[rudiment274]

I have a question about calculating rates and determining orders:

Three runs were used to study the reaction: A + B :rarrow:C + D

The following data was obtained (initial concentrations of C and D = 0

Initial Conentration (M)
___________________
Run  [A]       [ B] 
1    3.000   2.000
2    6.000   2.000
3    3.000   4.000


Concentration after 150 s (M) (I had to fill out this table based on the reaction stoichiometry)
_____________________
Run [A]       [ B]      [C]
1    2.970   1.970   0.09
2    5.940   1.940   0.18
3    2.880   3.880   0.360


(?) Calculate the rate in M/s for each of the three runs.

I know that Rate=-1/a x Δ[A]/Δt

But I do not know what values to substitute.

Also, when I'm finding the order, do I first write the rate law expression, choose 2 sets of data that depend on one reactant while the rest are held constant, set up ratio of rates, and take the log of the expression?

[mjc123]

I know that Rate=-1/a x Δ[A]/Δt
But I do not know what values to substitute.

I don't see what you don't understand. You have 3 runs, for each you have the values of [A] at two different times. Plug them in.

Also, when I'm finding the order, do I first write the rate law expression, choose 2 sets of data that depend on one reactant while the rest are held constant, set up ratio of rates, and take the log of the expression?

You can do that in principle, but in a case like this it's much simpler. Compare runs 1 and 2. [ B] is the same but [A] is doubled. What happens to the rate? What does that tell you about the order with respect to A?