Rate cannot be negative

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Rate cannot be negative

Postby JuliaPark2H » Sat Mar 12, 2016 8:04 pm

In the Winter 2015 Practice Final #7A, the question asks if the rate constant can ever be negative.
The answer says no, the rate constant can never be negative because the Rate can never be negative, since Rate=k[R]n.

I don't feel completely comfortable with this answer/reasoning, mostly because the rate law for reactants do seem to have a negative rate, where the concentration decreases over time. The rate constant can be considered as the slope of a linear rate law relationships, and negative slopes are possible. So what does the above answer mean when the rate& rate law cannot be negative?

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Re: Rate cannot be negative

Postby Chem_Mod » Sat Mar 12, 2016 10:09 pm

I guess it's like the rate is always positive, but you have the reverse reaction that also has a positive rate. So if your reaction is A --> B, then your forward reaction rate law is going to be "Rate = kforward[A]". At the same time, you also have the reverse reaction happening, where the reverse reaction rate law is "Rate = kreverse[B]". In both cases, the rates are positive and the rate constants (k) are positive. But depending on how fast each reaction is going, the rate of consumption or rate of formation of each compound (d[A]/dt, d[B]/dt) may be positive or negative.

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