The units of a rate constant for zero, first and second order reactions are given in the textbook. If the time units are minutes, what would you expect the units to be for a third order reaction?
a. 1/min3 b. mol2/L2-min c. L/mol-min2 d. L2/mol2-min e. L2/mol2-min2
help help!
rate laws:
zero order is rate = k so units are M/s or moles/l*s
1st order is rate = k*(A)
k = rate/(A) = moles/L*ws/(moles/L) = 1/s or s^-1.
Here is a site that will give you the units of zero, 1st, 2nd, and a formula for calculating any order units for k. Just scroll to that part of the site.
http://en.wikipedia.org/wiki/Rate_equation
To determine the units of a rate constant, we can use the rate equation for each type of reaction.
For a zero-order reaction, the rate equation is:
Rate = k[A]^0 = k (as [A]^0 = 1)
Since the rate has units of concentration per unit time, the rate constant (k) for a zero-order reaction has units of concentration per unit time as well. In this case, the units will be mol/L-min.
For a first-order reaction, the rate equation is:
Rate = k[A]^1 = k[A]
Again, the rate has units of concentration per unit time, so the rate constant (k) for a first-order reaction also has units of concentration per unit time. The units will be 1/min.
For a second-order reaction, the rate equation is:
Rate = k[A]^2
The rate has units of concentration per unit time, so the rate constant (k) for a second-order reaction has units of concentration per unit time per concentration. The units will be L/mol-min.
Now, let's consider a third-order reaction. The rate equation is:
Rate = k[A]^3
The rate has units of concentration per unit time, so the rate constant (k) for a third-order reaction has units of concentration per unit time per concentration squared. The units will be L^2/mol^2-min.
Therefore, the correct answer is d) L^2/mol^2-min.