Question

The rate of a first-order reaction is followed by spectroscopy, monitoring the absorption of a colored reactant at 520 nm . The reaction occurs in a 1.29-cm sample cell, and the only colored species in the reaction has an extinction coefficient of 5700 cm-1M-1 at 520 nm .

a) Calculate the initial concentration of the colored reactant if the absorbance is 0.564 at the beginning of the reaction.

b) The absorbance falls to 0.254 at 30.0 min . Calculate the rate constant in units of s-1.

c) Calculate the half-life of the reaction. (in sec)

d) How long does it take for the absorbance to fall to 0.104? (in sec)

Answer 1

a) A = ebc
A = 0.564
e = 5700
b = 1.29
c = ?

b) A = ebc
A = 0.254
e = 5700
b = 1.29
c = ? at 30 min.
Convert delta C to delta C/second and rate = k*c

c)
k = 0.693/t_1/2
You have k, solve for t1/2.

d) A = ebc
A = 0.104
e = 5700
b = 1.29
c = ?

Then ln(No/N) = kt
No = from part a.
N = from part d.
k = from part b.
t = ? in seconds if b is in sec.