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15.5 Positive vs Negative

Posted: Fri Feb 24, 2017 4:48 pm
by Michael_Johanis_2K
C2H2+3O2--->2CO2+2H2O
The unique rate of combustion is .44 M/s.
Find the rate at which oxygen reacts.

I solved this by using:
-1/3 * d[O2]/dt = .44 M/s
d[O2]/dt = -1.32 M/s

However, the answer is (positive) 1.32 M/s. Why is my answer wrong? Am I incorrectly using the unique reaction rate rule, which says that for aA ---> bB, then -1/a d[A]/dt = 1/b d[B]/dt?

Re: 15.5 Positive vs Negative

Posted: Fri Feb 24, 2017 11:33 pm
by Marie_Bae_3M
I'm not completely sure but I think the negative in the rate of decomposition for O2 (-d[O2]/dt) is in terms of the general reaction rate and merely indicates that the concentration of O2 is being used up. Since "reacts" already implies the negative (O2 concentration decreasing), I think another negative would cancel out "reacts" and make it seem as though O2 is forming? I think it was mentioned before that rates are always positive, and maybe the reason for that is because they ask for it in the respective terms of whether the substance is a reactant or product?

Re: 15.5 Positive vs Negative

Posted: Fri Feb 24, 2017 11:45 pm
by Kayla_Black_2B
Marie_Bae_3M wrote:I'm not completely sure but I think the negative in the rate of decomposition for O2 (-d[O2]/dt) is in terms of the general reaction rate and merely indicates that the concentration of O2 is being used up. Since "reacts" already implies the negative (O2 concentration decreasing), I think another negative would cancel out "reacts" and make it seem as though O2 is forming? I think it was mentioned before that rates are always positive, and maybe the reason for that is because they ask for it in the respective terms of whether the substance is a reactant or product?

Yeah, rate constants are never negative since the rate of reaction is never negative. (Course Reader, Pg. 199).