AP Chemistry
Chapter 15 Outline
15.1Chemical equilibrium occurs when opposing reactions are proceeding at equal rates.
15.1.1The rate at which products are formed from the reactants equals the rate at which reactants are formed from the products.
15.1.1.1No macroscopic changes (such as concentration changes) are observable, but the reaction DOES NOT stop!
15.1.1.2How fast a reaction reaches equilibrium is a matter of kinetics.
15.1.1.3Neither reactants nor products can escape the system.
15.1.2For forward reaction: R P ratef = kf[R]
15.1.2.1For reverse reaction: P Rrater = kf[P]
15.1.2.2 At equilibrium, ratef =ratef , so kf[R]= kr[P]
15.1.2.2.rearranged, the equilibrium constant, K
15.2The equilibrium constant
15.2.2.1Equilibrium can be reached from either direction: starting with all reactants, starting with all products, or some of both products and reactants
15.2.3Law of Mass Action: for a generic reaction aA + bB cC + dD
15.2.3.1 Kc, because we’re using molar concentrations
15.2.3.2in general, K = products/reactants
15.2.3.3value of K depends on the on the particular reaction and the temperature; equilibrium constants are often typically recorded as dimensionless
15.2.3.4for gaseous systems, Kp, because we’re using partial pressures of each gas in atmospheres
15.2.3.5Kc and Kp are usually numerically different, but there is a relationship between them
15.2.3.5.1 where n = (moles of gaseous products) – (moles of gaseous reactants) KNOW AND BE ABLE TO USE THIS EQUATION!
15.3Interpreting and working with equilibrium constants KNOW THIS SECTION!
15.3.2Value of Keq gives information on system at equilibrium
15.3.2.1If Keq > 1: products predominate; equilibrium lies “to the right”
15.3.2.2Keq < 1: reactants predominate; equilibrium lies “to the left”
15.3.3Equilibrium constant expressions can be manipulated in several ways
15.3.3.1If you write a reaction backwards, its Keq value is equal to the reciprocal of the forward reaction Keq
15.3.3.1.1Example: A + B C Keq = Y
15.3.3.1.2 For C A + B Keq = 1/Y
15.3.3.2If an equation is multiplied through by a constant value, the Keq expression is raised to a power equal to that constant.
15.3.3.2.1Example: A + B C Keq = Y
15.3.3.2.1.13A + 3B 3C Keq = Y3
15.3.3.3 If two or more reactions are summed together, then the overall Keq expression is the product of the equilibrium constants for the individual steps.
15.3.3.3.1 Example: A + B C Keq = Y
C + D E + FKeq = Z
A + B + D E + F Keq = YZ
15.4Heterogeneous Equilibria = when substances are not all in the same phase
15.4.2Whenever a pure solid or a pure liquid is included in a heterogeneous equilibrium, its concentration is not included in the Keq expression.
15.4.3As long as the solid components of the heterogeneous equilibrium are present, the system will reach the same ratios!
15.5Calculation Equilibrium Constants Be able to use the ICE strategy!
15.5.2From the balanced equation, create a table: initial, change, equilibrium
A + 2 B 3C if no products are initially present:
A / B / CI / [A] / [B] / 0
C / -x / -2x / +3x
E / [A] – x / [B] – 2x / +3x
15.5.3Use the given information and stoichiometric relationships to figure out the amount of change for each species and substitute into the Keq expression
15.6Applications of Equilibrium Constants KNOW AND BE ABLE TO DO THIS!
15.6.2The Reaction Quotient, Q: Predicting the Direction of Reaction
15.6.2.1Q: the ratio of products to reactants at this moment (not necessarily at equilibrium)
15.6.2.1.1Substitute “current” concentrations into the Keq expression and evaluate
15.6.2.2Compare Q to K to determine the direction of the reaction
15.6.2.2.1If Q = K, the reaction is already at equilibrium
15.6.2.2.2If Q > K, then there are too many products; products will react to form reactants, causing the reaction to move “to the left”
15.6.2.2.3If Q < K, then there are not enough products; reactants will form more products, causing the reaction to move “to the right”
15.6.2.2.4If both reactants and products are present initially, compare Q to Keq to determine the signs when using the ICE approach
15.7LeChatelier’s Principle: If a system at equilibrium is disturbed by a change in temperature, pressure, or concentration, the system will shift its equilibrium position so as to counteract the effect of the shift.
15.7.2Changing Concentrations:
15.7.2.1Add reactant: forward reaction will proceed faster, making products; system will “shift right” to re-establish equilibrium
15.7.2.2Add product: reverse reaction will proceed faster, making reactants; system will “shift left”
15.7.2.3Remove reactant: forward reaction will slow down; reverse reaction will make more reactant; system will “shift left”
15.7.2.4Remove product: reverse reaction will slow down; forward reaction will make more product; system will “shift right”
15.7.3Volume and Pressure Changes
15.7.3.1Remember: fewer moles of gas reduced pressure
15.7.3.1.1If the pressure is increased (or volume is decreased), the system will shift towards the side with FEWER moles of gas
15.7.3.1.2If the pressure is decreased (or volume is increased), the system will shift toward the side with MORE moles of gas
15.7.4Temperature changes: think about the energy change as if it were a chemical species
15.7.4.1Endothermic: reactants + heat products
15.7.4.2 Raising the temperature is like adding more reactants; system will shift right
15.7.4.3 Lowering the temperature is like removing reactants; system will shift left
15.7.4.4 Increasing T results in a larger Keq value.
15.7.4.5Exothermic: reactants products + heat
15.7.4.5.1 Raising the temperature is like adding more products; system will shift left
15.7.4.5.2 Lowering the temperature is like removing products; system will shift right.
15.7.4.5.3 Increasing T results in a smaller Keq value.
15.7.5Catalysts increase the rates of both the forward and reverse reactions.
15.7.5.1Catalysts will NOT change the value of Keq but will increase the rate at which equilibrium is achieved.