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Equilibrium

Equilibrium. IB Textbook Chapter 15. Reversible reactions. Some reactions will take place in both directions (reactants → products and reactants ← products) The reactions occur simultaneously in both directions. Chemical equilibrium.

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Equilibrium

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  1. Equilibrium IB Textbook Chapter 15

  2. Reversible reactions • Some reactions will take place in both directions (reactants → products and reactants ← products) • The reactions occur simultaneously in both directions

  3. Chemical equilibrium • Equilibrium: the forward reaction and reverse reaction take place at the same rate • Aka: The Boring Point • Written like this: A + B  C • The word “rate” is important because • There is not the same AMOUNT of product & reactant • You can speed up and slow down the reaction, but it’s still going to go at the same RATE forward and backwards

  4. Keq • Equilibrium is a predictable thing • Scientists have studied equilibrium systems and determined a constant for EACH system • The call it… .Keq • K = [products]power of the # of moles [reactants]power of the # of moles • It’s often shown like this: aA + bB  cC + dD Keq = [C]c[D]d [A]a[B]b

  5. Keq Examples (more? See p. 548 #13) • Try to write K for this equilibrium: 2NO2 N2O4 Keq = [N2O4] [NO2]2 Note: K actually comes in several forms: Kc Ksp Ka Kb Kw or Kp but, you don’t need all that mess yet…

  6. Keq Examples (more? See p. 548 #13) • And this one… But, K can be a little tricky because you never include liquids (l) or solids (s): CaCl2(s) Ca2+ (aq) + 2Cl- (aq) Keq = [Ca2+][Cl-]2 • Finally… Ca(s) + 2HOH(l) Ca(OH)2(aq) + H2(g) Keq = [Ca(OH)2][H2]

  7. Interpreting K Page 548 #19 • So… What does it mean if • K < 1 • K > 1 • K = 1 Bottom of the ratio is bigger; There are more reactants than products Top of the ratio is bigger; There are more products than reactants The top and bottom are the same; The amounts of products and reactants are equal

  8. Le Châtelier’s Principle • If an equilibrium system is stressed, it will adjust/shift to relieve the stress • There are 3 ways it can be stressed • Concentration • If the con. of reactants or products is increased, the equilibrium will shift away from the increase • If the con. Is decreased, it shifts toward the decrease • Pressure • If the pressure on a system is increased, equilibrium will shift toward the side with least moles of gas • Temperature • Heat can be added or removed, and should be treated as a reactant (endothermic) or product (exothermic)

  9. Try it… NH4OH(g) + heat  NH3(g) + HOH(g) • What will happen if… • Heat is added? • The system is cooled? • More NH4OH is added? • HOH is added? • Pressure is decreased? • Volume is decreased?

  10. Le Châtelier’s Principle and K? • K can only change when • the temperature changes • the pressure changes • Changes in concentration still balance out to the same K (power point #2!)

  11. Try it…with K NH4OH(g) + heat  NH3(g) + HOH(g) • What will happen if… • Heat is added? • The system is cooled? • More NH4OH is added? • HOH is added? • Pressure is decreased? • Volume is decreased? K K K same K same K same K same

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