When to include H2O in the rate law


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Glenda Marshall DIS 3M
Posts: 68
Joined: Sat Sep 27, 2014 3:00 am

When to include H2O in the rate law

Postby Glenda Marshall DIS 3M » Fri Feb 20, 2015 10:04 pm

Question #14.105 gives the following section mechanism:

ClO- + H2O ----> HClO + OH- (fast)
HClO + I- ----> HIO + Cl- (slow)
HIO + OH- ----> IO- + H2O (fast)

and asks for the rate law. By using the pre-equilibrium process, I wrote the rate law as

[ClO-][H2O][I-] / [OH-]

The solutions manual has the same answer except they left out H2O because it is a solvent. How do we know when to consider H2O as a solvent and thus make it irrelevant? And how would we know how to recognize a different solvent if it wasn't H2O?

Viridiana G [3M]
Posts: 6
Joined: Fri Sep 26, 2014 2:02 pm

Re: When to include H2O in the rate law

Postby Viridiana G [3M] » Sat Feb 21, 2015 12:35 am

Referring to page 73 (Chemical Kinetics, page 21) in the course reader:
H2O can be included in the rate law if it is present in the reaction mechanism, but we usually leave out H2O because its concentration is much higher. So its concentration doesn't affect the rate.

Justin Le 2I
Posts: 142
Joined: Fri Sep 26, 2014 2:02 pm

Re: When to include H2O in the rate law

Postby Justin Le 2I » Wed Feb 25, 2015 1:29 am

Leave out H2O unless the problem explicitly says that water is not the solvent.


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