[research]

hi
I always thought that the reaction order of a chemical reaction is equal to coefficient of primary substances (in reactions with molecularity of one, of coarse) but in the following reaction I see that in contrast of my exception (two order), the order is zero!! so how I should determine the reactions order?
every help will be appreciated 
2NO₂ (g)---> 2N₂ (g) + O₂ (g)
rate=k

[ContinuousProcess]

IIRC, reaction order is determined experimentally or by observing a table of concentrations vs time.

There is no way to just guess looking at the equation.

[research]

IIRC, reaction order is determined experimentally or by observing a table of concentrations vs time.

thanks for your reply. would you explain me please what is IIRC?

[Borek]

http://www.urbandictionary.com/define.php?term=iirc

[research]

thank you borek

[demoninatutu]

A quick Google will answer your question, research.

That said, the reaction order will be related to the stoichiometry of the chemical equation only for very simple reactions, e.g. X+Y —>Z where X and Y collide and form molecule Z. If you add more of either X or Y, the collisions are more frequent and the reaction is faster. This gives us the rate equation d[Z]/dt=k[X][Y]. The reaction order is the sum of the indices above the square brackets. Note, we could write the previous equation as d[Z]/dt=k[X]¹[Y]¹ so, as 1+1=2, this reaction is second order, as is 2X —>Y, where d[Y]/dt=k[X]².

Now let's suppose that molecule Y in the first reaction is water and we're conducting the reaction in water, for example, CO₂ + H₂O —> H₂CO₃

Because water is the solvent, the CO₂ molecules are always colliding with water molecules — adding more water won't increase the rate of production of H₂CO₃. However, adding more CO₂ will increase the reaction rate so the reaction looks like it's first order with respect to CO₂. This reaction will also appear to be zero order with respect to water, (as we said, adding more water will not affect the rate of the reaction). in other words, it's pseudo-first order: d[H₂CO₃]/dt = k[CO₂].

A zero order reaction will not depend on the concentration of the chemical at all. The rate of a thermal decomposition, for instance, will often depend on the temperature of the system rather than the concentration of the reactant.

In addition, many reactions are more complex than molecules simply knocking into one another and these may have third order kinetics or even fractional orders.

For all of these reasons, the order of a reaction is only related to stoichiometry for very simple reactions and (generally) only for first or second order reactions.