Average rate of consumption

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

Kelly Kiremidjian 1C
Posts: 62
Joined: Fri Sep 29, 2017 7:04 am

Average rate of consumption

When finding average rate of consumption of R, I know the equation is (- delta[R]/delta t) because the concentration of R would be decreasing during the reaction. But when stating the rate, is it always positive? Textbook question 15.3 part a has the rate of consumption of NO2 as a positive rate so I wasn't sure if rate is always expressed as a positive number.

Sungyoon_Baek_1A
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Re: Average rate of consumption

Rate will always be positive, rate will never be expressed as a negative number. As you said the equation for R is R = - delta[R]/delta t, but remember that R can also be expressed as R = +delta[P]/delta t. The negative when looking at the forwards rate is there to account for the smaller final reactant concentration than initial concentration (to make sure we have a positive rate). Using the second equation, since the concentration of products of increasing the final concentration is greater than the initial concentration (giving us a positive number) and therefore giving us a positive rate value.

Chem_Mod
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Re: Average rate of consumption

I would use a positive value but specify whether it is being formed at that rate or being consumed at that rate.