Understanding the Rate Law for Chemical Reactions with Variable Exponents

[eax]

for
a + b -> c

the rate law would be
rate = [a]^m * ^n

Does m & n neccessarly have to be whole numbers? Could they be fractions?

 

[d_leet]

My ap chem teacher says that they have to be whole numbers, she didn't explain why but she did say that so I'm not 100% sure that this is the case but it kind of makes sense... maybe...

 

[Haxx0rm4ster]

Yes.
Either 0, 1 or 2... and occassionally/rarely 3.

 

[ksinclair13]

I think my teacher gave us one that came out to [A]^{.5}. If I find the example in my notes I will post it here.

Edit: Well I think I just thought of one (and I just remembered that I don't have my chemistry with me tonight).

Let's say that as A quadruples while B is held constant, the rate law only doubles. Wouldn't that give A an order of .5? Or is this just not physically possible?

 

[GCT]

pretty sure its possible, for simple cases such as this where you're given one rate equation, the rate law is related to the coefficients for the reactants or products. You're simply relating each of the reaction agents with each other. With weird molecules, you can also have strange rate laws, there may be one involving oxygen gas O2 which would has a coefficient of .5 for O2. Rate laws are more or less accurate in relating to experimental results, the're not actually laws, they're just used to model the dynamics...the equation itself may be adequate for some set of conditions but it is not the perfect description of it. That is real world does not comply to rate laws, at times it's fortunate that it can be used quite effectively.

 

[siddharth]

Does m & n neccessarly have to be whole numbers? Could they be fractions?

adding to what GCT said, they can be fractions or even negative.

For example, in
CH_3CHO \rightarrow CH_4 + CO
the rate law is rate =k [CH_3CHO]^{1.5}
And in
2O_3 \rightarrow 3O_2
the rate law is rate=k [O_3]^2 [O_2]^{-1}

 

[GCT]

yeah, rate laws can get pretty nasty