I read in my book that negative rates do not have any significance. Can't we interpret negative rate simply as decrease in concentration of reactant? What's wrong with that and why do we put an additional negative sign with the change in concentration of reactant to make it positive?
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2$\begingroup$ Negative rate means the negative net rate of the reaction in forward sense and positive net rate in backward sense. But the rates of forward and backward reactions are never negative. $\endgroup$– PoutnikCommented Jul 1 at 10:07
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2$\begingroup$ If $A\to B$ do you mean why do we write $dA/dt=-kA$ or do you mean why is the rate constant $k$ not negative? $\endgroup$– porphyrinCommented Jul 1 at 11:03
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$\begingroup$ The reaction rate provides you with the change of concentration of reactants and products. A reactant decrease in concentration while a product increases its concentration. The rate of disappearance of a reactant is negative, while the rate of formation of a product is positive. You can calculate the rate of reaction using the concentration of reactants or products, but it must give you the same number. $\endgroup$– PAEPCommented Jul 1 at 15:15
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2$\begingroup$ A reaction rate has to be positive. If a reaction rates is negative, it would mean that the amount of reactant is increasing. This cannot happen, by definition. $\endgroup$– MauriceCommented Jul 1 at 15:23
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Why do we put an additional negative sign with the change in concentration of reactant to make it positive?
To make both sides of the equation $$\frac{dA}{dt}=-k[A]$$ consistent in terms of mathematical sign. The constraints are that the concentration of reactant [$A$] cannot be negative, however, the sign of the change in the concentration of species $A$, $\Delta A$ in necessarily negative, because its concentration is decreasing as the reaction is progressing ($\Delta A= (A_{final} - A_{initial}) < 0$) or in in other words $A_{final} < A_{initial}$. Consequently, the constant $k$ has to be a positive number. These are all man-made consistent set of rules in modeling the rate of reaction.
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$\begingroup$ Unnecessary down voting serves no purpose to the community. Have courtesy and appreciate the time of others and point out the issue if any. Being lazy and pressing buttons won't work. $\endgroup$– ACRCommented Jul 2 at 4:33