Equilibrium Systems and Stress

Phase Equilibrium Liquid to Gas: When water vapor evaporates at the same rate it condenses in a closed container, dynamic equilibrium exists

Phase Equilibrium Melting/Freezing Equilibrium  Occurs when the rate of solid to liquid equals the rate of liquid to solid

Phase Equilibrium Solution Equilibrium

Chemical Equilibrium  When the rates of the forward and reverse reactions are equal in a chemical reaction  The concentration (amount) of reactants and products of the reaction remains the same  Ex: 2CO (g) + O 2(g) ↔ 2O 2(g)

Chemical Equilibrium

Le Chatlelier’s Principle Stress: something that causes a change in a system at equilibrium However, the system will adjust to this new stress and come back into equilibrium again This is LE CHATLELIER’S PRINCIPLE

Le Chatelier’s Principle This is LE CHATELIER’S PRINCIPLE  When a system at equilibrium is disturbed, the system adjusts in a way to reduce the change

Le Chatelier’s Principle Chemical equilibria responds to three kinds of stress:  Changes in the concentration of reactants or products  Changes in temperature  Changes in pressure

Le Chatelier’s Principle 1. Changes in concentration of reactants and products This is a model of the system at equilibrium.

Changes in concentration of reactants and products If more reactants are added to the system, the equilibrium changes. The system will respond by making more product. Stress is being added to the system here

Changes in concentration of reactants and products 1 CO (g) + 2H 2(g) ↔ CH 3 OH (g) System at equilibrium: System with added reactant CO: To reach equilibrium again, the system will make more products: The reaction will shift to the right (the forward reaction)

Changes in concentration of reactants and products 1 CO (g) + 2H 2(g) ↔ CH 3 OH (g) System at equilibrium: System with added product (CH 3 OH) To reach equilibrium, the system will make more reactants The reaction will shift to the left (reverse reaction)

Effects of Temperature Temperature effects equilibrium the same way as concentration changes For exothermic forward reactions:  Increasing the temperature of an equilibrium system usually leads to a shift in favor of the reactants  For endothermic forward reactions: Increasing the temperature usually leads to a shift in favor of the products

Effects of Temperature N 2(g) + O 2(g) ↔ 2NO (g) System at Equilibrium System with added heat on the product side To reach equilibrium, the system will make more reactants The reaction favors the formation of reactants (reverse rxn)

Effects of Pressure For solutions, pressure has almost no effect on the equilibrium Gases are greatly effected by changes in pressure An increase in pressure causes the system to shift its equilibrium position to REDUCE THE PRESSURE To reduce pressure, the system needs to reduce the number of gas particles

Effect of Pressure Rule: At constant temperature, increasing the pressure on a gas causes the system to shift in the direction that reduces the number of moles of gas

Effects of Pressure Example: N 2 O 4(g) ↔ 2NO 2(g) How many moles of gas are on the reactant side?_____ How many moles of gas are on the product side?_____ Which side has the greater amount of moles?________ If adding pressure to this system causes the system to respond by reducing the number of moles of gas, which way will the reaction go?

Real Life Applications The “Bends”  Nitrogen and other gases are dissolved in our blood  nitrogen (g) ↔ nitrogen (dissolved)  As the diver comes up from the high pressures of the ocean, the equilibrium shifts to the left  If they come up too fast, nitrogen bubbles form in the blood and other body fluids cause severe abdominal pain and maybe death