F322: Equilibria

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Introduction

  • A reversible reaction is one that can go both ways; occuring in different directions.
    E.g. Haber Process: N2 + 3H2 <-> 2NH3
  • This means that the reaction is in equilbrium.
  • Dynamic equilibrium requires:
    -System must be closed.
    -Rate of forward reaction must = rate of backward reaction. 
    -Macroscopic (what you can see) properties must remain the same.
  • Le Chatilier's principle states: "when any of the conditions affecting the position of equilbrium change, then the position changes to minimise that change". 
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Changing Concentration

  • CH3COOH + CH3CH2 <-> CH3COOCH2CH2CH3 + H2O 
    -Increasing concentration of ethanoic acid (CH3COOH), will make the equilibrium shift to the right to mimise the change; so more H2O (etc).
    -BUT reducing concentration of H2O will make the equilibrium shift to the right again to minimise the change; so more products.
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Changing Pressure

  • Only applies to equilibria with gases (check state symbols).
    2SO2(g)   +   O2(g)   <->          2SO3(g) 
    -Direction depends on number of moles of gases (3 on the left here, 2 on the right).
    -So increasing pressure here will make it shift to the right, as there are fewer moles.
    -And decreasing will make it shift to the left, as there are more moles.
     
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Changing Temperature

  • INCREASING TEMP ALWAYS FAVOURS THE

ENDOTHERMIC             REACTION     DH = positive

  • DECREASING TEMP ALWAYS FAVOURS THE

EXOTHERMIC                REACTION     DH = negative


E.g.             A   +   B    <->      C          DH  =   -190 kJmol –1

DH  = + 190 kJmol-1 <---------

-So increasing temperature will cause the equilibrium to shift to the left, creating more A + B.

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Catalysts

  • Adding a catalyst will increase rate of reaction, BUT
  • Will not affect the position of equilibrium.
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Haber Process

  • N2(g)   +   3H2(g)      <->  2NH3(g)   DH = -92kJmol-
  • So:
    -Increasing the pressure will make it shift to the right to minimise change.
    -And decreasing temperature will make it shift to the right also; exothermic.
  • Problems:
    -Low temperature = slow reaction.
    -High pressure = dangerous/expensive. 
  • Thus:
    -Comromise on both pressure and temperature.
    -Use Fe (catalyst). 
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