I don't see why someone would use the average rate with initial concentrations. The rate has it's biggest value at the beginning of the reaction, if you use the average rate, there would be a big error. Seems like no integration this time, you can just memorize the rate laws.

Good. So then average rate over the whole reaction must indeed be 0 M/s (because the reaction never completely finishes until t=∞)?

0, 1st and 2nd order aren't very difficult to integrate anyway. A+B

Products though is worth memorizing.

I have another question. As the concentration of peroxide is first order and the reaction has a second order overall, is it correct to use the k from: r=k[H_{2}O_{2}][KI] in this equation: ln[H_{2}O_{2}]=ln[H_{2}O_{2}]_{0}-kt? I think not, as I found a equation for the second order reaction here http://en.wikipedia.org/wiki/Rate_equation#Zero-order_reactions (the one with A and B), but there are two unknowns.

I haven't actually done the problem yet but I'm sure you got r=k[H

_{2}O

_{2}][I

^{-}] right.

Of course you can't treat it as first-order, only pseudo first-order. You will want to look this up if you haven't already. But to do this, you will have to forget about modelling [H

_{2}O

_{2}] as a function of time, and model [I

^{-}] instead. This makes sense because even if all the I- were consumed (and it never

* quite* gets there) most of H

_{2}O

_{2} will be left over - i.e. its concentration is said to be unchanging and can be taken as constant throughout the experiment. So you will use r=k

_{mod}[I

^{-}] where k

_{mod} = k*[H

_{2}O

_{2}]

_{0}. This is an ugly approximation as I'm sure you can see, because H

_{2}O

_{2} is not that huge this time - I will test it using exact integral, after my first round is done.

If you have learnt partial fractions you can integrate r=k[A][B] exactly. This is what is done in the section "Second-order reactions" on your Wikipedia page. As for the presence of two variables, you can easily express [A] in terms of [B] and substitute.

However, looking at the initial concentrations in the question it seems that this is not expected and all they expect is you to use pseudo-first order. Many times in IChO I have seen the same problem - you either use constant-concentration approximation, or you can do exact integration. I have seen r=k[A]

^{2}[B] which is a more challenging integration but still can be done. (Don't ask for derivation, I haven't yet, just seen the result in my book)