Le Chatelier’s Principle Changing concentration Changing volume/pressure Changing temperature

Can a chemist change the equilibrium position to make a reaction run in one direction? YES! In 1888, French chemist Henri-Louis Le Chatelier discovered that there are ways to control equilibria to make reactions more productive. He proposed what is now called Le Chatelier’s Principle: if a stress is applied to a system at equilibrium, the system shifts in the direction that relieves the stress. A stress is any kind of change in a system at equilibrium that upsets the equilibrium.

Stress 1: Changes in Concentration Adding reactants immediately increases the number of effective collisions between reactant molecules and the rate of the forward reaction increases. This shifts the equilibrium to the RIGHT (toward making more product) Over time the reactants will decrease and the products will increase until a new equilibrium position is established. Removing products will act in the same way as adding reactants There isn’t enough product so the reactant molecules try to create more, which is a shift to the RIGHT Adding products will shift the reaction to the LEFT to create more reactant until a new equilibrium is reached.

Stress 2: Changes in Pressure or Volume Volume and pressure only apply to gaseous equilibria and they follow Boyle’s law: if volume goes down, pressure goes up, and if volume goes up, pressure goes down Pressure is a result of the number of collisions of gas molecules, so it’s a function of the number of moles (coefficients) of the molecules. STRESS: increased pressure (decreased volume) HOW TO FIX: decrease pressure SHIFT: to the side of the reaction that has the least number of TOTAL moles STRESS: decreased pressure (increased volume) HOW TO FIX: increase pressure SHIFT: to the side of the reaction that has the most number of TOTAL moles

Stress 3: Temperature Change Each reversible reaction will represent one endothermic process and one exothermic process It is easiest to think of “heat” as a physical reactant and place it on a side of the equation that makes the most sense and then treat it according to the concentration rules Remember that we read the “forward” reaction as left right. If ΔH is negative, it’s exothermic and “heat” is a product (according to the “forward” reaction) If ΔH is positive, it’s endothermic and “heat” is a reactant (according to the “forward” reaction)

C2H2 (g) + H2O (g)  CH3CO (g) if ΔH = -151 kJ HOMEWORK QUESTIONS Explain how a system at equilibrium responds to a stress and list factors that can be stresses on an equilibrium system. Explain how decreasing the volume of the reaction vessel affects each equilibrium: 2 SO2 (g) + O2 (g)  2 SO3 (g) H2 (g) + Cl2 (g)  2 HCl (g) 3) Decide whether higher or lower temperatures will produce more CH3CO in the following equilibrium: C2H2 (g) + H2O (g)  CH3CO (g) if ΔH = -151 kJ