# Change in concentration vs. reaction rate

1. Sep 1, 2014

### Maylis

Hello,

I am wondering, why is it that

$\frac {d[C]}{dt} \ne k[C]$ in general, where $C$ is a chemical species, and the product $k[C]$ is the reaction rate, $r$. $r_{c} = k[C]$

My thoughts is that because the units aren't necessarily the same, therefore they can't be the same. But I was wondering about a more physical explanation.

2. Sep 1, 2014

### Einstein Mcfly

What you're describing is a unimolecular reaction. If you have a reaction that is more than just one thing changing on its own, you have at least a bimolecular reaction and you can't talk about it just in terms of a single concentration.

The units of the rate constant are whatever they have to be for whatever type of reaction you have. You'll learn all about this in P-chem.

3. Sep 2, 2014

### DrDu

What do the square brackets stand for, exactly?

4. Sep 2, 2014

### Maylis

Concentration of the species

5. Sep 2, 2014

### DrDu

You have to be careful, here. The velocity of the reaction may in deed be formulated as the change of the concentration of a species with time. However, even in unimolecular reactions, the expression on the right hand side depends rather on the chemical activity than on concentration.