Determine the average rate of change of B from 𝑡=0 s to 𝑡=242 s. A⟶2B

Question

Time (s). Concentration of A (M)

0. 0.650

121. 0.395

242. 0.140

Answer 1

Average rate of change will be change in concentration / change in time

Since we are given the concentrations of reactant (A), we can find the average change in A first.

Change in [A] = 0.140 - 0.650 = -0.510 M

Change in time = 242 s - 0 s = 242 s

Average Rate = -0.510 M / 242 s = -0.00211 M/s

Note this has a - sign indicating the [A] is decreasing over time.

To find the average rate of change of B, we use the stoichiometry of the equation...

Note that for every A that is used, 2 B's are produced. So,...

average rate of change of B = 2 x 0.00211 = +0.00422 M/s (positive since B is being formed)

If you have learned to write rate equations as

-∆[A]/∆t = +1/2∆[B]/∆t you can use this to obtain the same answer...

-0.00211 = +∆B/2

∆B = 2x 0.00211 = 0.00422 M/s