Equilibria

Equilibria

1. The idea

   • Reversible reaction
   • Closed container, so as soon as products are formed they react together and form reactants again
   • Forms a mix of products and reactants
     • Called an equilibrium mixture
   • Setting up an equilibrium
     • Looking at it like evaporation of water
     • In a closed container, some water will evaporate
     • The volume of liquid will decrease, the number of vapour molecules will increase
     • As more molecules enter the vapour stage, some gas molecules will turn back to liquid
     • After a time, the rate of condensation and evaporation will become equal
     • This is called dynamic equilibrium
   • Conditions
     • Can only be reached in a closed system
     • Can be approached from either direction and the final equilibrium position will be the same as long as conditions remain the same
     • Equilibrium is reached when the rates of two opposing processes are the same
     • We know equilibrium has been reached when the macroscopic properties of the system do not change with time
       • Density
       • Concentration
       • Colour
       • Pressure
       • Properties that don't depend on the total quantity of matter
     • An equilibrium mixture can have ANY proportion of reactants and products

2. Changing conditions

   • Equilibrium mixture
     • We can change the proportion of reactants to products in an equilibrium mixture
     • We can obtain a greater yield of products this way
     • Said to be changing the position of the equilibrium
     • If the proportion of products is increased, the equilibrium has moved to the right
     • If the proportion of reactants is increased, the equilibrium has moved to the left
     • We can move the position left or right by varying conditions like temperature, concentration and pressure
   • Le Chatelier
     • If a system at equilibrium is disturbed, the equilibrium will move in the direction that tends to reduce the disturbance
     • This principle doesn't tell us how far the equilibrium moves so we can't predict the quantities involved
   • Concentration
     • Increasing the concentration of reacctants favours the forward reaction - equilibrium moves to the right
     • Removing product as it is formed also moves the equilibrium to the right to produce more product
   • Pressure
     • Pressure changes only affect reactions involving gases
     • Changing the overall pressure will only affect the position of equilibrium if there are a different number of molecules on either side of the equation
     • If we increase the pressure, the equilibrium will shift to the side where there are fewer moles as there is less pressure
   • Temperature
     • A negative sign for enthalpy means heat is given out in the forward direction
     • If temperature is increased, the equilibrium will move to decrease the temperature - it will favour the endothermic reaction
   • Catalysts
     • Have no effect on the position of equilibrium so they don't alter the composition of the equilibrium mixture
     • They allow equilibrium to be reached more quickly
     • They provide an alternative route for the reacition, so they affect both reactions equally

3. Examples

   • Ammonia
     • Nitrogen and hydrogen react together by a reversible reaction
     • N2 (g) + 3H2 (g) <=> 2NH3 (g) ΔH° = -92kJmol-1
     • Works best at low temperature and high pressure
     • Produced mostly by the Haber process
     • Methane produces the hydrogen in the reaction CH4 (g) + H2O (g) -> CO2 (g) + 3H2
     • 80% of ammonia is used in fertiliser, and some is used in making nylon
   • Ethanol, C2H5OH
     • Ethanol can be made by fermentation from sugars using the enzymes in yeast as a catalyst
     • C6H12O6 (aq) -> 2C2H5OH + 2CO2
     • It can be produced by hydration of ethene from crude oil, obtained by fractional distillation and cracking
     • The hydration of ethene is a reversible reaction
     • Spreeded up by a catalyst of phosphoric acid absorbed on silica
     • H2C=CH2 (g) + H2O (g) <=> CH3Dh2OH (g) ΔH° = -46jkJmol-1
     • All reactants and products are gaseous at the temperature used
     • Maximum yield
       • High pressure
         • Can cause ethene to polymerise
         • Increases costs
       • Low temperature
         • Reduce reaction rate and how fast equilibrium is reached
       • Excess steam
         • Dilutes catalyst
       • Due to these problems, the conditions in practice are
         • 570K
         • 6500kPa
   • Methanol, CH3OH
     • Used principally as a chemical feedstock - starting material for making other chemicals
     • CO (g) + 2H2 (g) -> CH3OH (g) ΔH° = -91
     • Compromised conditions are used
     • 500K and 10000kPa