Questions 2013-03-03

Email questions...

"I am having trouble with determining orders of reactions.  I am pretty confused on the entire concept of them.  I am confused on questions like 3-5 on the practice tests.  I also don't understand what it means when the reaction increases by factors."
Some of this confusion may be a result of unfamiliar terminology. First, remember that for our class at this point, the only orders we are going to use are 0, 1, & 2. We determine those orders by changing the concentration of one reagent and seeing how the observed initial rate of the reaction changes. If we double the concentration of "A" {that's changing it by a factor of 2}, the observed initial rate of the reaction will either be unchanged {change by a factor of 2⁰} if the reaction is zero-order with respect to "A", or it will double {change by a factor of 2¹} if the reaction is first-order w.r.t. "A", or it will quadruple {change by a factor of 2²} if the reaction is second-order w.r.t. "A". Textbooks tend to really like just doubling concentrations, but there's nothing magic about multiplying by 2. You could determine the orders of a reaction by dividing the concentrations by 2 {this is also a change by a factor of 2, it's just dividing instead of multiplying}, or multiplying/dividing the concentrations by a factor of 3 or 4 or 72, it should all work the same way.

"I was looking at last spring's old chem exam 2a and for problem number 27 I got a different answer. I believe I did the math right but maybe I didn't. To get t, I did:  ln(1.03)-ln(1.67)/(-3.63x10^-2) and my answer was 14.2 and not 13.3 like you got."
This is a good calculator warning. Most importantly, when you are answering exam questions, show your work clearly and as completely as possible. If you have a calculation clearly set up correctly and just make a math/calculator error, you won't lose as many points as if you don't have your equation clearly set up. For this specific question, be sure to use parentheses on your calculator to make sure the math is being done in the order you intend. In the absence of parentheses, your calculator will evaluate multiplication/division before addition/subtraction, so if I put in the implied parentheses:
ln(1.03) - {ln(1.67)/(-3.63x10^-2)} = 14.2
But we really want that to be:

{ln(1.03) - ln(1.67)} / (-3.63x10^-2) = 13.3

By the way, if you see something on a posted answer key that seems incorrect, please let me know. I think I have everything done correctly on the keys, but there definitely could be some mistakes.