If slope for ln(A) vs. t is negative is k?


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Nicole Leppi 2K
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If slope for ln(A) vs. t is negative is k?

Postby Nicole Leppi 2K » Tue Feb 24, 2015 9:44 pm

For example if a slope for a first order reaction (ln(A)) vs. t) was -3 would k=-3 or would k=3?

Nicole Leppi 2K
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Re: If slope for ln(A) vs. t is negative is k?

Postby Nicole Leppi 2K » Tue Feb 24, 2015 10:02 pm

that's what I thought, thanks!

Justin Le 2I
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Re: If slope for ln(A) vs. t is negative is k?

Postby Justin Le 2I » Wed Feb 25, 2015 1:37 am

Since the slope is equal to -k and the slope is -3, then k = 3. k is always positive.

martha-1I
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Re: If slope for ln(A) vs. t is negative is k?

Postby martha-1I » Wed Feb 25, 2015 9:47 pm

Rate constants (k) are always positive.
Rates can be negative or positive depending on whether it's for formation or decomposition. This is so the rates of formation and rates of decomposition could cancel out.

Neil DSilva 1L
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Re: If slope for ln(A) vs. t is negative is k?

Postby Neil DSilva 1L » Wed Feb 25, 2015 11:52 pm

Martha,

I don't think rates can be negative either. If k is always positive and concentrations are always positive, the rate is always positive as well. Correct me if I'm wrong, but even if the rate is for decomposition / consumption and has a negative sign, the rate is still actually positive, because the rate is given by (-1) x [(change in concentration) / (change in time)] where the change in concentration is given by the final concentration minus the initial concentration. Since the initial concentration is greater than the final concentration, the change is given with a negative value that is canceled out by the negative sign in front.

Chem_Mod
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Re: If slope for ln(A) vs. t is negative is k?

Postby Chem_Mod » Thu Feb 26, 2015 1:06 pm

Justin Le 2I wrote:Since the slope is equal to -k and the slope is -3, then k = 3. k is always positive.


Correct.
Slope = -k
-3 = -k
Therefore k = 3

Chem_Mod
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Re: If slope for ln(A) vs. t is negative is k?

Postby Chem_Mod » Thu Feb 26, 2015 1:13 pm

Equilibrium constants, rate constants, and the rate of a reaction are always positive numbers.


Since is negative due to decreasing reactant concentration the rate is positive.

Writing the rate with respect to the product gives,
and this is also a positive value.

martha-1I
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Re: If slope for ln(A) vs. t is negative is k?

Postby martha-1I » Thu Feb 26, 2015 3:29 pm

Oh I understand. Thank you for the clarification!


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