Equilibrium

  • Created by: dgavan
  • Created on: 22-05-19 15:13
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  • Equilibrium
    • Kc
      • Kc = [products]^p /[reactants]^r
      • Units: write out expression but with the moldm^-3 units for the conc and cancel out to get Kc units
      • Two types of equilibria: homogeneous vs heterogeneous
        • Homogeneous
          • When all species are in the same physical state; all species get included in Kc
        • Heterogeneous
          • When some species are a diff physical state to others; only aq and gaseous are included in the Kc as the others are judged to have a constant state
      • To calculate from exp.al data, draw a RICE table; Ratio, Initial mols, Change in mols, End mols
        • Dont forget that at the start (i.e. initial mols), there's gonna be 0 mols of the RHS stuff of the eqn
        • Magnitude of change is proportional to stoichiometric ratios; just reverse the sign for the other half of the eqn
    • Kp
      • An eq constant used for gases only
        • This is because it's easier to measure pressure than conc
      • Mole fraction (x) = mols of gas/ total mols of gas mixture
        • Partial pressure (p) = mole fraction x total pressure
      • Kp = partial pressure (products)^a/partial pressure (reactants)^b
        • e.g. for H2(g) +I2(g) ->(eq) 2HI(g)...
          • Kp = p(HI)^2 / p(H2) x p(I2)
    • Controlling the position of eq
      • Kc/Kp tells us the position of eq
        • When K = 1, [products] = [reactants]
        • When K > 1, [products] > [reactants]
        • When K < 1, [products] < [reactants]
      • Only temp can affect K; it's not affected by pressure, conc or any catalysts
        • Catalysts get you to eq quicker, but don't change it
      • If rxn is exo (i.e. heat is a product) then increasing temp will decrease K and so decrease eq yield of products
        • Now that K has decreased, the ratio of [P]/[R] is too product-heavy, so to make up the new K value, eq shifts to the LHS to make more reactants
      • If rxn is endo (i.e. heat is a reactant) then as temp increases, K increases and so increase eq yield of products
        • Now that K has increased, the ratio of [P]/[R] is too reactant heavy; the eq must shift to the RHS to make more products to make it up to the new K value
      • What happens when we change conc?
        • If we increase [R] (denominator), the system is knocked out of eq, BUT K can't be messed up by conc, so to keep K the same, we need to increase the numerator i.e. [P]
          • So... eq shifts to RHS to keep K the same
      • What happens when we change pressure?
        • When pressure increases, all the partial pressures in Kp are affected. If the numerator has a higher total power, then the ratio between [P]/[R] is too P heavy (and vice versa)
          • So... eq shifts to LHS here to make more reactants to keep Kp the same

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