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Henri Le Chatelier (1850–1936)

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).

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Henri Le Chatelier (1850–1936)

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  1. 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)

  2. N2(g) + 3 H2(g) 2 NH3(g) new eq. orig. eq. H2 added at this time system establishing new eq. (A) change in concentration 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.

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

  4. 2 SO2(g) + O2(g) 2 SO3(g) e.g., More specifically, if the partial pressures are affected, then there will be shifting. Otherwise...no. “Ne!” “Ne!” “Ne!” “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.

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

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

  7. as T ... K shift , as T ... K shift , 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)

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

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