15.3

[Kayla Danesh 1F]

15.3 In 20. s, the concentration of nitrogen dioxide, NO2, decreases from 450 mmol L 1 to 320 mmol L 1 in the reaction 2 NO2(g)--> 2 NO(g) + O2(g). (a) Determine the rate of reaction of NO2.

Why is the correct answer a positive value, and not negative? shouldn't it be a negative value since the reactant concentration is decreasing?

[Janine Chan 2K]

I think the reaction rate is always written as positive. In the solutions manual, it makes up for the decreasing concentration by adding a negative sign in the equation (-∆conc/∆time). So, you would just say that the reactant concentration is decreasing at (input rate here).

[Lauren Seidl 1D]

Rate is similar to speed vs. velocity. Though velocity can be positive or negative because it has direction, speed can only be a positive value. The same goes for rate of a reaction, it can only be a positive value because it is simply how fast the reaction is proceeding.

[Alex Nechaev 1I]

Since there is already a negative sign in the formula for determining the reaction rate of a reactant, the subtraction of concentrations that would come out negative becomes positive. I think that reaction rates are just always expressed as positive values.

[Jacob Cho 2L]

The reaction that breaks apart the NO2 progress in the forward direction. Thus, the rate of the reaction is negative. On the flip side, if the reaction rate was negative, then reactants would be forming (i.e. NO2).