General Chem 1120 Study Guide Test 2
General Chem 1120 Study Guide Test 2 CHEM 1120
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This 6 page Study Guide was uploaded by Hayden Massey on Sunday March 20, 2016. The Study Guide belongs to CHEM 1120 at Auburn University taught by Ria Yngard in Spring 2016. Since its upload, it has received 38 views. For similar materials see General Chemistry II in Chemistry at Auburn University.
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Date Created: 03/20/16
Chemical Equilibrium: when the concentrations of reactants and products are persistent Equilibrium – highly dynamic; not static – no net change in products and reactants When reactants collide to make products, concentrations of reactants decrease, and the reaction rate decreases For a reverse reaction, the concentrations of reactants build up and the rate increases Injecting element into reactants = Speed Up BC of more collisions between molecules Reasons why concentrations remain unchanged when mixed: 1. Already at chemical equilibrium 2. Forward/Reverse reactions are so slow that you cant see the change When molecules have strong chemical bonds, mixtures exist where no change is seen over time BC the rates are so slow Law of Mass Action: jA + kB lC + mD l m [C] [D] K = [A] [B] Equilibrium Expression Where K is called the equilibrium constant Some Characteristics of the Equilibrium Expression: 1. Reverse reaction is reciprocal of original 2. Balanced Equation x N ; K new = ( K orininal ¿¿ 3. Apparent units of K are found by powers of various concentration The equilibrium constant always remains the same for a given equation, however the equilibrium concentrations may change depending on initial concentrations Equilibrium Position: a variation of equilibrium concentrations C = molar concentration of the gas = n/V = moles / volume jA + kB mC P m K p c PAP Bk Kp is the equilibrium constant in terms of partial pressures K pK(RT ) Δ n where Δ n= L+M −(J+K) the “true” equilibrium constant involves the ratio of equilibrium pressure for a given substance to a reference pressure for that substance (ratio is activity of substance) Pi =a Activity = P i reference jA + kB lC + mD aB ¿ ¿k a j¿ ( A ( )a ) m K= c D ¿ PcP Dm KP= j k PAP B K ≠ K P except for when the sum of the powers in the numerator and denominator are the same Homogeneous Equilibria: all products and reactants are gases Heterogeneous Equilibria: products and reactants are solids, liquids, aqueous, and/or gases A heterogeneous equilibrium’s position doesn’t depend on amount of pure solids or liquids present in equation Activity of a pure solid or liquid is always equal to 1 When pure solids and liquids are in a chemical reaction, their concentrations are not included in equilibrium expressions. The Extent of a Reaction If K is much greater than 1… - At equilibrium, the reaction is mostly products - Equilibrium is to the right - Essentially goes to completion If K is less than 1… - At equilibrium, the reaction is mostly reactants - Equilibrium is to the left - Reaction doesn’t occur to any significant extent The size of K and time required to reach equilibrium are not related Reaction Quotient Concentration of 1 product or reactant = 0, than system will shift in direction of the component that = 0 jA + kB mC A ¿ ¿0 ¿ B ¿ where 0 is the initial concentration ¿ ¿0 ¿ ¿ m Q= [C]0 ¿ If Q = K, then system is at equilibrium If Q > K, then system shifts left to reach equilibrium If Q < K, then system shifts right to reach equilibrium 1. Write a balanced reaction and an equilibrium expression 2. List initial concentrations 3. Calculate Q and find shift 4. Define change needed to shift equilibrium with X’s and apply to initial concentration 5. Substitute new equations into equilibrium expressions and solve for X 6. Plug X value into equations to find equilibrium concentrations Initial Concentration Change Equilibrium Concentration A -X A – X B -X B - X C +2x C + 2x If 2 results are found, plug them into the equilibrium concentration expression. The equilibrium concentration expression cannot be a negative number. When X is small, you can assume that x = 0 and cancel it out of the equation. This only works if the value you get is within 5 % error If a change in conditions is imposed on a system at equilibrium, the equilibrium will shift in the direction that tends to reduce that change in conditions Increase concentration of Reactants = Q < K = Shift Right Increase concentration of Products = Q > K = Shift Left Decrease concentration of Reactants = Q > K = Shift Left Decreased concentration of Products = Q < K = Shift Right If reactant or product is a gas and is added to the system, the system will shift away from the added component If reactant of product is a gas and is removed from the system, the system will shift toward the removed component Effect of a Change in Pressure: 1. Add/Remove a gaseous reactant or product at constant volume 2. Add an inert gas not already in equation at constant volume 3. Change the volume of the container The addition of an inert gas at a constant volume causes no affect on the equilibrium position. It changes the total pressure, but not partial pressure or concentrations of reactants or products When the volume of a container is reduced, the system responds by reducing its volume by decreasing the total number of gaseous molecules in the system At constant temperature and pressure: volume of gas α number of moles of gas When the volume increases, the system will shift in the direction that increases its volume. Since the value of K changes with temperature, the effect of temperature on equilibrium is different If energy in the for of heat is added to the system, the shift will be in the direction that consumes energy Side with energy = decrease in concentration of elements on this side Side without energy = increases in concentration of elements If side with energy is with products, K decreases If side with energy is with reactants, K increases Endothermic Reactions: Increase in Temperature = Equilibrium Shift Right = K Value Increases Decrease in Temperature = Equilibrium Shift Left = K Value Decreases Exothermic Reactions: Increase in Temperature = Equilibrium Shift Left = K Value Decreases Decrease in Temperature = Equilibrium Shift Right = K Value Increases Treat energy as a reactant (for endothermic) or product (for exothermic) and predict the direction of the shift as if an actual reactant or product is added or removed jA + kB mC P ¿ P obs B ¿ ¿ j (¿A ¿obs) ¿ ¿ ¿ (P ob) m KPObser=ed C ¿ KPobs is expected to increase with increasing total pressure BC the extra powers in the denominator greatens the error in pressures compared to numerator To find “true” value of K p , we measure K p at many various values of total pressure obs yiP i obs ai= P where yi is the activity coefficient for correcting Pi ref For equilibrium pressures > 1 atm, the value of Kp calculated from observed equilibrium pressures is expected to be within 1% of true value
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