rate question

[joonkimdds]

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2A + B --> C

if A is 1st order and B is 1st order, what's the correct rate equation?
rate = k[A]^1 ^1
rate = k[A]^2 ^1
rate = k[2A]^1 ^1

 

[mx41]

First one

 

[joonkimdds]

First one

but isn't A consumed twice faster than B?

 

[whawha]

but isn't A consumed twice faster than B?

Have you decided to retake it? I thought you were keeping the first one...


Anyways,

When you deal with reaction kinetics, you have to remember that the order is independent of the stoichiometric coefficients. Since the problem states that it is first order wrt A and first order wrt B as well, the answer should be A.

 

[mx41]

but isn't A consumed twice faster than B?

It doesn't matter what the coefficients are
The rate law isn't determined by the stoichiometry of the equation, it's determined by performing an experiment. Since the problem says it's first order, that means that the experiment was performed and the calculated order (or exponent in the rate law) is 1. That is what you use in the rate law

If the problem asks:
What is the rate law?
2A + B --> C

Then the equation would be:
rate = k[A]^2*
Because it doesn't mention what order they are.

 

[Valigator00]

It doesn't matter what the coefficients are
The rate law isn't determined by the stoichiometry of the equation, it's determined by performing an experiment. Since the problem says it's first order, that means that the experiment was performed and the calculated order (or exponent in the rate law) is 1. That is what you use in the rate law

If the problem asks:
What is the rate law?
2A + B --> C

Then the equation would be:
rate = k[A]^2*
Because it doesn't mention what order they are.

What if one of the choices is "cannot be determined from the given info?" Would that be the "theoretically" correct choice?

 

[joonkimdds]

If the problem asks:
What is the rate law?
2A + B --> C

Then the equation would be:
rate = k[A]^2*
Because it doesn't mention what order they are.

In this case, is [A] 2nd order because coefficient is 2 or is it not certain although we put 2 as a power of A.

 

[mx41]

What if one of the choices is "cannot be determined from the given info?" Would that be the "theoretically" correct choice?

You're right, theoretically, but I remember one of my chemistry professors doing a problem similar to this one, and she used the coefficients as the exponent in the rate law when not given any orders in the question for A, B, and C

When a question asks
What is the rate law?
2 H2 + O2 -> 2 H2O

And no order was mentioned.
Then in this case, the answer "cannot be determined" would be true because the rate consists of actual elements, and not fake ones like "A, B, or C"