the value of $-\frac{d\left[I^{-}\right]}{dt}$ at the same time t ?

• A. $-6.6\times {10}^{-2}M/s$
• B. $6.6\times {10}^{-2}M/s$
• C. $-3.3\times {10}^{-2}M/s$
• D. $3.3\times {10}^{-2}M/s$

Answer 1

The correct option is D $3.3\times {10}^{-2}M/s$

For a given reaction, if:
$aA+bB\to cC+dD$

Overall rate $r$ can be expressed as:

$r=\frac{-1}{a}\times \frac{d\left[A\right]}{dt}=\frac{-1}{b}\times \frac{d\left[B\right]}{dt}=\frac{+1}{c}\times \frac{d\left[C\right]}{dt}=\frac{+1}{d}\frac{d\left[D\right]}{dt}$

$\therefore \frac{-1}{3}\times \frac{d\left[I^{-}\right]}{dt}=\frac{+1}{2}\times \frac{d\left[S{O}_4^{2-}\right]}{dt}$

$-\frac{d\left[I^{-}\right]}{dt}=\frac{3}{2}\times 2.2\times {10}^{-2}$

$-\frac{d\left[I^{-}\right]}{dt}=3.3\times {10}^{-2}M/s$