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Chemical Kinetics: determining order of reaction

  • #1

Homework Statement



Good day! I have a question from my chem HW that is stumping me. Although the answer may be simple, it has eluded me for quite a bit. It regards rate laws and constants of reactions.

Question is as stated: This reaction was monitored as a function of time: AB --> A + B

A plot of 1/[AB] versus time yields a straight line
with slope= - 0.055 M-1s-1

What is the value of the rate constant (k) at this temp?





Homework Equations



SINCE it stated 1/[AB] vs. time, its implying an order 2 reaction since order 2 reaction
has the integrated rate law:
1/[AB]t = kt + 1/[AB]0

y axis = 1/[AB].......


The Attempt at a Solution



When a order 2 reaction is plotted against time, k = slope. When the question stated it was talking about a plot of 1/[AB] its referring to a order 2 reaction, and since order 2 reactions (when plotted against time) have a y axis = 1/[AB], im deducing the reaction is order 2.

I'm confused since the given slope is negative, and every graph of an order 2 reaction vs. time in my book has a straight line with a positive slope.

Like I said, its an easy problem, with an easy solution, but i'm not seeing it.
 

Answers and Replies

  • #2
19,666
3,979

Homework Statement



Good day! I have a question from my chem HW that is stumping me. Although the answer may be simple, it has eluded me for quite a bit. It regards rate laws and constants of reactions.

Question is as stated: This reaction was monitored as a function of time: AB --> A + B

A plot of 1/[AB] versus time yields a straight line
with slope= - 0.055 M-1s-1

What is the value of the rate constant (k) at this temp?





Homework Equations



SINCE it stated 1/[AB] vs. time, its implying an order 2 reaction since order 2 reaction
has the integrated rate law:
1/[AB]t = kt + 1/[AB]0

y axis = 1/[AB].......


The Attempt at a Solution



When a order 2 reaction is plotted against time, k = slope. When the question stated it was talking about a plot of 1/[AB] its referring to a order 2 reaction, and since order 2 reactions (when plotted against time) have a y axis = 1/[AB], im deducing the reaction is order 2.

I'm confused since the given slope is negative, and every graph of an order 2 reaction vs. time in my book has a straight line with a positive slope.

Like I said, its an easy problem, with an easy solution, but i'm not seeing it.
If [AB] is decreasing with time, then 1/[AB] must be increasing with time. That's why the slope is positive.

Chet
 
  • #3
Thanks for the reply, but 1/[AB] has a negative slope not positive in this problem. That's why i'm confused. The graph would indicate that 1/[AB] is decreasing with time which means [AB] is increasing which makes no sense.
 
  • #4
19,666
3,979
Thanks for the reply, but 1/[AB] has a negative slope not positive in this problem. That's why i'm confused. The graph would indicate that 1/[AB] is decreasing with time which means [AB] is increasing which makes no sense.
Oh. I didn't notice that. It must just be a mistake in the problem statement.

Chet
 

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