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### Topic: Question on rate of reaction using concentration  (Read 1421 times)

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#### purplecinnamon

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##### Question on rate of reaction using concentration
« on: February 18, 2016, 01:49:18 PM »
Hello! So I have a question about concentration. We are given this balanced equation.

2Al(CH3)3(g)+3H2O(g) Al2O3(s)+6CH4(g)
The question says: "While the reaction number one is running, which of the following does not describe the rate of the reaction?"

A. -(1/3)(Δ[H2O]/Δt)

B. -(1/2)(Δ[Al(CH3)3]/Δt)

C. (-1/6)(Δ[CH4]/Δt)

D. Δ[Al2O3]/Δt

E. All of the above apply

So I'm confused for two reasons first, how is it that any of these answers for that matter can express the speed of the reaction? Shouldn't the rate of the reaction be based on all of the concentrations?
And the second thing I am confused about is, what does it mean by "while number 1 is running"?

Thank you very much for your time.

#### mikasaur

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##### Re: Question on rate of reaction using concentration
« Reply #1 on: February 18, 2016, 02:20:30 PM »
I imagine when they say "number 1" they're referring to your reaction and there may be some other reactions in following questions that are reactions 2+. I imagine you don't have to worry too much about that.

As for your actual question, there are many ways you can measure the rate at which a reaction is proceeding. Imagine you had a very good way of detecting H2O but were unable to detect any of the other species in the reaction. Would you be able to use that to measure the rate over a period Δt? What if you could only detect CH4?

::EDIT:: You may also want to look up the difference between rate expressions and rate laws. I think that's where you're getting tripped up.
Or you could, you know, Google it.

#### purplecinnamon

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##### Re: Question on rate of reaction using concentration
« Reply #2 on: February 18, 2016, 02:37:08 PM »
Oh I see. So the answer mus be C, because C has a negative sign in front of it. And looking at how the reactants have the negative sign, it would make sense that the products should positive. But how can we get those expression from our balanced equations?

#### mikasaur

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##### Re: Question on rate of reaction using concentration
« Reply #3 on: February 18, 2016, 02:40:11 PM »
http://chemistry.stackexchange.com/questions/5872/difference-between-reaction-rate-and-rate-law

I googled a bit and found the link above. Note that the question asked mixes the terms slightly but the answerer gives a nice explanation.
Or you could, you know, Google it.

#### purplecinnamon

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##### Re: Question on rate of reaction using concentration
« Reply #4 on: February 18, 2016, 04:42:12 PM »
Oh that makes so much sense! Thank you!