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Overall Order Of Reaction?

How do you figure out the overall order of reaction?

For example;
If one reactant was first order and another was second order what would the overall order of reaction be?

Do you just total them up?

Seems simple but i couldnt find it in any textbooks
Reply 1
Avatar for Wenzel
Wenzel
Yup, total them up. So for your example total order would be 3.
Reply 2
Avatar for mkchelsea
mkchelsea
1
oh I have a question on this topic too, What if you have a reaction:

A + B ----> C + D

You have already found out that it is first order wrt to A, and you've been told that B is in large excess. What can you assume about the total order of the reaction?
A question like this came up on a paper i was doing, i think the reaction was between an ester and water, with water being in excess. Thanks for the help
Reply 3
Avatar for walkjj
walkjj
The total order is 1, if it is in excess it wont effect the rate equation. Or at least I think thats right.
Reply 4
Avatar for Wenzel
Wenzel
mkchelsea
oh I have a question on this topic too, What if you have a reaction:

A + B ----> C + D

You have already found out that it is first order wrt to A, and you've been told that B is in large excess. What can you assume about the total order of the reaction?
A question like this came up on a paper i was doing, i think the reaction was between an ester and water, with water being in excess. Thanks for the help


Can't be certain without seeing the question, but since B is in large excess, I'd say that we can assume that the overall order is 1 (since rate of reaction is the change in concentration per unit time, and B is in large excess - so adding more of B will have little to no effect on the overall rate of reaction).

I'm pretty positive that would be right.
Reply 5
Avatar for mkchelsea
mkchelsea
1
walkjj
The total order is 1, if it is in excess it wont effect the rate equation. Or at least I think thats right.

ok thnx, that question was really bothering me!
Reply 6
Avatar for walkjj
walkjj
No problem :smile: Glad to help.
Wenzel
Can't be certain without seeing the question, but since B is in large excess, I'd say that we can assume that the overall order is 1 (since rate of reaction is the change in concentration per unit time, and B is in large excess - so adding more of B will have little to no effect on the overall rate of reaction).

I'm pretty positive that would be right.


yeah if something is in excess, then it doesnt change approximatly, and hence is not a variable in the equation (its actually absorbed into the rate constant k(new) = K(old)[excess])

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