## writing rate law

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

Anika_Patel_1G
Posts: 32
Joined: Fri Sep 29, 2017 7:06 am

### writing rate law

Why are we able to write a rate law using the slow elementary step but not the general balanced reaction? Also, how do we use the elementary step to write the rate law?

Hannah Chew 2A
Posts: 76
Joined: Fri Sep 29, 2017 7:05 am
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### Re: writing rate law

While the general reaction is definitely needed to see what's happening in the bigger picture, we specifically look at the slowest step to determine the rate law because the slowest step determines the reaction rate. In class, Dr. Lavelle used the example of baking brownies; it doesn't matter how fast everyone else is working to measure ingredients / mix them all together. The limiting step is the one person who has to put the brownies in the oven for a set period of time. The rate of baking brownies is dependent on the baking time. So similarly, the rate law is based on the slow step.

To make the rate law, it's similar to previous problems. You have to find the order with respect to each reactant in the slow step, but it really depends on the context of the problem to figure out how to approach the problem.

David Minasyan 1C
Posts: 54
Joined: Thu Jul 13, 2017 3:00 am

### Re: writing rate law

The slow elementary step is much like the concept of a limiting reactant from last quarter.

Sirajbir Sodhi 2K
Posts: 47
Joined: Sat Jul 22, 2017 3:00 am

### Re: writing rate law

You can't just use the balanced equation to write the rate law because the reaction has many steps that are not reflected in the reaction (except in an elementary reaction). The rate law is an expression of the MECHANISM of a reaction -- each individual step -- which the stoichiometric coefficients don't tell you much about.

The slow step of a reaction is the rate-limiting step (explained well in the above answer) so that determines the rate of the reaction.