Le Chatelier’s Principle Equilibrium Chemical Equilibrium Kc vs. Kp Reaction Quotient Le Chatelier’s Principle Ksp

Reversible Reaction (review) A reversible reaction is one whose reactants and products are interchangeable: A and B can react to form AB, and AB can react to form A and B. “What’s the point?”, you may ask. Well, an equilibrium reaction is a reversible reaction whose reactants and products are forming at the same rate, meaning there is no net change in the concentration of reactants or products.

Chemical Equilibrium Keep in mind that an equilibrium reaction continues to constantly react, merely at the same rate – therefore, the concentration of reactants and products does not change, unless acted upon by an external factor Even when equilibrium is reached, the forward and reverse reactions continue, but at the same rate. Lastly, just because equilibrium means the forward and reverse reaction happens at the same rate doesn’t mean the concentration of reactants has to be the same as the concentration of products.

Equilibrium Expression For every equilibrium reaction, there is an equilibrium constant, Kc. The equilibrium constant is only dependent upon temperature. For the general reaction: aA + bB  cC + dD The equilibrium expression is Kc = [C]c[D]d [A]a[B]b CaCl2  Ca2+ + 2Cl- Kc = [Ca2+] [Cl-]2 [CaCl2]

Heterogeneous Equilibria vs. Homogenous Equilibria Homogeneous equilibrium involves only one phase All reactants and products are included in the equilibrium expression Heterogeneous equilibrium involves more than one phase CaCO3 (s)  CaO (s) + CO2 (g) Pure solids and liquids do not appear in the equilibrium expression Kc = [CO2] The size of the equilibrium constant indicates which side of a reaction is favored at equilibrium. Kc > 1 : lies to the right Kc< 1 : lies to the left

KC Examples The following equation represents the reversible decomposition of PCl5. PCl5(g)  PCl3(g) + Cl2 (g) At 250 C, the equilibrium concentrations of the substances are as follows: [PCl5]=1.271M; [PCl3]=0.229 M; [Cl2]=0.229M. What is the value of Kc for this rxn? The following equilibrium reaction is used in the manufacture of methanol. The equilibrium constant at 400 K for the reaction is 1.609. CO(g) + 2H2 (g)  CH3OH(g) At equilibrium, the mixture in the reaction vessel has a concentration of 0.818M of CH3OH and 1.402 M of CO. Calculate the [H2] in the vessel.

Reaction Quotient When we substitute reactant and product partial pressures or concentrations into an equilibrium-constant expression, the result is known as the reaction quotient, Q. Q = Kc only when the system is at equilibrium. Q > Kc rxn shifts to the left to reach equilibrium Q < Kc rxn shifts to the right to reach equilibrium

Le Chatelier’s Principle Le Chatelier simply says that when the environment of the equilibrium equation changes, the reaction will shift in order to reduce the stress introduced into the system. There are three situations you need to know about Le Chatelier’s Principle: - change in concentration - temperature - pressure

Change in concentration When there is a change in the concentration of a reagent, Le Chatelier’s Principle says that the reverse change occurs on the other side of the equation. Example: 2CO (g) + O2 (g)  2CO2 (g) If extra CO were added, Le Chatelier says that there would be more CO2 produced in the reaction. This means the reaction shifts to the right – the right side has gained while the left side has lost reagent. What happens to the concentration of O2 ?

Concentration, continued If CO were decreased, then the opposite will happen to the other side; there would be less CO2 in the reaction. This means that the reaction now shifts to the left. Practice Problem: 2CO (g) + O2 (g)  2CO2 (g) What direction does the equilibrium equation shift when more O2 is added?

Change in temperature This one is a bit more complicated; there are two things to take into account. 1.) Exothermic reaction vs. Endothermic (DH is negative or DH is positive, respectively) 2.) Temperature increase or decrease If you remember one situation, you can figure out the rest just by switching them around. Just remember this: The thumb and your pointer finger of your left hand represents an exothermic reaction; the thumb (since it points down) means temperature goes down, and the pointer finger (since it points to the right) means the reaction goes to the right.

Temperature, continued Example time: 2CO (g) + O2 (g)  2CO2 (g) , DH is (-) If the temperature decreases, the reaction goes to the right. Verify this with your fingers… Now, what if DH is (+)? What if the temperature increases and DH is (-)? What if the temperature increases and DH is (+)?

Change in pressure Le Chatelier says that when pressure increases, the reaction goes toward the side with the lesser amount of moles of gas.

Pressure, continued 2CO (g) + O2 (g)  2CO2 (g) There are three moles of gas on the left and only two on the right, so if the pressure was increased, the reaction would go towards the right. Conversely, if the pressure was decreased, the reaction would go towards the left.

Solubility Product Ksp Ksp, or solubility product, is the product of the molar concentration of the constituent ions, each raised to the power of its stoichiometric coefficient in the equilibrium expression Molar solubility: the number of moles of solute in one liter of a saturated solution (mol/L) Solubility: the number of grams of solute in one liter of a saturated solution (g/L) Both terms can be used to figure out Ksp

Solubility, and establishing a saturated solution is an equilibrium process. At equilibrium the rate of dissolution of molecules is equal to the rate of crystallization of the solid Keep in mind: crystallization may result in a orderly lattice arrangement of molecules- known as a precipitate

Ksp examples Calculate the water solubility of silver acetate, AgC2H3O2, in mol/L, given that the Ksp value is 1.9x10-3.

Ksp examples Calculate the water solubility of calcium fluoride, CaF2, in mol/L, given that the Ksp value is 3.9x10-11.