## pseudo first order rxns

$\frac{d[R]}{dt}=-k[R]^{2}; \frac{1}{[R]}=kt + \frac{1}{[R]_{0}}; t_{\frac{1}{2}}=\frac{1}{k[R]_{0}}$

Ruiting Jia 4D
Posts: 65
Joined: Fri Sep 28, 2018 12:27 am

### pseudo first order rxns

Hello! I am confused about the concept of pseudo-first-order reactions. When would we need to use them, and why is it that some reactants would be insignificant to the rate law?

WilliamNguyen_4L
Posts: 65
Joined: Fri Sep 28, 2018 12:28 am

### Re: pseudo first order rxns

You use pseudo rate laws are used when all reactant concentrations change. When all reactant concentrations are changing it is difficult to study the rate of the reaction. So in order to study it we assume one reactant concentration is small and the others are so much larger that the concentration depends only on the reaction with the small concentration. Because the concentrations of the other reactants are so large when they react their concentrations essentially don't change so that's why they are insignificant to the rate law.

Ruiting Jia 4D
Posts: 65
Joined: Fri Sep 28, 2018 12:27 am

### Re: pseudo first order rxns

WilliamNguyen_4L wrote:You use pseudo rate laws are used when all reactant concentrations change. When all reactant concentrations are changing it is difficult to study the rate of the reaction. So in order to study it we assume one reactant concentration is small and the others are so much larger that the concentration depends only on the reaction with the small concentration. Because the concentrations of the other reactants are so large when they react their concentrations essentially don't change so that's why they are insignificant to the rate law.

How do we assume one is small over the other (how do we choose which one is smaller)?

Melissa Bu 1B
Posts: 36
Joined: Fri Sep 28, 2018 12:19 am

### Re: pseudo first order rxns

At the kinetics review session today, the facilitator said that we likely will not be tested mathematically on pseudo-first order reactions. We just need to know conceptually that pseudo-first order reactions are second order reactions which under some particular conditions (as described above), look like first order reactions (the integrated rate law looks like a first order integrated rate law).