AP Chemistry Unit 7 - Equilibrium

Unit 7 - Equilibrium At equilibrium, concentration of reactants and products remain constant. The concentrations do not have to be in equal amounts. The forward and reverse reactions occur at equal rates, so there is no observable change in concentration. Use 𝐾𝑒𝑞 to describe equilibrium, and use Q to compare any time to equilibrium. [𝑝𝑟𝑜𝑑𝑢𝑐𝑡 1]𝑐𝑜𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑡 1 [𝑝𝑟𝑜𝑑𝑢𝑐𝑡 ]𝑐𝑜𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑡 2 𝑄 [𝑟𝑒𝑎𝑐𝑡𝑎𝑛𝑡 1]𝑐𝑜𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑡 1 [𝑟𝑒𝑎𝑐𝑡𝑎𝑛𝑡 2]𝑐𝑜𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑡 2 [𝑝𝑟𝑜𝑑𝑢𝑐𝑡 1]𝑐𝑜𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑡 1 [𝑝𝑟𝑜𝑑𝑢𝑐𝑡 ]𝑐𝑜𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑡 2 = [𝑟𝑒𝑎𝑐𝑡𝑎𝑛𝑡 1]𝑐𝑜𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑡 1 [𝑟𝑒𝑎𝑐𝑡𝑎𝑛𝑡 2]𝑐𝑜𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑡 2 (𝑃𝑝𝑟𝑜𝑑𝑢𝑐𝑡 1 )𝑐𝑜𝑒𝑓.1 (𝑃𝑝𝑟𝑜𝑑𝑢𝑐𝑡 2 )𝑐𝑜𝑒𝑓.2 𝐾𝑝 = (𝑃𝑟𝑒𝑎𝑐𝑡𝑎𝑛𝑡 1 )𝑐𝑜𝑒𝑓.1 (𝑃𝑟𝑒𝑎𝑐𝑡𝑎𝑛𝑡 2 )𝑐𝑜𝑒𝑓.2 Solids and liquids should not be included in K or Q expressions. Raise the concentration or pressure of each species to its coefficient. K is constant for all concentrations at a single temperature. 𝐾𝑒𝑞 = Write and solve 𝐾𝑎 and 𝐾𝑏 just like 𝐾𝑒𝑞 problems. Remember to exclude liquid water in the K expression and ICE table. Small K = reactant favored(reverse reaction), large K = product favored(forward reaction) Q>K = reactant favored(reverse reaction), K>Q = production favored(forward reaction) Q=K - equilibrium Similar to Hess’s Law, you can manipulate reactions to get an ideal reaction and get a new K. ● If you add two equations, multiply each K. ● If you flip an equation, invert K(1/K) ● If you multiply an equation by a coefficient, raise K to the power of the coefficient. If you are given K and an initial concentration, create an ICE table and use the expressions at equilibrium in the table with x to create an equilibrium expression for the given K. Solve for x and plug x back into the table if needed. Le Chatelier’s Principle - a system will shift to equilibrium if it experiences an external stress such as change in concentration, temperature, pressure To justify Le Chatelier’s Principle on the AP exam, discuss the effect of the stress on Q compared to K and say the direction which equilibrium will shift. Stress Shift Increase concentration Away from substance Decrease concentration Toward substance Increase pressure Toward side with fewer moles of gas(compare coefficients of gases) Decrease pressure Toward side with more moles of gas Increase volume Toward side with more moles of gas Decrease volume Toward side with fewer moles of gas Increase temperature Endothermic reaction(ΔH>0) - remember that flipping an equation flips the sign of ΔH Decrease temperature Exothermic reaction(ΔH<0) Add a catalyst or inert gas No shift Solubility reactions have an ionic solid as reactants and its ions as products. High solubility means the forward reaction is favored(K>Q). To write solubility equilibrium expressions, break the solid into its ions. Multiply the concentrations of each ion to get 𝐾𝑠𝑝 . To solve for solubility from 𝐾𝑠𝑝 , plug x back into the solubility equilibrium expression for an ion that appears once, 2x for an ion that appears twice, etc. For MgOH₂, 𝐾𝑠𝑝 = [𝑀𝑔][𝑂𝐻]2 = 𝑥 ∗ (2𝑥)2 Common Ion Effect - if a solid is dissolved in a solution with one of the solid’s ions already present, the solubility of the solid goes down. If you know the solubility of the ion in the solution, you can plug that number and 𝐾𝑠𝑝 in the solubility equilibrium expression to solve for the other ion. A precipitate will form in a reaction is Q>𝐾𝑠𝑝 . Calculate the concentrations of each ion and plug them into a Q expression to see if Q is larger than the given K.