Equilibrium

Equilibrium

1. reversible reactions

   • can take place in either the forward or the reverse direction
     • 2SO2 + O2 <=> 2SO3

2. Dynamic Equilibrium

   • when a system is in a state of equilibrium, there is no observable change, however, the system is still dynamic (in constant motion). As fast as the reactants are converted into products, the products are being converted back into reactants.
   • concentrations of the reactants and the products remain the same.
   • the rate of the forward reaction is the same as the rate of the reverse reaction.
   • only applies if the system remains 'isolated'
     • no materials are being added or taken away
     • no external conditions (temperature or pressure) are being altered.

3. factors affecting the position of equilibrium

   • only remains in equilibrium when isolated in a closed system (nothing allowed in or out)
   • position can be altered by:
     • concentrations of reactants or products
       • increasing the concentration of a reactant causes the position to move in the direction that decreases this increased reactant concentration.
         • the system opposes the change by decreasing the concentration of the reactant by removing it
           • the position of equilibrium shifts to the right hand side, forming more products.
       • increasing the concentration of a product causes the position to move in the direction that decreases this increased product concentration.
         • the system opposes the change by decreasing the concentration of the product by removing it
           • the position of equilibrium shifts to the left hand side, forming more reactants.
     • pressure (in reactions involving gases)
       • changing the total pressure of a system will only change the position of equilibrium if there are gases present
         • the side with the greater moles of gas is the side with the higher pressure.
           • Increasing the total pressure of a system causes the position of equilibrium to move to the side with fewer gas molecules, as this will decrease the pressure.
           • Decreasing the total pressure of a system causes the position of equilibrium to move to the side with more gas molecules, as this will increase the pressure.
     • temperature
       • the effect on the position depends on the enthalpy sign
         • state whether the foward reaction is endothermic or exothermic
           • increasing the temperature of the system causes the position of equilibrium to move in the direction that decreases the temperature
             • the system opposes the change by taking in heat and the position of equilibrium moves in the endothermic (positive enthalpy change) direction
           • decreasing the temperature of the system causes the position of equilibrium to move in the direction that increases the temperature
             • the system opposes the change by giving out heat and the position of equilibrium moves in the exothermic (negative enthalpy change) direction
   • the effect of a change can be predicted using Le Chatelier's Principle
     • Le Chatelier's Principle: when a system in dynamic equilibrium is subjected to a change, the position of equilibrium will shift in order to minimise that change.
   • a catalyst increases the rate at which equilibrium is established(reached), but does not alter the position or composition of equilibrium

4. Equilibrium and industry

   • Haber Process
     • the preparation of amonia from nitrogen and hydrogen
       • N2 + 3H2 <=> 2NH3 (enthalpy change = -92 kJ/mol)
       • foward reaction produces fewer gas molecules; production of ammonia favoured by using high pressure
         • drawback: the high energy required to compress the gas adds significantly to the running costs of the process.
       • foward reaction is exothermic; production of ammonia is favoured by using low temperature
         • drawback: low temperature means the reaction would take place at a very low rate. comparitively few N2 and H2 molecules have the required activation energy
       • the modern ammonia plant
         • must produce a sufficient yield at a reasonable cost and in as short a time as possible
           • in practice a compromise must be made between yield and rate.
             • temperature: this must be high enough to allow the reaction to proceed at a realist rate, whilst still producing an acceptable equilibrium yield. 400 - 500 degrees celcius is typicaly used.
             • pressure: a high pressure must still be used, but it must not be so high that the workforce is put in danger or the environment threatened. 200 atmospheres is typically used.
             • catalyst: an iron catalyst is added to speed up the rate of reaction, allowing the equilibrium to be reached faster and for lower temperatures to be used. This also reduces costs.
             • unreacted hydrogen and nitrogen gases are passed through the reactor again. Eventually, virtually all the N2 and H2 will have been converted into ammonia.
   • Contact Process
     • the conversion of sulfur dioxide into sulfur trioxide
   • chemists strive to make the highest possible yield of a desired product as quickly and as cheaply as possible.