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Consider a reaction $$\ce{aA + bB -> cC + dD}$$ whose reactants are given by A and B and the products are C and D.

The rate of this reaction is given by $$\frac{dx}{dt} = -\frac{d[\ce{A}]}{adt}= -\frac{d[\ce{B}]}{bdt} = \frac{d[\ce{C}]}{cdt} = \frac{d[\ce{D}]}{ddt}$$

The rate of the reaction also depends on the surface area of reactants, nature of reactants, stirring, temperature. Does the rate equation account for all the factors (I could find that by changing these factors the change in concentration for a time interval would vary)? If in a lab we are given different samples of reactants and measure the rate of the reaction (but same concentration), wouldn't each get a different rate value? How is the rate uniquely defined?

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  • $\begingroup$ The rates above assume homogenous reactions, so surface and stirring is out. $\endgroup$
    – Poutnik
    Sep 9, 2022 at 13:42
  • $\begingroup$ So what if its homogenous? Say the reaction happens in the liquid state, stirring would still help ? $\endgroup$
    – Shashaank
    Sep 9, 2022 at 13:43
  • $\begingroup$ Well, stirring increases temperature..... but generally is used to reach a homogenous state. $\endgroup$
    – Poutnik
    Sep 9, 2022 at 13:44
  • $\begingroup$ Oh, I could do it slowly enough so that I could increase the collisions only and not much of the temperature? Isn't that negligible? $\endgroup$
    – Shashaank
    Sep 9, 2022 at 13:45

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The rate is given by the differentials you present and these have values depending on concentration, temperature etc. as you mention. What defines a reaction is the rate constant. In your example $$ rate = \cdots etc. =\frac{d[D]}{ddt}=k[A]^{\alpha}[B]^{\beta}\cdots$$ where $k$ is the rate constant and $\alpha+\beta\cdots$ define the reaction order. The rate constant and exponent powers have to be determined by experiment.

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  • $\begingroup$ Can those vary by changing the reaction conditions? $\endgroup$
    – Shashaank
    Sep 9, 2022 at 23:39
  • $\begingroup$ The Arrhenius eqn. describes how the rate constant depends on temperature for example. $\endgroup$
    – porphyrin
    Sep 12, 2022 at 7:10

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