Question

What are the rate of reaction units?

Answer 1

For any reaction? Concentration per unit time.

What units of time do you want? Seconds? Minutes? Hours? Well, whatever you can measure with a stopwatch I suppose... Know what makes sense given the numerical value of the rate you are looking at. Is it about the picosecond-time-scale autoionization of water? The 10-second time-scale of the reaction of crystal violet with sodium hydroxide? The billions-of-years-time-scale of the uranium-238 decay chain?

What unit of concentration do you want? Molecules per unit volume? Molarity? Your choice... depends on whether you are a statistical mechanical chemist or a more typical experimentalist.

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The rate of reaction is given as `r(t)` in a rate law as:

`r(t) = k[A]^m[B]^n`

`= -1/a(Delta[A])/(Deltat) = -1/b (Delta[B])/(Deltat) = 1/c(Delta[C])/(Deltat) = 1/d(Delta[D])/(Deltat)`

for the reaction

`aA + bB -> cC + dD`

where:

• `k` is the rate constant (constant with respect to temperature).
• `[" "]` is the concentration of a given reactant.
• `m` is the order of reactant `A`.

And we know that `[" "]` is concentration and `t` is time... so by definition of the rate of change of concentration over time

`(Delta[" "])/(Deltat)`

being the slope of a concentration vs. time plot, the units are clearly

`"concentration"/"time"`