Confusion about Stoichiometric coefficients

[Saladsamurai]

Hey folks! I am studying from a thermodynamics text (engineering thermo) and I am a little confused with the wording in the following development of the "Equation of Reaction Equilibrium":

Consider a closed system containing 5 components A, B , C, D and E. We will assume tha E is inert and thus does not appear in the rxn:

$v_AA +v_BB \leftrightharpoons v_CC + v_DD \qquad(1)$​

where the v's are the stoichiometric coefficients. Note that the stoichiometric coefficients $v_A, v_B, v_C, v_D$ do not correspond to the respective number of moles present. The amounts of components are designated $n_A,n_B,n_C,n_D$. However the, changes in the amounts of components present do bear a relationship to the values of the stoichiometric coefficients. That is,

$$-\frac{dn_A}{v_A}=-\frac{dn_B}{v_B}=\frac{dn_C}{v_C}=\frac{dn_D}{v_D}\qquad(2)$$​

I am a little confused as to the distinction between the n's and the v's. Is it saying that the n's are the actual amounts present whereas the v's are the theoretical amounts needed for a balanced reaction?Thanks!
~Casey

 

[espen180]

Hey folks! I am studying from a thermodynamics text (engineering thermo) and I am a little confused with the wording in the following development of the "Equation of Reaction Equilibrium":



I am a little confused as to the distinction between the n's and the v's. Is it saying that the n's are the actual amounts present whereas the v's are the theoretical amounts needed for a balanced reaction?


Thanks!
~Casey

Stochiometric coefficients describe the relative rates of reaction for the different reactants and products, you could say.

Another way to look at it is that for every v_a moles of A participate in the reaction, v_b moles of B will also participate, v_c moles of C and v_d moles of D will be produced.

Hope that clears things up.

 

[chemisttree]

I think you understand the text perfectly.
It is very like an extensive property vs and intensive property in this description.