Kinetics problem TBR

[allatrope]

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The problem asks: Why is the initial rate observed in reaction studies?

A. As a reaction proceeds, it slows. The same period in the reaction must be compared to be consistent.

B. As a reaction proceeds, its rate increases. The same period in the reaction must be compared to be consistent.

C. Only the initial concentration can be accurately measured.

D. Any rate can be measured, but the initial one is most convenient.

The correct answer is A and I don't understand why. In terms of kinetics, do all reactions slow? Usually TBR does a good job explaining, but this time it was weak.

Many Thanks!

 

[Jepstein30]

The problem asks: Why is the initial rate observed in reaction studies?

A. As a reaction proceeds, it slows. The same period in the reaction must be compared to be consistent.

B. As a reaction proceeds, its rate increases. The same period in the reaction must be compared to be consistent.

C. Only the initial concentration can be accurately measured.

D. Any rate can be measured, but the initial one is most convenient.

The correct answer is A and I don't understand why. In terms of kinetics, do all reactions slow? Usually TBR does a good job explaining, but this time it was weak.

Many Thanks!

If we think of this in terms of rate laws, we can take a sample reaction: A -> B

If the rate law for that reaction is first order-> rate = k[A]

So we can see that as [A] decreases (as its made into B), the rate will also decrease.

The only caveat is if the reaction is zero order, then its rate is constant (rate = k) regardless of [reactant] until it runs out and completely stops.

 

[allatrope]

Thanks, that was a great explanation.