*Le Châtelier’s Principle and Equilibrium

*Reaction Basics Reactants create products Reactant A + Reactant B Product AB As reactants are consumed, their concentration decreases. As products are made, their concentrations increase. Reactions have rates. Reactions may be reversible and their forward and reverse reactions have rates.

Reaction slows down as time passes as reactants are used up Rate of reaction graph Reaction slows down as time passes as reactants are used up Amount of product formed Rxn stops making products after a certain time – graph levels out after a reactant has been used up. Reaction begins rapidly Time

*Reaction Basics *Processes that proceed in both the forward and reverse direction are said to be reversible reactions. reactants Û products

Dynamic Equilibrium *Dynamic equilibrium is where the rates of the forward and reverse reactions are equal.

Identify the reactants and the products

APPEARS to remain

*Equilibrium A state of balance at which the rates of the forward and reverse reactions are equal. At this point, an equilibrium expression can be written for the reaction: Constant “K”= [Products] [Reactants] The value of K will only change with a temperature change

* HONORS ONLY

* HONORS ONLY

HONORS ONLY

HONORS ONLY

HONORS ONLY

HONORS ONLY

*Equilibrium Position At equilibrium, the relative concentrations of reactants and products don’t change. Unless a stress is applied

*Le Châtelier’s Principle* If a stress is applied to a system in dynamic equilibrium, the system changes to relieve the stress.

*Disturbing a Chemical Equilibrium The equilibrium may be disturbed in 3 ways: (1) change in concentration of a reactant (or product) (2) change in temperature (3) change in volume or pressure (for gases only!!)

*Equilibrium  Equal [ ] *The rates of the forward and reverse reactions are equal at equilibrium. *But that does not mean the concentrations of reactants and products are equal. If the temperature remains the same, K (the equilibrium constant) does not change. K ONLY CHANGES IF YOU CHANGE THE TEMPERATURE!

*Disturbing and Restoring Equilibrium If conditions are changed (a stress,) concentrations of all the chemicals will change until equilibrium is restored once again. NOTE: The concentrations do not restore back to their original levels…they only adjust until the forward and reverse rates are equal again. The mathematical relationship K will re-establish itself to be the same mathematical relationship as before…the same K…K remains constant

Enough kinetic energy / force What makes an effective collision in the collision theory? Enough kinetic energy / force In other words: must have enough energy to overcome the activation energy barrier Correct orientation

*Activation energy – minimum amount of energy required for a reaction to occur

Effect of Concentration of Reactants Stress: Increasing [reactants] Relief: Decreasing [reactants] Z2Y (g) + XY2 (g) Z2XY3 (g) Example: increasing [Z2Y (g)] would cause the reaction to shift to the ____. Shift: to the right (products’ side) Adding [reactants] shifts the reaction toward making products.

Effect of Concentration of Products Stress: Increasing [products] Relief: Decreasing [products] Z2Y (g) + XY2 (g) Z2XY3 (g) Example: increasing [Z2XY3 (g)] would cause the reaction to shift to the ____. Shift: to the left (reactants’ side) Adding [products] shifts the reaction toward making reactants.

*Le Châtelier’s Principle Changes in Concentration continued Remove Add Add Remove aA + bB cC + dD Change Shifts the Equilibrium Increase concentration of product(s) left Decrease concentration of product(s) right Increase concentration of reactant(s) right Decrease concentration of reactant(s) left

*Change in concentration summary: Reaction shifts away from the added [ ] Reaction shifts toward the removed [ ]

Le Chatelier’s Principle Effect of a Change in Temperature Exothermic reaction – produces heat (heat is a product) Adding energy shifts the equilibrium to the left (away from the heat term). Endothermic reaction – absorbs energy (heat is a reactant) Adding energy shifts the equilibrium to the right (away from the heat term).

Effect of Temperature Increasing the temperature causes the equilibrium to shift in the direction away from heat. Stress: Increase in Temp Relief: Remove heat Shift: Towards the left SO2 (g) + O2 (g) 2SO3 (g) + heat

Stress: Heat is added Relief: get rid of heat by reacting with reactants to create products

*Change in temperature summary: Reaction shifts away from the added [heat] Reaction shifts toward the removed [heat]

The Effect of Temperature Changes on the Equilibrium constant for Endothermic Reactions Adding heat to an endothermic reaction will shift the reaction to the right. This decreases the [ ] of reactants and increases the [ ] of products. Adding heat to an endothermic reaction will increase the value of K because the reaction shifts to making more products (which makes K a larger value.)

The Effect of Temperature Changes on the Equilibrium constant for Exothermic Reactions Adding heat to an exothermic rxn will shift the reaction to the left. This decreases the [ ] of products and increases the [ ] of reactants.

*The Effect of Catalysts Catalysts affect the rate of the forward and reverse reactions by the same factor. Therefore, catalysts do not affect the position of equilibrium. Catalysts do one main thing: lower the activation energy of the reaction

Conditions That Affect Reaction Rates *Catalyst – helps a reaction happen easier without being consumed itself, *Enzyme – catalyst in a biological system

Conditions That Affect Reaction Rates Concentration – increases rate because more molecules lead to more collisions Temperature – increases rate would increases kinetic energy & collisions Volume of a reaction vessel (the container) – decreasing volume would increases # of collisions

For a container of gases, what does decreasing the volume really mean? N2 + 3H2 2 NH3

Decreasing the volume really means increasing pressure Why??

MOLES!!! What is pressure? A force per square area. In chemistry, the particles exert the force on the square area of the container. Increase # of particles in the container, increase the pressure. How do you measure particles in chemistry??? MOLES!!!

Effect of Pressure 2NO2 (g) N2O4 (g) Stress: Increase Pressure Relief: Decrease the Pressure (duh!) How does a reaction decrease pressure? By having less particles (moles) in the container. If pressure is increased, the equilibrium will shift to reduce the number of particles by shifting to the side with less moles. (Shifts right here.) This only affects the gases in the reaction!!! If there were equal number of moles of reactants and products, no shift occurs.

*Effect of Volume/Pressure Change on Equilibrium N2(g) + 3H2(g) 2 NH3(g) The system is initially at equilibrium. Stress is decreased volume. What happens?

Effect of Volume/Pressure Change on Equilibrium STRESS: Decrease volume of container According to Le Châtelier’s Principle, the equilibrium should shift to remove that pressure. Must examine equation to figure out relief: The way the system reduces the pressure is to reduce the number of gas moles in the container. N2(g) + 3H2(g) 2 NH3(g) The system shifts in the direction that gives the fewest number of gas moles (this rxn would shift to the right)

Example 2 When the pressure is decreased by increasing the volume, the position of equilibrium shifts toward the side with the greater number of molecules. Which way does the rxn shift?

Example 2 When the pressure is decreased by increasing the volume, the position of equilibrium shifts toward the side with the greater number of molecules. Which way does the rxn shift?

***Le Châtelier’s Principle Changes in Volume and Pressure A (g) + B (g) C (g) Change Shifts the Equilibrium Increase pressure Side with fewest moles of gas Decrease pressure Side with most moles of gas Increase volume Side with most moles of gas Decrease volume Side with fewest moles of gas When the number of gas moles on either side is the same, there is no effect.

What happens? 2NO2 (g) N2O4 (g) Stress: pressure increases Relief? Relief: decreasing the pressure So how would the reaction reduce the pressure by making less particles? Shift? Shift: to the right (side with less moles)