Why we are dividing rate of reaction with respect to a species by its stoichiometry coefficient to find rate of that particular reaction

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?

• 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.
– Ed V
Sep 3 '21 at 18:09
• 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) .??? Sep 3 '21 at 18:18
• Well, so long as you are consistent and others know what you are doing, no problem. The 2 has to go somewhere.
– Ed V
Sep 3 '21 at 18:54
• 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? Sep 4 '21 at 7:16
• 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 Sep 4 '21 at 7:39