Le Chatelier’s Principle: “If a system at equilibrium is disturbed by a change in temperature, pressure, or concentration of a component, the system will shift its equilibrium position to counteract the effect of the disturbance.” Henri Le Chatelier (1850–1936)

(A) change in concentration N2(g) + 3 H2(g) 2 NH3(g) Diagram shows what happens when we add H2 to an equilibrium system… H2 shift  to est. new eq. NH3 Suppose we remove NH3... N2 shift  to est. new eq. In industrial settings, the ammonia would be removed from the system by applying high pressure and low temperatures to induce liquefaction of the ammonia. orig. eq. system establishing new eq. new eq. H2 added at this time

(B) change in pressure – for gaseous equilibrium systems only 2 NO2(g) N2O4(g) SHIFT to est. new eq. If we increase pressure, the system “wants” to... get smaller SHIFT to est. new eq. If we decrease pressure, the system “wants” to... get bigger For H2(g) + I2(g) 2 HI(g), pressure changes result in... NO SHIFT

“Ne!” “Ne!” “Ne!” “Ne!” “Ne!” More specifically, if the partial pressures are affected, then there will be shifting. Otherwise...no. “Ne!” “Ne!” “Ne!” e.g., 2 SO2(g) + O2(g) 2 SO3(g) “Ne!” If we add neon into reaction vessel... Partial pressures of SO2, O2, and SO3 are unchanged. “Ne!” NO SHIFT As long as temperature doesn’t change, K is... NOT affected by P-V changes.

– + (C) changes in temperature These always result in... shifts in eq. AND changes in K. For exothermic reactions: – R P + heat (DH is ____) -- as T increases... shift , K Don’t memorize… THINK! -- as T decreases... shift , K For endothermic reactions: + heat + R P (DH is ____) -- as T increases... shift , K -- as T decreases... shift , K

For PCl5(g) PCl3(g) + Cl2(g), DHo = 87.9 kJ. Predict shifts for... PCl5(g) + 87.9 kJ PCl3(g) + Cl2(g) Rewrite this as… SHIFT (a) adding Cl2 SHIFT (b) increasing temperature SHIFT (c) decreasing volume SHIFT (d) adding PCl5

Using standard molar enthalpies, predict the eq. shifts and changes in K with changes in temperature for 2 POCl3(g) 2 PCl3(g) + O2(g) DHfo (kJ/mol) –542.2 –288.1 0 Strategy: 1) Use DHfos to estimate DHrxn. 2) Use Le Chatelier to predict shifts. DHrxn = 2(–288.1) – 2(–542.2) = +508.2 kJ (endothermic) as T ... shift , K as T ... shift , K

-- the forward and reverse reaction rates are both increased Effect of a Catalyst -- the forward and reverse reaction rates are both increased Catalytic converters in cars help to quickly convert nitrogen oxides into nitrogen and oxygen and carbon monoxide into carbon dioxide. -- NO SHIFT -- Catalysts increase the rate at which eq. is achieved, but the final composition of the system is unchanged, as is K.