PCl 5 ( g ) <-----> PCl 3 ( g ) + Cl 2 ( g ) has an equilibrium constant in term

Question

PCl5(g) <-----> PCl3(g) + Cl2(g)

has an equilibrium constant in terms of pressures Kp = 2.15.

(a) Suppose the initial partial pressure of PCl5 is 0.319 atm, and PPCl3 = PCl2 = 0.659 atm. Calculate the reaction quotient Qp and state whether the reaction proceeds to the right or to the left as equilibrium is approached.

Qp = _______
Reaction proceeds to the _______(right or left)

(b) Calculate the partial pressures at equilibrium.

PPCl5 = ______atm

PPCl3 = ______ atm

PCl2 = ______ atm

(c) If the volume of the system is then decreased, will there be net formation or net dissociation of PCl5?

There will be net _________(formation or dissociation.)

Explanation / Answer

for the reaction PCl5(g)<-------->PCl3(g)+Cl2(g)

Reaction coefficient, Q = PPCl3*PCl2/PPCl5, where P indicates partial pressure of the component.

given initially, PPCl5 =0.319 atm and PPCl3=PPCl2= 0.659 atm

Q= 0.659*0.659/0.319 =1.36 < K, the equilibrium value. So the reaction proceeds to the right so as to increase the partial pressure of Cl2 as well as PCl3.

Let P= change in presssure of PCl5 to reach equilibrium

At Equilibrium, PPCl5= 0.319-P and PPCl3= PPCl2=0.659+P

So KP= (0.659+P)*(0.659+P)/ (0.319-P)=2.15

when solved using excel, x= 0.0711

so at equilibrium, PPCl5=0.319-0.0711=0.2479 atm, PPCl3= PPCl2= 0.659+0.0711=0.7301 atm

In the reaction, PCl5<------->PCl3+Cl2, there is an increase in number of moles of gaseous products in the forward direction. So as the volume is decreased, since at constant pressure from gas law, PV= nRT

Volume is proportional to no of moles, as per Lechatlier Principle, as the volume is decreased, the reaction shifts in a direction so as to increase the volume ( i.e no of moles). so the reaction moves to the right. so more PCl5 dissociates.

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