[eleventhxhour]

One way to remove rust stains from ceramic bathroom fixtures is to apply a
solution of oxalate ion, as shown in the following equation.

C2O4^-2(aq) + 2Fe^+3(aq) --> 2CO2(g) + 2Fe^+2(aq)

What is the average rate of reaction for this system over the specified period if the
concentration of oxalate ion changes from 0.80 mol/L at 3.0 min to 0.20 mol/L at
8.0 min?

So I understand that you have to do the change in concentration divided by change in time. However, the textbook has a positive answer (0.12 mol/(L.min) and I got -0.12mol/(L.min). I'm not sure why, because I thought you'd do the second concentration/time divided by the initial concentrtion/time, which would get you a negative number...

Can someone help? Thanks

[Mark S 2014]

Rate of reaction is (Change in Concentration)/(Change in Time) (0.60 mol dm^-3)/(5 min^-1) = 0.12 mol dm^-3 min^-1. If you have done it by using (0.2-0.8 )/(3-8 ) remember both the numerator and denominator will both be negative, so therefore the answer will be positive.

[mjc123]

No it won't, it will be (0.2 - 0.8 )/(8-3) = -0.12 mol/L/min
However, "rate of reaction" is conventionally a positive quantity, equal to either the rate of production of product or minus the rate of loss of reactant, taking into account stoichiometric coefficients. Thus in this case
Rate of reaction = -d[C₂O₄^2-]/dt = -1/2 d[Fe³⁺]/dt = 1/2 d[CO₂]/dt = 1/2 d[Fe²⁺]/dt
I have said before that I don't like talking about "rate of reaction"; I far prefer to talk in terms of the rate of change of a specific reagent.