[mythonline]

I am just starting to learn this subject, was wondering if anyone could help me understand why those equations are true:

when the reaction is aA + bB <-> cC + dD
V = -1/a* d[A]/Dt = -1/b * d/Dt =  -1/c * d[C]/Dt =  -1/d * d[D]/Dt =

[RandoFlyer]

Here's an example to help:

4 PH_3(g)  P_4(g) + 6H_2(g)

Rate(V) = (1/4) :delta:[PH₃]/ :delta: t  =  :delta:[P₄]/ :delta: t  =  (1/6) :delta:[H₂]/ :delta: t


In this case:
P₄ forms at 1/4 the rate that PH₃ "disappears."
P₄ forms at 1/6 the rate that H₂ "appears"

Hope that helps!

[Juan R.]

I am just starting to learn this subject, was wondering if anyone could help me understand why those equations are true:

when the reaction is aA + bB <-> cC + dD
V = -1/a* d[A]/Dt = -1/b * d/Dt =  -1/c * d[C]/Dt =  -1/d * d[D]/Dt =

The equations for C and D are incorrect.

You have a system n_A molecules of A, n_B molecules of B, n_C molecules of C, and n_D molecules of D. The composition can be written as a vector

N = (n_A, n_B, n_C, n_D)

This composition will change if there are chemical reactions. For instance, the above reaction changes the number of molecules as

(a, b, 0, 0) <-> (0, 0, c, d)

From left to right in each instance, a molecule of A and b molecules of B disappear and produce c molecules of C and d molecules of D and vice versa.

The change of the number of molecules will be given by another vector

w = (0, 0, c, d) - (a, b, 0, 0)

w = (-a, -b, c, d)

Notice the signs (negative are reactants positive are products)!

The rate of change of the number of molecules will be the product of the velocity R of the reaction and of the vector of molecular change

dN / dt = w R

That is

d n_A / dt = -a R

d n_B / dt = -b R

d n_C / dt = c R

d n_D / dt = d R

If volume L is constant divide all

d [A] / dt = -a V

d [ B] / dt = -b V

d [C] / dt = c V

d [D] / dt = d V

Compare with what you wrote and note the change of sign in C and D.

Ask if you have doubts.

[mythonline]

thanks a lot