# Predicting rate equations from data

I am having trouble wit the following question on my problem sheet:

"The reaction $\ce{Br- + ClO^- \to BrO^- + Cl^-}$ occurs with a rate constant of $\text{0.35 dm}^3 \text{mol}^{-1} \text{s}^{-1}$

at 298 K. For the following initial conditions calculate the time taken for the Br^- concentration to fall to half its original value:

i) $[\ce{Br-}]=[\ce{ClO-}]=0.0017$

ii) $[\ce{Br-}]=0.0017$ and $[\ce{ClO-}]=0.084$"

I am sure I could do this question if I knew the rate equation but it isn't given. From the units of the rate constant the reaction must be second order but how am I supposed to know whether it's second order with respect to one of the reactants (and thus zeroth order w.r.t the other) or first order with respect to both? Also, what's the significance of the concentrations being equal (there's another question that stresses that the concentrations of both the reactants are equal).

• I don't see a reasonable mechanism by whom the rate equation would have second order with respect to one reactant. – RBW Feb 1 '15 at 16:37
• Expanding on @Marko 's point, if there were a couple of potential mechanisms, one for first order in both reactants and one for zeroth/second order, then you wouldn't know. There's no particular reason the concentrations should be equal, or reason why it's important (unless perhaps there is a method of initial rates question elsewhere in your problem set.) – Jason Patterson Feb 2 '15 at 3:10