## Units of K

$aR \to bP, Rate = -\frac{1}{a} \frac{d[R]}{dt} = \frac{1}{b}\frac{d[P]}{dt}$

Posts: 23
Joined: Wed Sep 21, 2016 3:00 pm

### Units of K

I was somewhat confused on the sign of k.
Since the General Rate equation for RXN aA --> bB is
Rate = -1/a d[A]/dt = 1/b d[B]/dt, and we know d[A]/dt = k[A]^(molecularity)
would the sign of k change whether we are considering the product or reactant? Or is it always positive? Is there something I'm missing here?

Natalie Rotstein 3J
Posts: 20
Joined: Wed Sep 21, 2016 2:59 pm

### Re: Units of K

It's positive for the thing being considered; for the equation you mentioned, the reason one side is negative is because the concentration of the reactants decreases while the concentration of the products increases, making the change in reactants negative and products positive. To counter this so that k is the same value/sign on either side, the - is added to the equation.