Rate law

[idkididk]

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Scientist A proposes a mechanism for a certain reaction, and uses that mechanism to derive a rate law for the reaction. Scientist B then determines the rate law for the reaction experimentally, using the method of initial rates. If the two rate laws are the same, what can be concluded?

My answer: If scientist B determined the correct rate law, then the mechanism proposed by scientist A is correct.

The answer is: Assuming that the mechanism proposed by scientist A is correct, then scientist B determined the correct rate law.

Why is it not the other way around? I thought raw laws have to be determined experimentally most of the time, especially if many different mechanisms can be proposed for the reaction. So the experimentally determined rate would be more accurate, therefore telling us which of the mechanisms is correct

 

[gettheleadout]

There's a logical subtlety here. Scientist A proposes a mechanism and derives a rate law. Scientist B gathers data and derives a rate law. The fact that B's approach more closely aligns with real-world experimental practices is irrelevant. Consider the two alternative conclusions if the rate law's found are identical:

1) If B's rate law is correct, then A's mechanism is correct.

This is not a deductively valid conclusion. The only proper reasoning is as follows:

P1) Rate laws A and B are identical
P2) Rate law B is correct.
___
C) Rate law A is also correct.

The following argument, which this answer choice is making, is unsound:

P1) If rate law A is correct, then mechanism A is correct.
P2) Rate law A is correct.
___
C) Mechanism A is correct.

This is an unsound argument because, although the reasoning (argument structure) is valid, premise 1 is not true. Many different theoretical mechanisms may yield the same mathematical contribution to the rate law. Proposal of a correct mechanism requires not only correct mathematical relationships between species such that a correct rate law is derived, but also correct identification of each and every species and step.

2) If mechanism A is correct, then rate law B is correct.

This is a deductively valid conclusion. Consider the following argument:

P1) If mechanism A is correct, then rate law A is correct.
P2) Mechanism A is correct.
___
C1/P3) Rate law A is correct.
P4) Rate laws A and B are identical
___
C2) Rate law B is correct.

The argument holds.

 

[idkididk]

There's a logical subtlety here. Scientist A proposes a mechanism and derives a rate law. Scientist B gathers data and derives a rate law. The fact that B's approach more closely aligns with real-world experimental practices is irrelevant. Consider the two alternative conclusions if the rate law's found are identical:

1) If B's rate law is correct, then A's mechanism is correct.

This is not deductively valid. The only proper reasoning is as follows:

P1) Rate laws A and B are identical
P2) Rate law B is correct.
___
C) Rate law A is also correct.

The following argument, which this answer choice is making, is invalid:

P1) If rate law A is correct, then mechanism A is correct.
P2) Rate law A is correct.
___
C) Mechanism A is correct.

This is an invalid argument because premise 1 is not true. Many different theoretical mechanisms may yield the same mathematical contribution to the rate law. Proposal of a correct mechanism requires not only correct mathematical relationships between species such that a correct rate law is derived, but also correct identification of each and every species and step.

2) If mechanism A is correct, then rate law B is correct.

This is a deductively valid conclusion. Consider the following argument:

P1) If mechanism A is correct, then rate law A is correct.
P2) Mechanism A is correct.
___
C1/P3) Rate law A is correct.
P4) Rate laws A and B are identical
___
C2) Rate law B is correct.

The argument holds.

Funny thing is, I'm taking a philosophy class called Reasoning and Argumentation next semester, where I'll learn this exact kind of logic. I'm trying to wrap my ahead your reasoning still, but thank you!

 

[gettheleadout]

Funny thing is, I'm taking a philosophy class called Reasoning and Argumentation next semester, where I'll learn this exact kind of logic. I'm trying to wrap my ahead your reasoning still, but thank you!

You're welcome! I had to make some edits to correct my logic terminology.