Chapter 17 “Reaction Rates and Equilibrium” Part 2: Equilibrium Reactants ⇌ Products Equilibrium #4.ppt Equilibrium #4.ppt

Today you will learn… What factors affect chemical equilibrium How to predict the effects of changing conditions in an equilibrium system according to Le Chatelier’s Principle

Le Chatelier’s Principle The French chemist Henri Le Chatelier (1850-1936) studied how the equilibrium position shifts as a result of changing conditions Le Chatelier’s principle: If stress is applied to a system in equilibrium, the system changes in a way that relieves the stress

Le Chatelier’s Principle What items did he consider to be stress on the equilibrium? Concentration Temperature Pressure Each of these will now be discussed in detail

Le Chatelier’s Principle Concentration – adding more reactant produces more product, and removing the product as it forms will produce more product (See-saw demo)

Le Chatelier’s Principle Temperature – increasing the temperature causes the equilibrium position to shift in the direction that absorbs heat If exothermic, heat is on the products side Cooling an exothermic reaction will take away product This favors products Example: C + O2(g) ⇌ CO2(g) H = -393.5 kJ

Le Chatelier’s Principle Temperature – increasing the temperature causes the equilibrium position to shift in the direction that absorbs heat If exothermic, heat is on the products side Increasing temperature adds product This would shift the reaction to the reactant side Example: C + O2(g) ⇌ CO2(g) H = -393.5 kJ

Le Chatelier’s Principle Pressure – changes in pressure will only effect gaseous equilibria P is on the side with more moles of gas N2(g) + 3H2(g) ⇌ 2NH3(g) For every two molecules of ammonia made, four molecules of reactant are used up – P is on the left side Increase P means… shifts to the right

Le Châtelier’s Principle Adding a Catalyst does not change K does not shift the position of an equilibrium system system will reach equilibrium sooner uncatalyzed catalyzed Catalyst lowers Ea for both forward and reverse reactions. Catalyst does not change equilibrium constant or shift equilibrium.

Le Chatelier’s Principle