Elementary reaction's order of reaction

[Tahmeed]

According to textbook, elementary reactions have an order of reaction equal to the summation of the stoichiometric coefficients. why is this so?

And there is another problem, we can balance an equation in different ways and get different stoichiometric coefficient sets, for rates, which set is prefered?

 

[Kevin McHugh]

Not necessarily, if the reaction is first order, it only involves the coefficient of one reactant. The reaction rate is independent of the concentration of the other reactant. As far as I know, there is only one way to balance a reaction.

 

[Borek]

According to textbook, elementary reactions have an order of reaction equal to the summation of the stoichiometric coefficients. why is this so?

That's true for elementary reactions. In most cases the overall stoichiometry of a process (one that we describe with a reaction equation) is different.

And there is another problem, we can balance an equation in different ways and get different stoichiometric coefficient sets, for rates, which set is prefered?

The one in which all coefficients are the smallest integer numbers possible. So of the three reaction equations:

H₂ + ½O₂ → H₂O

2H₂ + O₂ → 2H₂O

4H₂ + 2O₂ → 4H₂O

the middle one is - by convention - considered "the right one".

 

[Tahmeed]

That's true for elementary reactions. In most cases the overall stoichiometry of a process (one that we describe with a reaction equation) is different.
The one in which all coefficients are the smallest integer numbers possible. So of the three reaction equations:

H₂ + ½O₂ → H₂O

2H₂ + O₂ → 2H₂O

4H₂ + 2O₂ → 4H₂O

the middle one is - by convention - considered "the right one".

That's true. But why exactly it is so? why elementary reactions follow this? what is the physics behind this?

 

[Kevin McHugh]

It's not physics, but convention. Chemists don't like 1/2 coefficients in stoichiometry, because 1/2 of a molecule doesn't exist (for long). And the balanced equation is always reduced to its lowest common factor.

 

[Ygggdrasil]

That's true. But why exactly it is so? why elementary reactions follow this? what is the physics behind this?

Elementary reactions are reactions that occur in a single step with no intermediates. In the reaction equations, you would interpret them as saying, in your example, x molecules of hydrogen react with y molecules of oxygen. Obviously, non-integer coefficients do not make sense in this context. Similarly, unless four molecules of hydrogen and two oxygen molecules must all come together at one place to make two water molecules, this is not an elementary reaction.

In terms of reaciton kinetics, the rate equation follows from the definition of an elementary reaction as a reaction occurring in a single step. For a unimolecular reaction (A --> P), the rate is just v = k[A]. Each molecule of A has the same probability of reacting per unit time, so the more A that is present, the more molecules of A will undergo the reaction (though the same fraction of molecules of A will under go the reaction per unit time).

For bimolecular reactions to occur, the molecules involved in the reaction must first collide. The rate of collision is just the product of the concentration of two reactants. So if it's A + C--> P, then v = k[A][C] or if the reaction is 2A --> P, then v = k[A]². Trimolecular reactions follow a similar logic.