Question

In the reaction PCl3 + Cl2 ---> PCl5.

If the rate doesn't increase when the concentration of PCl3 is doubled and the rate increases by a factor of four when the concentration of Cl2 is doubled, how would I write the rate law for the reaction?

Answer 1

rate = k(PCl3)^x*(Cl2)^y
would x = 0; y = 2 make sense to you?

Answer 2

I understand why PCl3 is zero order, but I don't get why Cl2 is second order.

Answer 3

rate 1 = k*(PCl3)^0(Cl2)^y
rate 2 = k*(PCl3)^0(Cl2)^y

The problem says rate increases by 4 when we double Cl2. So let's do that. Let's call rate 1 = 1 to make things simple; also, since (PCl3)^0 = 1 we can dispense with that, too. Then we will call (Cl2)= 1 with rate 1, again, to make things simple, then (Cl2) = 2 with rate 2.
rate 1 = 1, (Cl2)^y = (1)^y
rate 2 = 4, (Cl2)^y = (2)^y
Now divide the two equations, we get

1/4 = (1)^y/(2)^y = (1/2)^y so
what must y be to raise 1/2 to that power in order to obtain 1/4. y must = 2. Does this help? It may be easier to see if you divide rate 2 by rate 1, in which case we have
4/1 = (2/1)^y and y must be 2 so that 2^2 = 4.