chemical equilibria

• reversible reaction

  products can react to reform the original reactants
• dynamic equilibrium

  -reactants and products are dynamic - constantly moving

  -rate of the forward reaction is the same as the rate of the backward reaction in a closed system and the concs of both is constant
• Le Chatelier's Principle

  if a change is made to a system in dynamic equilibrium, the position of equilibrium will move to counteract this change
• effects of conc on equilibrium

  INCREASE - equm shifts to the right (counteract effect of increase in conc of a reactant)

  
  

  DECREASE - equm shifts left (counteract effect of increase in conc of product)
• effect of pressure on equilibrium

  INCREASE - equm shift to side with fewer moles to decrease pressure again

  
  

  DECREASE - equm shifts to side with more moles to increase pressure again
• effects of temperature on equilibrium

  INCREASE - equm moves in endothermic direction to reverse change

  
  

  DECREASE - equm moves in exothermic direction to reverse change
• ffect of catalyst on equilibrium

  no effect

  -only cause reaction to reach equm faster
• Kc

  [products]/[reactants]

  -solids are ignored

  -Kc of reaction is specific and only changes if temp changes
• Haber process

  -an industrial process for producing ammonia from nitrogen and hydrogen by combining them under high pressure in the present of an iron catalyst

  -N2(g) + 3H2(g) reversible 2NH3(g) deltaHr =-97Jmol-1
• haber process - maximising NH3 yield - pressure

  -increase in press = equm shifts to fewer moles

  -in this case equm shifts right so yield of ammonia increases

  -increase in press will cause particles to be closer together - more successful collisions - increased ror

  -very high press are expensive to produce so compromise pressure of 200atm is used
• haber process - maximising NH3 yield - temp

  -to get max yield, equm should be shifted as far right

  -haber is exo, equm will shift right if temp is lowered

  -at low temps, the gases wont have enough kinetic energy to have successful collisions

  -a compromise temp of 400-450 degrees is used
• haber process - removing ammonia

  -removing NH3 by condensing it to a liquid cause equm to shift right to replace the ammonia causing more ammonia to be formed

  -recovered ammonia is stored at very low temps with no catalyst so the decomp of ammonia will be too slow to be a problem
• haber process - catalyst

  iron catalyst
• Contact process

  Industrial manufacture of sulfuric acid using the raw materials of sulfur and air

  2SO2(g) + O2(g) reversible 2SO3(g) deltaHr = -197kJmol-1

  SO3 +H2SO4 ---> H2S2O7

  H2S2O7 + H2O ---> 2H2SO4
• contact process - maximising H2SO4 yield - pressure

  -increase in press means equm will shift to sode with fewer moles

  -in this case equm shifts towards right so yield of sulfur trioxide increases

  -reaction is carried out at 1atm

  -Kc for reaction is already very high meaning pos of equm is already far right

  -higher press than 1atm is unnecessary and expensive
• contact process - maximising H2SO4 yield - temperature

  -to get max yield, equm should be shifted as far right

  -haber is exo, equm will shift right if temp is lowered

  -at low temps, the gases wont have enough kinetic energy to have successful collisions

  -a compromise temp of 450 degrees used
• contact process - removing sulfuric acod

  -SO3 is removed by absorbing it in 98% sulfuric acid

  -adding S03 straight to water produces an uncontrollable and hazardous exothermic reaction

  -SO3 reacts with sulfuric acid to make oleum, H2S2O7 which is then carefully diluted to make more H2SO4
• contact process - catalyst

  Vanadium(V) oxide
• If Kc is very large

  Kc>>>1 then equilibrium lies to the right therefore reaction mixture is mainly products
• If Kc is very small

  Kc<<<1 then equm lies to the left so reaction mixture is mainly reactants
• if Kc is close to 1

  if Kc 0.1<Kc<10 then reaction mixture contains similar conc of products and reactants