Oder with respect to a reactant

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haleyf 2I
Posts: 12
Joined: Fri Sep 25, 2015 3:00 am

Oder with respect to a reactant

Postby haleyf 2I » Thu Feb 25, 2016 6:38 pm

Could someone do a quick run-through of the steps needed to find the order of a particular reactant if you're given a table with initial concentrations and reaction rates? I understand that you look for two trials where only the concentration of the desired reactant is changed and the other concentrations stay constant but then what?

Cristian Yanes 1L
Posts: 17
Joined: Fri Sep 25, 2015 3:00 am

Re: Oder with respect to a reactant

Postby Cristian Yanes 1L » Thu Feb 25, 2016 7:52 pm

Hopefully this helps--
Let's say reactants = A B and C
Step 1:
Look for two trials where nothing but the concentration of a single reactant changes (as you mentioned) and observe the rate change between those two trials. Did it double as the concentration of the reactant doubled? First-order. Did it quadruple as the concentration of the reactant doubled? Second-order. Did it half when the reactant of the concentration doubled? Half-order. Was it unchanged when the concentration increased/decreased? Zero-order. For the sake of this example, let's say the reactant C goes from 0.015 to 0.030 and the rate doubles, therefore it is first order.

Step 2:
Now that you've determined that reactant C is first-order, look for another trial where A or B changes and C either stays constant or changes (most likely it will change). Now that you've determined C's effect on the rate (it will double/half it as it doubles/halves), look for the effect of A or B on the concentration. For this example, let's say A doubles and C halves and the rate is doubled. Therefore, A would be a second-order reaction, as A doubling alone would quadruple the rate but since C halving halves the rate, the rate only overall doubles.

Step 3:
Now you repeat Step 2 to determine what order B is. Let's say it's zero-order.

Step 4:
Generally, they will now ask you to find the rate of some unknown reaction. In this case, our Rate Law would equal Rate=[A]^2[B]^0[C] or [A]^2[C]. And now you just plug in the given concentrations to find the missing rate!

Hope I helped! :-)

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