Textbook Problems 15.3 and 15.5

[mrao25]

I was wondering as to why in 15.3, we divide by the number of moles from the equation (such as part c where we divide by 2 for the moles of NO2), whereas in 15.5 we multiply by the number of moles (such as part a where we multiply by 3 for the moles of O2)?

Thanks!

[Carmille Vega 1C]

Part c of 15.3 asks for the the unique rate of reaction, which is the rate of change of the reactant(s)/stoichiometric coefficient. So you divide the rate of change of 2NO2(g), -6.5, by the stoichiometric coefficient of 2NO2(g), which is 2. Part a of 15.5 asks for something different, which is simply the rate of reaction (notice how it's not the unique rate of reaction).

[Rachel Lipman]

In the solutions manual, 15.3 part c is depicted as the second b.

[Helen Shi 1J]

How come the textbook answer for 15.3 part a) is 6.5 x 10^-3 mol/L*s? Why did they not divide by the number of moles (2)?

[Adrian Lim 1G]

I think the answer in the solutions manual shows the rate of reaction of NO2 in this specific reaction, so it doesn't divide by 2 for part a

[Katelyn 2E]

also regarding this question, is there a difference in how we should calculate rate of reaction and rate of formation?

[William Xu Dis 1D]

Just remember that the rate of the reaction (unique rate of the reaction) is always the rate of either formation of a product or consumption of a reactant divided by the coefficient of that product or reactant in the balanced chemical equation. For calculating the rate of a formation, you simply divide concentration change by change in time. Coefficients are only used when relating rates of one reactant or product to another reactant or product, such as when calculating the unique rate of the reaction.

[Cristina Sarmiento 1E]

So when you are calculating the rate of reaction that is not the unique rate of reaction, you multiply the concentration by the coefficient?