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If there is a reaction like this

$$\ce{ H2 + Cl2 -> 2HCl}$$

I can understand that each individual species (reactant or product) can have different rates and we can compare them like this,

$$-\frac{d[\ce{H2}]}{dt} =- \frac{d[\ce{Cl2}]}{dt} = \frac{d[\ce{HCl}]}{2dt}$$

But I'm having problem with defining the rate of reaction (for the chemical reaction shown above) as the above expressions.

Can someone please explain me why it is like this?

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  • $\begingroup$ You are missing negative signs, but think about this: the rate of decrease, in either reactant concentration, is half the rate at which product concentration increases. Simply, when one hydrogen molecule bites the dust, two HCl molecules are produced. $\endgroup$
    – Ed V
    Sep 3 '21 at 18:09
  • $\begingroup$ Yeah but why rate of reaction of this reaction is [HCl] /2dt why it can't be - 2[Cl2]/dt.why we have to go for simplest ratio(dividing by stoicheometry) .??? $\endgroup$
    – Zayden
    Sep 3 '21 at 18:18
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    $\begingroup$ Well, so long as you are consistent and others know what you are doing, no problem. The 2 has to go somewhere. $\endgroup$
    – Ed V
    Sep 3 '21 at 18:54
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    $\begingroup$ Reaction rate is convention, there is no true reaction rate. Important thing is to keep this convention clear, not deliberate. If there was 2A + 3B -> C + 4D, which (absolute value ) of component rates would you choose as reaction rate? $\endgroup$
    – Poutnik
    Sep 4 '21 at 7:16
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    $\begingroup$ I think i got it. Its a rule we have to divide the reaction rate with related to a component by its stoicheometry. Thank you soo muchhh $\endgroup$
    – Zayden
    Sep 4 '21 at 7:39

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