Winter 2013 Final Q4C reaction rate

$aR \to bP, Rate = -\frac{1}{a} \frac{d[R]}{dt} = \frac{1}{b}\frac{d[P]}{dt}$

Keirsten Andersen 3L
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Winter 2013 Final Q4C reaction rate

Given the initial concentration of a product, reaction quotient, time, and change in concentration of product, how do we find the reaction rate? This solution takes the change in concentration and divides it by the time but what does the change in product have to do with the reaction rate?

Chem_Mod
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Re: Winter 2013 Final Q4C reaction rate

Yes. Recall the definition of reaction rate is the change of a substance participate in the reaction over a unit of time. It could be either reactants or products. If it is reactants then it is negative (decreasing over time) whereas if it is product the rate will be positive.

Lily Wei 1D
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Re: Winter 2013 Final Q4C reaction rate

And can someone explain why the answer for this question is supposed to be positive even though we are using the reactant (O2) concentration to determine the average rate?