A reaction is second order respect to the reactant A. If the reaction is 85% complete in 12 minutes, how long would it take for the reaction to be 15% complete?
a. 110s
b. 27s
c. 62s
d.130s
e. 22s

e. 22s

Explanation:

Initial Concentration = 100

Final concentration = 15 ( Since the reaction is 85% complete)

Time = 12 minutes = 720 seconds

The integrated rate law for second order reactions is given as;

1 / [A] = (1 / [A]o) + kt

Making k subject of interest, we have;

kt = (1 / [A] ) - (1 / [A]o )

k = (1 / [A] ) - (1 / [A]o ) / t

k = [(1/ 15) - (1 / 100) ] / 720

k = (0.0667 - 0.01 ) / 720

k = 0.0567 / 720

k = 7.875e^-5

How long would it take for the reaction to be 15% complete?

Final concentration = 85 (Since the reaction would be 15% complete)

The integrated rate law for second order reactions is given as;

1 / [A] = (1 / [A]o) + kt

Making t subject of interest, we have;

kt = (1 / [A] ) - (1 / [A]o )

t = (1 / [A] ) - (1 / [A]o ) / k

t = [(1 / 85 ) - (1 / 100) ] / 7.875e^-5

t = (0.0117 - 0.01 ) / 7.875e^-5

t = 0.001765 / 7.875e^-5

t = 22.41 seconds

Corect option = E.

1. the question asks the most appropriate, which seems to rule out "all of the above." your choice appears to be the correct one.

2. the protocol described is good for identifying correlation, but not cause and effect. for cause/effect, you need to devise a protocol that will assign the treatment (or not) to groups that have essentially the same "demographics" with respect to all the other variables. your choice regarding control group seems appropriate. random assignment of treatment (choice d) may be appropriate also. (i think you can argue that b and d are just different descriptions of the same method.)

3. d will randomize the birds, but may not guarantee there are 10 in each group. e will do both, so seems the better choice.

comment on question 2

based on the problem statement, it seems clear that confident, attentive drivers are more likely to use daytime running lights. so, the protocol might need to be devised in such a way that the lights would be turned on or off without the driver's knowledge. (an attentive driver will figure it out.) assigning a confident, attentive driver to a group required to drive without running lights might mess with his/her confidence and attentiveness. you don't want your experiment to do that. (nor do you want the experiment to subject a driver to greater risk of accident as a consequence of driving without running lights.) experiments involving human safety are always problematic.