if a closed system at equilibrium is subjected to a change, processes will occur that tend to counteract that change
a.k.a.
"nature tries to undo what you do"
"SHIFTS" IN AN EQUILIBRIUM SYSTEM
You will be able to predict the results of changing the temperature, concentration and pressure conditions by using Le Chatelier's Principle. Consider the phrase used above, "nature tries to undo what you do". The term "shift" is what nature does to keep the system at equilibrium. When you decrease the temperature of the equilibrium system, the reaction will "shift" to produce more heat. (Examples below!)
Meanwhile, if you increase the concentration of a reactant, the reaction will "shift" towards the product side, so that it will be able to use the added reactant. (Examples below!)
However, if you decrease the volume of a system, you will increase the pressure, therefore increasing the concentration of both the reactants and the products. Le Chatelier's Principle then predicts that the reaction will "shift" to lower the overall pressure. (Examples below!)
Meanwhile, if you increase the concentration of a reactant, the reaction will "shift" towards the product side, so that it will be able to use the added reactant. (Examples below!)
However, if you decrease the volume of a system, you will increase the pressure, therefore increasing the concentration of both the reactants and the products. Le Chatelier's Principle then predicts that the reaction will "shift" to lower the overall pressure. (Examples below!)
HOW WILL THE FOLLOWING STRESSORS CAUSE SHIFTS IN AN EQUILIBRIUM SYSTEM?
TEMPERATURE CHANGE:
When the temperature is increased, the endothermic reaction rate will increase, and the equilibrium will shift toward the side without the heat term. (That means the side with less enthalpy! But that doesn't really have anything to do with this...)
However, when the temperature is decreased, the exothermic reaction rate will increase and the equilibrium will shift toward the side with the heat term. (maximum enthalpy! yay!)
For example, consider the equation N2O6 + heat <=> 2NO3. The forward reaction is endothermic. Meanwhile, the reverse reaction is exothermic. As you increase the temperature, nature will counteract your actions and decrease the temperature by removing heat. Therefore, the reaction will shift to the right.
However, when the temperature is decreased, the exothermic reaction rate will increase and the equilibrium will shift toward the side with the heat term. (maximum enthalpy! yay!)
For example, consider the equation N2O6 + heat <=> 2NO3. The forward reaction is endothermic. Meanwhile, the reverse reaction is exothermic. As you increase the temperature, nature will counteract your actions and decrease the temperature by removing heat. Therefore, the reaction will shift to the right.
CONCENTRATION CHANGE:
When the concentration of a reactant is increased, the equilibrium will shift towards the product side. Meanwhile, when the concentration of a product is increased, the equilibrium will shift to the reactant side.
Consider this! You want to make hot chocolate, but you don't seem to have any milk in the fridge, so you decide to use powdered milk! The powdered chocolate and the powdered milk are your reactants (you'll need hot water too, but let's pretend you don't need it to make hot chocolate), and the finished hot chocolate is your product. Unfortunately, you live in an alternate universe that turns your hot chocolate back into powdered chocolate and powdered milk. The more powders that you have, the more hot chocolate you will have in the end.
However, that means that the universe has more hot chocolate to "un-mix". Therefore, when you increase the concentration of the chocolate powder and milk powder, the universe will have more hot chocolate to "un-mix", and your hot chocolate equilibrium will shift towards the universe's side, which is the product's side.
Consider this! You want to make hot chocolate, but you don't seem to have any milk in the fridge, so you decide to use powdered milk! The powdered chocolate and the powdered milk are your reactants (you'll need hot water too, but let's pretend you don't need it to make hot chocolate), and the finished hot chocolate is your product. Unfortunately, you live in an alternate universe that turns your hot chocolate back into powdered chocolate and powdered milk. The more powders that you have, the more hot chocolate you will have in the end.
However, that means that the universe has more hot chocolate to "un-mix". Therefore, when you increase the concentration of the chocolate powder and milk powder, the universe will have more hot chocolate to "un-mix", and your hot chocolate equilibrium will shift towards the universe's side, which is the product's side.
VOLUME CHANGE:
When the total volume of a system is decreased (eg. inside a depressing syringe (not like a sad one, but one that's getting smaller)), then the equilibrium will shift towards the side of the reaction which contains fewer moles of gas. As the volume is decreased, the pressure will then increase, which also increases the concentration. For example when O2(g) > 2O(g) (impossible reaction, but whatever) is compressed in a syringe, then the point of equilibrium will shift towards the O2 side, to decrease the pressure in the system.
WHAT IS THE EFFECT OF A CATALYST ON DYNAMIC EQUILIBRIUM?
A catalyst reduces the amount of activation energy needed for both the forward and reverse reaction to take place. This does not affect equilibrium. The rates of both the forward AND reverse reactions will increase by the same factor, and therefore the overall equilibrium will not be changed at all.
Say for example Ea for NaCl > Na + Cl is +50kJ for the forward reaction, and +80kJ for the reverse reaction. Adding a catalyst decreases the Ea for both the forward and reverse reaction by -20kJ! (a miracle!). Though this may appear to change the microscopic properties of this reaction, the rates of the forward and reverse reaction stay equal and opposite (albeit faster), regardless of the catalyst.
Say for example Ea for NaCl > Na + Cl is +50kJ for the forward reaction, and +80kJ for the reverse reaction. Adding a catalyst decreases the Ea for both the forward and reverse reaction by -20kJ! (a miracle!). Though this may appear to change the microscopic properties of this reaction, the rates of the forward and reverse reaction stay equal and opposite (albeit faster), regardless of the catalyst.
HOW DO REACTION RATES AND CONCENTRATIONS CHANGE WHEN A STRESS IS APPLIED?
When a stress is applied to an equilibrium system which changes the point of equilibrium, then the reactions rates will change to an unequal amount for a period of time until the concentrations in the reaction reach the new point of equilibrium.
Example 1: Temperature increase in an exothermic reaction - assume NaCl > Na + Cl is an exothermic reaction with Keq=2 (1 mole of NaCl, Na, and Cl). As the temperature increases the equilibrium will shift towards the NaCl side of the reaction, let's say Keq now is .75 (2 Moles of NaCl, 1.5 Moles of Na and Cl). In order to reach this new point of equilibrium, which has a higher concentration of NaCl, the reverse reaction will have to be faster than the forward reaction. Once the concentrations align with the equilibrium once more, the rates of the forward and reverse reactions become equal again.
Example 2: Increase in concentration of the reactants - in the reaction NaCl > Na + Cl, an injection of NaCl is put into a container with a system of NaCl > Na + Cl already at equilibrium.The rate of the forward reaction will have to be faster than the rate of the reverse reaction for a period of time until the concentrations of the products and reactants are equal to Keq.
Example 1: Temperature increase in an exothermic reaction - assume NaCl > Na + Cl is an exothermic reaction with Keq=2 (1 mole of NaCl, Na, and Cl). As the temperature increases the equilibrium will shift towards the NaCl side of the reaction, let's say Keq now is .75 (2 Moles of NaCl, 1.5 Moles of Na and Cl). In order to reach this new point of equilibrium, which has a higher concentration of NaCl, the reverse reaction will have to be faster than the forward reaction. Once the concentrations align with the equilibrium once more, the rates of the forward and reverse reactions become equal again.
Example 2: Increase in concentration of the reactants - in the reaction NaCl > Na + Cl, an injection of NaCl is put into a container with a system of NaCl > Na + Cl already at equilibrium.The rate of the forward reaction will have to be faster than the rate of the reverse reaction for a period of time until the concentrations of the products and reactants are equal to Keq.