### Determining Reaction Order

Posted:

**Wed Mar 14, 2018 3:16 pm**could someone further explain how to determine reaction orders?

Created by Dr. Laurence Lavelle

https://lavelle.chem.ucla.edu/forum/

https://lavelle.chem.ucla.edu/forum/viewtopic.php?f=148&t=29726

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Posted: **Wed Mar 14, 2018 3:16 pm**

could someone further explain how to determine reaction orders?

Posted: **Wed Mar 14, 2018 3:30 pm**

To determine reaction order you would add the coefficients of the concentrations in the Rate law. For example, if the rate law = k[A]^2 * [B]:

The Reaction is Second-Order in terms of A, First-Order in terms of B, and (2+1) Third-Order overall.

The Reaction is Second-Order in terms of A, First-Order in terms of B, and (2+1) Third-Order overall.

Posted: **Wed Mar 14, 2018 4:42 pm**

The reaction order is determined experimentally. Thus, the pattern is as follows:

If the reactant concentration is doubled and the rate is unchanged, then the reactant is zero order.

If the reactant concentration is doubled and the rate is doubled, then the reactant is first order.

If the reactant concentration is doubled and the rate is quadrupled, then the reactant is second order.

First find the individual order for each reactant and assign this number as the exponent in the rate law for the particular reactant. The overall order can be given by taking the sum of the exponents.

If the reactant concentration is doubled and the rate is unchanged, then the reactant is zero order.

If the reactant concentration is doubled and the rate is doubled, then the reactant is first order.

If the reactant concentration is doubled and the rate is quadrupled, then the reactant is second order.

First find the individual order for each reactant and assign this number as the exponent in the rate law for the particular reactant. The overall order can be given by taking the sum of the exponents.

Posted: **Wed Mar 14, 2018 10:25 pm**

You can also determine reaction order from the rate constant:

K=1/s first order

K=1/M*S second order

K=1/M^2*S third order

You get these units for k by doing this: K= M^(1-X)/S; where X is the order of the reaction. Just a helpful hint to find those units :)

K=1/s first order

K=1/M*S second order

K=1/M^2*S third order

You get these units for k by doing this: K= M^(1-X)/S; where X is the order of the reaction. Just a helpful hint to find those units :)

Posted: **Wed Mar 14, 2018 10:30 pm**

in general, the only way to determine the reaction order is experimentally or via the reaction mechanism.

Posted: **Fri Mar 16, 2018 2:43 am**

The reaction order is found experimentally, and is usually stated in the question itself. For zero order, rate = k; for first order, the rate is k*[A]; and for second order, the rate is k*[A]^2