confused with kinetic rate equations

[yoyohomieg5432]

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Why is it sometimes when you see an equation, i.e.:
aA +bB ----> cC

we say the rate is k[A]^a ^b

but other times we say k[A]^x ^y and then x and y need to be determined experimentally?

i thought i understood the rate laws but i'm totally confused now. i can think to times where there's been a reaction like A+B-->C and i just write rate= k[A] and it's right, but when can we just flat out write down a rate law and when does it need to be determined experimentally? or have i been wrong along and it always has to be determined experimentally?

 

[rfvbnmju]

Ok I think you're confusing rate with equilibirum

the equilibrium constant Keq for aA +bB ----> cC is
k eq = [C]^c / ([A]^a ^b)
This is when A, B and C are either aqueous or gases and are just the coefficent of the balanced equation.


The rate law = k[A]^x ^y, x and y are found experimentally based upon the initial rates.

You can also say that rate = delta C/(c)delta t or - delta A/(a)delta t or -delta B/(b)delta t

This just tells you that rate is at which your products/reactants appears/disappears.

 

[Synapsis]

A note my GChem professors always stressed was that if you have elementary steps in a reaction mechanism, the order with respect to each reaction does not need to be determined experimentally.

aA + bB ---> cC (fast)

cC + dD -----> eE (slow)

You could write the rate equation (without steady state approximation) as: rate = k[C]^c[D]^d, remembering that we write the rate equation using the slow step reactants.

While if you simply had

cD + dD -----> eE

you would need to determine the order experimentally.

 

[sakabato93]

Why is it sometimes when you see an equation, i.e.:
aA +bB ----> cC

we say the rate is k[A]^a ^b

but other times we say k[A]^x ^y and then x and y need to be determined experimentally?

i thought i understood the rate laws but i'm totally confused now. i can think to times where there's been a reaction like A+B-->C and i just write rate= k[A] and it's right, but when can we just flat out write down a rate law and when does it need to be determined experimentally? or have i been wrong along and it always has to be determined experimentally?

If you are only given a general equation in this form:

aA +bB ----> cC

then the rate law musts be determined by experimental data.

However, if you are given the reaction broken up into elementary mechanistic steps, then the rate law can be written using the coefficients of the reactants of the rate-determining (i.e. slow) step of the mechanism, shown as follows:

Reaction: aA +bB ----> dD

Mechanism:

aA +bB ----> cC (fast)

cC----> dD (slow)

rate = k[C]^c

BUT! You cannot leave the rate law in terms of a reaction intermediate. You have to rewrite the rate law in terms of the original reactants.

Here's a video showing how to do this process using actual reactions instead of vague letters: http://www.youtube.com/watch?v=0qYODKoKPrU

Good luck!