Question

In the presence of excess thiocyanate ion, SCN-, the following reaction is first order in iron(III) ion, Fe3+. The rate constant is 1.27/s. Fe3+(aq) + SCN-(aq) → Fe(SCN)2-(aq) If 46.7% of the reaction is required to obtain a noticeable color from the formation of the Fe(SCN)2- ion, how many seconds are required?

Answer 1

Answer:

0.60 s

Explanation:

Our strategy to solve this question is based on the fact that the reaction is first order.

For a first order reaction:

ln [A]t / [A]₀ = - kt

where [A]t is the concentration after a time t,

           [A]₀  is the original concentration of A,

           k is the rate constant,

           t is the time

Now we are told we need a 46.7 completion rate, therefore:

[A]t  = 0.467 [A]₀

ln ( 0.467 [A]₀ / [A]₀  ) =

( -1.27/s ) t  ⇒ t = ln ( 0.467  ) /(-1.27 ) s

                                                 = - 0.76/ (-1.27) s = 0.60 s