# A Question about Chemical Equilibrium

say we have a reaction:

N2O4 <====> 2NO2

for this reaction,if initial number of moles of N2O4 is 'a' can we write

t = 0 a -
at equilbrium a-x 2x

similarly can we do the same for pressure i.e.
if initial pressure is p,then can we say

t=0 p -
at equilbrium p-y 2y

if yes why? and is x=y,i.e is the number of moles x dissociated equal to decrease in pressure of reactant?
and is this method valid for all reversible reaction irrespective of the stoichiometric coefficient of reactants and products

please help.

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## Answers and Replies

James Pelezo
Gold Member
Physics Forums - Equilibrium N2O4(g) <=> 2NO2(g)

------------------- N2O4(g) ---- <=> --- 2NO2(g)
Ci: ------------------ a ------------------------ 0 ---
∆C:---------------- -x ---------------------- +2x ---
Ceq: ------------- a - x -------------------- 2x ---

Kc = [NO2]2/[N2O4] = (2x)2/(a – x) = 4x2/(a – x) => Kc(a – x) = 4x2 => Kc·a - Kc·x = 4x2

4x2 + Kc·x + (-Kc·a) = 0

x = [-b ± (b2 – 4ac)1/2]/2a

a = 4, b = Kc , c = -Kc·a

substitute into quadratic; solve for x …

[N2O4] = a – x

[NO22] = 2x