ShmoopTube

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Here’s your Shmoop du jour, brought to you by experimental rate law, the rule that made

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all speed limits Avogadro’s number for a day. [Policeman pointing to speed limit with Avogadro's number on it]

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It wasn’t popular.

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Anyway, here’s today’s question:

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What is the experimental rate law for the following reaction, given the data below?

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And here are your potential answers: Anybody feeling psychic? [Psychic woman appears]

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Let’s start off by trying to predict what that rate law should be based on the reaction [Stoichiometry appears on her crystal ball]

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stoichiometry.

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So the stoichiometric coefficients are often the same as the reaction

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order for each reactant. [Man working in a science lab]

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Since the reactants are two CO and one O2, the reaction will probably be second order [Finger points to the reactants in the equation]

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in CO and first order in O2.

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If this guess is correct, the rate equals the

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rate constant times the concentration of carbon monoxide squared times the concentration of [Psychic woman holding the rate constant]

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oxygen, which is answer (B).

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Shh…something is telling us we should open a psychic hotline…call now…only $10 a [Psychic woman doing a phone line advert]

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minute…

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But unfortunately, this isn’t a guessing game.

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To test our hypothesis, we’ll need to look at the table. [Wheel of fortune presenter chucks away his microphone and walks off]

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Between experiments 1 and 2, the initial concentration of CO is doubled while the initial concentration

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of O2 stays the same. [Man pointing to figures in a table]

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These two data points show us how the rate depends on the concentration of CO without

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the influence of O2.

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Seriously, no peer pressure here.

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In this case, when the CO concentration is doubled, the rate increases by a factor of

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four, which means the rate varies with concentration of CO squared. [Person using a pipette in a lab]

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Between experiments 1 and 3, the initial concentration of O2 is doubled while the concentration of

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CO stays the same.

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We can use these two data points to figure out how the rate depends on the concentration

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of O2 without worrying about CO.

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Here, we see that when the O2 concentration is doubled, the reaction rate is also doubled,

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so the rate is proportional to the O2 concentration. [Person taking blue liquid out of a flask]

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Putting this all together, our overall rate law is rate = k[CO]2 [O2] the rate equals [Man pointing to a blackboard with the answers on]

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the rate constant times the concentration of carbon monoxide squared times the concentration

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of oxygen.

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And, hey…that sounds pretty familiar, which means our guess was right and B is the correct [Psychic woman looks happy]

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answer.

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All right, well, this was fun, but we gotta get this hotline set up…shh…the spirits

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are speaking…they're telling you to phone in and stay on the line for an hour…. [Psychic hotline ad]

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on your parents credit card... yes, 1-800 shmoop, we'll tell you your future...

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