Exercise 7 | Chemical Kinetics: Rate Laws

Here is a reaction: NOBr (g) → NO (g) + 12 Br(g)

[NOBr] is measured at different time:

t = 0s ⇒ [NOBr]  = 0.0250 M

t = 6.2s ⇒ [NOBr]  = 0.0191 M

t = 10.8s ⇒ [NOBr]  = 0.0162 M

t = 14.7s ⇒ [NOBr]  = 0.0144 M

t = 20.0s ⇒ [NOBr]  = 0.0125 M

t = 24.6s ⇒ [NOBr]  = 0.0112 M

Calculate the initial half-life of the reaction.

Determine the order of the reaction:


If the reaction is a first-order reaction:

ln[NOBr] = ln[NOBr]0 – kt
⇒ we plot ln[NOBr] versus t
⇒ we don’t get a straight line (R2 = 0.9848)
⇒ this is not a first-order reaction

 

If the reaction is a first-order reaction:

1/[NOBr] = 1/[NOBr]0 + kt
⇒ we plot 1/[NOBr] versus t
⇒ we get a straight line (R2 = 1.000) and its equation is y = 2.0028x + 40
⇒ the reaction is a second-order reaction and k = 2.00 M-1.s-1

 

Calculate the initial half-life of the reaction:

for a second-order reaction, t1/2 = 1k [A]0 = 12.00 × 0.0250 = 20.0s