Chemical Kinetics: determining order of reaction

[BrettJimison]

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], I am 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.

 

[Chestermiller]

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], I am 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

 

[BrettJimison]

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.

 

[Chestermiller]

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