Changing the equilibrium position of the reaction
The equilibrium position of a reaction may be changed by:
•Adding or removing a reactant or product
•Changing the pressure by changing the volume (equilibria involving gases)
•Dilution (for equilibria in solution)
•Changing the temperature.
Le Chatelier's Principle
•Le Chatelier’s Principle is very useful in determining how the position of equilibrium can be changed to ensure more product is formed.
•Le Chatelier’s Principle states that if a system is at equilibrium and the temperature, pressure or concentrations of the species are changed, the reaction will proceed in such a direction as to oppose this change.
Changes in concentration and pressure
Changes in concentration and changes in pressure for gaseous systems will alter the equilibrium position but not the value of the equilibrium constant, K.
Addition of a reactant
The addition of a reactant moves the position of equilibrium forward, producing more product. The value of the equilibrium constant, K, is unchanged.
Increase in Pressure
An increase in pressure for a gaseous system moves the position of equilibrium to the side of the equation that has fewer moles of substance, and hence lower pressure. The value of the equilibrium constant, K, is unchanged.
Eg: For the reaction:
2SO2(g) + O2(g) <-----> 2SO3(g)
3 Particles 2 Particles
The reaction goes in the forward direction (a net forward reaction) to form 2 particles.
Dilution of solutions
When solution is diluted the reaction will go in the direction that will form the greater amount of particles.
Eg. Fe^3+(aq) + SCN^-(aq) <-----> Fe(SCN)^2+ (aq)
2 particles 1 particle
There will be a net back reaction.
The direction of the equilibrium will change for an increase in temperature depending on whether the reaction is endothermic or exothermic.
Increasing temperature will result in:
•A net back reaction for an exothermic reaction
•A net forward reaction for an endothermic reaction.
Addition of a catalyst
The addition of a catalyst does not change the position of equilibrium or the value of the equilibrium constant ,K. Equilibrium is simply reached more quickly.
Addition of an inert gas
The addition of an inert gas at constant volume does not change the position of equilibrium or the value of the equilibrium constant, K. The total pressure is increased.
→ indicates a net forward reaction
← indicates a net back reaction
•Add reactants: →
•Add Products: ←
•Increase Pressure: direction of the smaller amount of particles
•Dilution: direction of larger amount of particles
•Exothermic Reaction ←
•Endothermic Reaction →
•Catalyst: Increased rate but no change in equilibrium.
Yield and chemical industry
•There are two things we would like to do in industry to make processes more viable: increase the RATE and YIELD of the reaction.
•We can do this by altering five main variables:
Image courtesy of http://en.wikipedia.org/wiki/Image:Lechatelier.jpg, Public Domain, https://commons.wikimedia.org/w/index.php?curid=428795