# Rate of reaction and coefficient of the reactan

1. ### Entanglement

438
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

### Staff: Mentor

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.

3. ### Entanglement

438

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

### Staff: Mentor

Yes it is true - because we defined it this way. Order of the reaction equals sum of the coefficients from the elementary reaction equation.

4,514
6. ### Entanglement

438

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.

Last edited: Mar 17, 2014

### Staff: Mentor

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).