Equilibria

Important points for unit 4 Equilibria, Aqa chemistry.

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Kc = equilibrium constant

aA + bB + cC (reverse reaction sign) xX +yY +zZ

the above is valid for any system in eqm.

(X)eqm x (y)eqm y (Z) eqm z

(A) eqm a (B) eqm b (C) eqm c

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expression for an homogeneous system in eqm.

  • Homogenous = all in same state
  • units of Kc must be worked out by cancelling out.
  • 2A+B (reversible reaction sign) 2C
  • Kc =(C)2

(A)2(B)

  • = mol-1 dm3
  • value can only be found by experiment.
  • Method:
    • write out equation with mol values under reactants (products will be 0 mol initially)
    • question will tell you the mol value for one of the reactants at eqm.
    • The equation willl show you the ratio. (if 1:1 then the val at eqm will be the same if they have the same inital values)
    • Inital - eqm value = the volume of reactant used up.
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expression for an homogeneous system in eqm. 2

  • Look at ratio of products to reactants and use this to calculate vol of products
  • The question will tell the volume of the system. Divide the values of each volumes of products (a) and reactants (b) by the total volume of the system.
  • Kc = (a +a) / (b+b)
  • In some cases the units will cancel out and so Kc will have no units.
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Predicting effects of changes in temp and eqm cons

  • Type of reaction
    • endothermic with decrease in temp
      • Kc = decrease
      • Volume of products =decreases
      • Volume of reactants = increases
      • Eqm moves to the left.
    • endothermic with increase in temp
      • Kc = increase
      • volume of products = increases
      • volume of reactants = decreases
      • eqm moves to the right
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Predicting effects of changes in temp and eqm cons

  • exothermic with increase in temp
    • Kc = decrease
    • volume of products = decrease
    • volume of reactants = increase
    • eqm moves to the left
  • exothermic with decrease in temp
    • Kc = increase
    • volume of products = increase
    • volume of products = increase
    • volume of reactants = decrease
    • eqm moves to the right.
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General rule

  • For an exothermic reaction (enthalpy = -ve) increasing temp decrease eqm constant
  • For an endothermic reaction (enthalpy = +ve) increasing temp increases eqm constant.
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Changes in concentration or the addition of a cata

Concentration

  • addition to conc will cause eqm to remove changes to reduce disturbance
  • top and bottom values will change to produce the same values for Kc

Catalyst

  • effects forward and backward reaction equally
  • eqm position doesn't move
  • effects rate at which eqm is obtained.
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