## Finding the order

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

Michelle Ouk 3L
Posts: 8
Joined: Mon Jan 26, 2015 2:17 pm

### Finding the order

I'm having trouble figuring out what order things are in. I'm exactly sure what makes a a certain thing a particular order. I was wondering if someone can explain to me how to determine the order.

Chem_Mod
Posts: 19769
Joined: Thu Aug 04, 2011 1:53 pm
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### Re: Finding the order

If you're given a reaction that's the rate determining step (i.e. the slow step), then you just multiple all the reactants together.

Example 1:
A + B --> C
Rate = k*[A]*[B]

Example 2:
A + 2B --> C
Rate = k*[A]*[B]^2

But if you're given a data table, then you just have to compare the rate change with the reactant concentration change. For example, if reactant B was doubled (and all other reactants were kept the same), and you saw that the rate doubled, then the reaction is 1st order in B. If reactant A was doubled (and all other reactants were kept the same), and you saw that the rate increased by the square root of 2 (~1.4 times), then the reaction is 1/2th order in A.
You just keep repeating this until you get through every single reactant in your data table.

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