# Reaction Order Question

I already know the answer to this but I still have a question about how to understand this stuff for my upcoming quiz.

Two reactants, A and B, are mixed, and the reaction is timed until a color
change occurs. The data are as follows:
[A] Time (s)
0.100 0.140 25
0.050 0.140 50
0.100 0.070 100
The order of the reaction with respect to is ___?

Common sense tells me that since double the concentration (row 1 vs row 3 so [A] is constant) makes the time 4 times less, so the reaction is four times faster. That would make it a second order reaction.

But if I make it into rates I get something different. I divide the column B value by the Time column value (change in concentration over time, assuming the color change occurs when the concentration of B is 0). I get .0056 mol/L*s for the concentration of .140 mol/L and .0007 for the concentration of 0.07. This is a difference of a factor of 8 in with the reaction rates, which would indicate it is a third order reaction.

Obviously, the common sense approach was right... but I'm not exactly sure why it is wrong to hypothetically assume you have a liter of A or B, follow the concentration, and use that to make a rate.