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Le Chatelier’s Principle

Le Chatelier’s Principle. Or “How to Get Away from Stress”. Le Chatelier’s Principle. When a system at equilibrium is subjected to a stress, the equilibrium will tend to shift in the direction to relieve the stress. What kind of “stress” could a chemical reaction be subjected to??

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Le Chatelier’s Principle

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  1. Le Chatelier’s Principle Or “How to Get Away from Stress”

  2. Le Chatelier’s Principle • When a system at equilibrium is subjected to a stress, the equilibrium will tend to shift in the direction to relieve the stress. • What kind of “stress” could a chemical reaction be subjected to?? • Change in concentration • Change in temperature • Change in pressure • Change in pressure affects which state of matter? • Adding a catalyst? NO! Not a stress.

  3. Change in concentration – system shifts to get back to the same ratio of reactants and products. Add more N2? Equilibrium shifts to the right (). End with more NH3 Add NH3? Eq. shifts to the left (). End with more N2 and H2. Take out NH3? Eq. shifts . End with more NH3. What are the effects of each stress?

  4. Change in temperature – system shifts eq. so that heat is equalized. Raise the temperature Eq. shifts  to absorb the heat Lower the temperature Eq. shifts  to produce more heat NOTE: Lowering the temp. may shift the eq. in the direction of the products, but if the temp. is lowered TOO much, what happens to the rxn. rate? SLOOOOOOOW!!!!

  5. Change in Pressure – System shifts to equalize the number of moles of gas. Raise the pressure Eq. shifts  Shift is towards smallest total number of moles of gas, so the eq. shifts away from 4 moles (3 +1) towards 2 moles. Lower the pressure Eq. shifts  Shift equalizes pressure. NOTE: This is a famous reaction – the Haber process. It is extremely important in the agricultural industry to make fertilizers.

  6. Change amount of Catalyst? • NO! This does not affect the position of the equilibrium. It speeds the reaction in BOTH directions, so that it reaches equilibrium faster.

  7. Given: S8(g) + 12O2(g)  8 SO3(g) + 808 kcals What will happen when …… Oxygen gas is added? The reaction vessel is cooled? The size of the container is increased? Sulfur trioxide is removed? A catalyst is added to make it faster? Shifts to prodcuts Shifts to Products – to replace heat V increases, Pressure decreases, shifts to more particles – to reactants! Shift to products to replace it! No change!

  8. Given 2NaHCO3(s)  Na2CO3 (s) + H2O (g) + CO2(g) What will happen when . . . . . . . Carbon dioxide was removed? Sodium carbonate was added? Sodium bicarbonate was removed? Shift to products – to replace it No Change – solids do not affect equilibrium No Change

  9. Given Ca5(PO4)3OH(s)  5Ca2+(aq) + 3PO43-(aq) + OH- (aq) What will happen when. . . . . . Calcium ions are added? NaOH is added? 1 M HCl is added? Na3PO4(aq) is added? Shift to the reactants Adding OH- , shifts to reactants H+ + OH- H2O (removes OH-, shifts to products) Adds PO43- ions, shifts to reactants

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