Page 61 - A Working Method Approach For Introductory Physical Chemistry Calculations

Equilibrium I 45
Le Chitefier's Principfe states that when a system at equilibrium is
disturbed, the system will counteract as far as possible the effect of
the disturbance on the system.

Changes in Temperature
Consider the following two reactions:
(a) N2(g) + 3H2(g~* 2NH3(g),
K~ = P(NH~) /(P(N~>'P(H~)~I.
This is an exothermic reaction, i.e. AH" is -ve.
AG = AGO + RTln Kp
At equilibrium, AG = 0
+- lnKp = (-AG")/(RT) = (-AH"/RT) + (TAS"/RT), since
AGO = AH" - TAS".
Hence, cancelling Tin the second term, the expression rearranges to:
In Kp = ( -AH"/RT) + (ASo/R).
Therefore, if the temperature is increased, the (- AH"/Rr) term is
decreased (since AH" is -ve for an exothermic reaction), and so
Kp = P(NH~)~/(P(N~)'P(H~)~} decreased, i.e. if the temperature is
is
increased, the system can absorb heat by the dissociation of NH3(,)
into N2(g) and Hz(~). Hence, Kp will be decreased, i.e. the reaction
shifts in an endothermic direction to the left, as predicted by Le
Chiitelier's Principle. Similarly, for the abovti reaction, if the tempera-
ture is decreased, Kp is increased and the equilibrium shifts to the
right.
(b) N2(g) + 02(g) * 2NO(,),
Kp = P (NW2/(P "2)P (02))
This is an endothermic reaction, i.e. AH" is +ve.
At equilibrium, In Kp = ( -AH"/RT) + (ASo/R), as above.
Therefore, if the temperature is increased, the (- AH"/RT) term is
increased (made less negative, since AH" is +ve), and so Kp =
is
~(NO)~/(p(N~)p(02)) increased. The equilibrium then shifts to the
right. If Tis decreased the equilibrium shifts to the left.

Changes in Pressure

Consider the reaction
N(,) + 3H2(,2 + 2NH3(,),
K~ = pw3) I(P(WP(H~)~).
If the pressure of an equilibrium mixture of N2(g), H2(,) and NH3(,> is