## definition of first order

$\frac{d[R]}{dt}=-k[R]; \ln [R]=-kt + \ln [R]_{0}; t_{\frac{1}{2}}=\frac{0.693}{k}$

Sarah Blake-2I
Posts: 153
Joined: Fri Aug 30, 2019 12:16 am

### definition of first order

In class today, Dr. Lavelle was mentioning how to tell if a reaction is first order or second order. What does this mean exactly? Also, when using the rate law, how can we tell the order of the reaction based on our answer? Any help is much appreciated. Thank you !

Diana Chavez-Carrillo 2L
Posts: 122
Joined: Fri Sep 28, 2018 12:18 am

### Re: definition of first order

First order is when the rate in a reaction is proportional to the first power of the molar concentrations of the reactants. Second order is a reaction that has a rate that is proportional to the square of the molar concentrations of a reactant. You can tell which order to use if they directly say which order in the problem or by looking at the units given in the problem because each order uses different units of k.

Kassidy Ford 1I
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Joined: Sat Aug 17, 2019 12:16 am
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### Re: definition of first order

You can also tell by looking at the graphs of the reactions concentration over time. For a zero order reaction, the graph will be linear if you plot concentration over time. For first order, the graph will be linear if you plot ln(concentration) over time, implying an exponential relationship. For 2nd order, if you plot 1/(concentration) then you will get a linear graph.