Question

x + 2y ---> products is first order in c and 2nd order in y. what are the units of K, the specific rate constant, if the reaction rate is expressed in units of moles per liter per sec.

I know the answer is M.s^-1^
but I don't understand this concept. could you explain to me how to write this expression given any order?

Answer 1

I suppose you could work the 2-4 combinations out and memorize them; however, it's relatively easy to do one. For example, for the rate = k(x)^1(y)^2 for a third order reaction, you have
k = rate/(x)^1(y)^2 =
rate is M/t where M is molar and t is time.
Tben k = (M/t)/(M)(M)2
k = (M/t)/(M)(M)(M)
k = (M/t)*(1/M^3)
k = M on top cancels.
That leaves 1 on top and M^2 and t on the bottom or 1/M^2*t which often is written as M^-2*time^-1.