The answer to your question is a 12
Answer:

Explanation:
A reaction is spontaneous at all temperatures by the following combinations:
=> A negative enthalpy change (
)
=> A positive entropy change (
)
See the attached file for more better understanding!
The reaction is given as follows:

The plot of ln[a] verses t is linear with slope
.
The plot for ln[a] verses t is linear for first order reaction. The integrated rate law equation is as follows:
![[A]=[A_{0}]e^{-kt}](https://tex.z-dn.net/?f=%5BA%5D%3D%5BA_%7B0%7D%5De%5E%7B-kt%7D)
Here, k is rate constant, t is time of the reaction,
is concentration of reactant at time t and
is initial concentration of reactant.
Taking ln both sides,
...... (1)
Comparing with equation for linear graph, y=mx+c
Thus, slope =-k
Or, 
Now, if final concentration become half of its initial value,
and t becomes 
Putting the values in equation (1)
![ln\frac{[A_{0}]}{2}=ln[A_{0}]-(0.860 s^{-1})t_{1/2}](https://tex.z-dn.net/?f=ln%5Cfrac%7B%5BA_%7B0%7D%5D%7D%7B2%7D%3Dln%5BA_%7B0%7D%5D-%280.860%20s%5E%7B-1%7D%29t_%7B1%2F2%7D)
On rearranging,

Therefore, it takes 0.806 sec to react half of the reactant a.