# Rate of reaction and coefficient of the reactan

by ElmorshedyDr
Tags: coefficient, rate, reactan, reaction
 P: 403 Suppose in the reaction 2 NO + O2 --------> 2NO2 Why when the the concentration of NO is doubled, the rate increases 4 times, I know that the rate is directly proportional to the square of NO concentration (I don't know why) and it's order of reaction is 2 ( I don't why ). but I still can't understand a reason for all of that it seems foggy to me why the coefficient affects the proportionality. It should be intuitive to a chemistry student, I think a simple analogy would be helpful, thanks
P: 23,597
 Quote by ElmorshedyDr Why when the the concentration of NO is doubled, the rate increases 4 times, I know that the rate is directly proportional to the square of NO concentration (I don't know why) and it's order of reaction is 2 ( I don't why )
While everything you wrote is correct, it is the wrong way around.

First, we should check what is the reaction order - and the only sure way of doing it is an experiment. Once we know order is 2 everything else follows as a series of conclusions.
P: 403
 Quote by Borek While everything you wrote is correct, it is the wrong way around. First, we should check what is the reaction order - and the only sure way of doing it is an experiment. Once we know order is 2 everything else follows as a series of conclusions.

But in elementary reactions, the order is equal to the coefficient, isn't that true and why ???

P: 23,597
Rate of reaction and coefficient of the reactan

 Quote by ElmorshedyDr But in elementary reactions, the order is equal to the coefficient, isn't that true and why ???
Yes it is true - because we defined it this way. Order of the reaction equals sum of the coefficients from the elementary reaction equation.
 Sci Advisor P: 3,596 Looking at the article http://www.sciencedirect.com.1361-0....8986039990232X there are two possible mechanisms: ## \rm 2NO \leftrightarrows N_2O_2## (fast) ##\rm N_2O_2+O_2\rightarrow 2NO## (slow, rate-determining) or ##\rm NO+O_2\leftrightarrows NO_3## (fast) ##\rm NO_3+NO\rightarrow 2NO_2## (slow, rate-determining)
P: 403
 Quote by Borek Yes it is true - because we defined it this way. Order of the reaction equals sum of the coefficients from the elementary reaction equation.

You really misunderstand me, I know because it's defined like that, I mean why does the number of moles in a balanced question affects the proportionality in the rate equation for an example 2NO + O2 ------> 2NO2 , I2 + H2 -----> 2HI

Supposing that they are elementary reactions
Why in the first equation on doubling the conc of NO the rate increases 4 times but on doubling I2 it increases only 2 times

I already know the conception of the order of reaction and this stuff

but I want an analogy that really clarifies it, or a detailed easy explanation that make it clear and intuitive to a high school student.
 Admin P: 23,597 If you are looking for intuition - reactions occur when the molecules collide, the more molecules, the more frequent the collisions. Number of collisions for a given kind of a molecule is proportional to its number (or concentration).

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