F322: Equilibria

• 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". 

• 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.

• Only applies to equilibria with gases (check state symbols).
  2SO_2(g)   +   O_2(g)   <->          2SO_3(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. 

• 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.

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

• N_2(g)   +   3H_2(g)      <->  2NH_3(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).}