Le Châtelier Principle

1. Le Châtelier Principle If a reaction is at equilibrium and we alter the conditions so as to create a new equilibrium state, then the composition of the system will tend to change until that new equilibrium state is attained. (We say "tend to change" because if the reaction is kinetically inhibited, the change may be too slow to observe or it may never take place.)

2. History • In 1884, the French chemical engineer and teacher Henri Le Châtelier (1850-1936) showed that in every such case, the new equilibrium state is one that partially reduces the effect of the change that brought it about.

3. Le Châtelier principle: • If a system at equilibrium is subjected to a change of pressure, temperature, or the number of moles of a component, there will be a tendency for a net reaction in the direction that reduces the effect of this change.

4. Equilibrium Shift • Movement of a system at equilibrium, resulting in a change in the concentrations of reactants and products.

5. How do changes in Temperature affect equilibria? • Chemical reactions are accompanied by the liberation or uptake of heat. If we regard heat as a "reactant" or "product" in an endothermic or exothermic reaction respectively, we can use the Le Châtelier principle to predict the direction in which an increase or decrease in temperature will shift the equilibrium state. Thus for the oxidation of nitrogen, an endothermic process

6. How do changes in Pressure affect equilibria? • Recall that if the pressure of a gas is reduced, its volume will increase; pressure and volume are inversely proportional. • In equilibrium at some arbitrary temperature and pressure, and that we double the pressure, perhaps by compressing the mixture to a smaller volume. From the Le Châtelier principle we know that the equilibrium state will change to one that tends to counteract the increase in pressure.

7. In your notes make a table describing how each variable affects the equilibrium pg. 450-456 • Key point to know…. • Concentration • Temperature • Gas Volume Change • Write what variables that do not affect chemical equilibria.

9. Summary • If a stress such as a change in concentration, pressure, temperature etc. is applied to a system at equilibrium , the equilibrium will shift in such a way as to lessen/ counteract the effect of the stress.

10. Summary • The equilibrium shifts to use up/lessen the substance that was added or to replenish the substance that was removed. • If the [ ] of a reactant is increased or if a product is decreased the system shifts in the forward direction to the right • If the [ ] of a reactant is decreasen of in a product is increased the system shifts in the reverse direction to the left

11. Summary • Changing Volume or Pressure • The pressure of all the substances in a gas equilibrium can be changed by changing in the volume of the container. • The rxn shifts to compensate for the pressure change • If the volume is decreased the pressure increases

12. Summary in Depth • A shift occurs to reduce the total # of gas phase molecules in order to reduce the pressure, if shifts in the direction that produces the least gas phase substances • Changing the pressure by changing the volume shifts an equilibrium ONLY if the sum of the co-efficients for gas reactants is different from the sum of the coefficients for gas phase products • Adding an inert gas has no effect as long as the volume remains the same. Changing the pressure of an equilibrium must change the [ ] of at least on substance in the equilibrium expression

13. Summary with Temperature • Effect of temperature on an equilibrium if you know whether the rxn is endothermic or exothermic • Endo ( Δ H° > 0) Exo( Δ H° <0) • Endo rxnan increase in temp always means an increase in Kc the rxn becomes more product favored • Exorxn an increase in temp always means a decrease in Kc the rxn becomes more reactant favored