Writing and Solving Equilibrium Constant Expressions

Objectives: Today I will be able to: Write and solve equilibrium constant expressions for reactions Formal assessment – analyzing student responses to the practice problems and closure Informal assessment – monitoring student questions and interactions as they complete the practice problems

Lesson Sequence Evaluate: Warm Up Explain: Equilibrium Notes Review Elaborate: Practice Problems Evaluate: Closure

Warm Up

Objectives Today I will be able to: Write and solve equilibrium constant expressions for reactions

Homework Study for Ch. 5/ 19 Exam Tomorrow

Agenda Warm Up Equilibrium Review Notes Equilibrium practice problems Closure

Equilibrium Notes Chapter 15

What is equilibrium?

Equilibrium is … The forward and reverse reaction rates are equal Referred to as dynamic Equilibrium DOES NOT mean the concentrations of reactants and products are equal

Writing Equilibrium Constant Expressions Based on the Law of Mass Action a A + b B  c C + d D Law of Mass Action – there is a relationship between concentrations of reactants and products at equilibrium

What is the difference between heterogeneous equilibrium and homogeneous equilibrium?

Heterogeneous vs. Homogeneous Homogeneous – all substances involved in equilibrium system are in the same phase N2(g) + 3H2(g)  2NH3(g) Heterogeneous – some substances in the equilibrium system are in a different phase PbCl2(s)  Pb2+(aq) + 2Cl1- (aq)

What does this mean for writing equilibrium expressions?

In heterogeneous equilibrium… Liquids and solids are omitted from the equilibrium constant expression Why can they be omitted? Think about the molarity relationship Double mass, double the volume so the ratio stays the same

Two types of equilibrium constant expressions Concentrations Symbol = Kc Use [ ] to represent molarities in expression Partial Pressures Symbol = Kp Use ( ) to represent partial pressures in expression Note: Kp ≠ Kc

What do K values mean? K >1 K = 1 K < 1 Reaction is product favored at equilibrium K = 1 Reaction has equal concentrations of reactants and products at equilibrium K < 1 Reaction is reactant favored at equilibrium Think about the magnitude of the value in the reaction

Equilibrium Problems Write the equilibrium expression for the following reaction 2 SO2(g) + O2(g)  2 SO3(g) [SO3]2 [SO2]2 [O2] _______________

Keq = .18 (reactant favored) Equilibrium Problems If .0172 M of O2, .0250 M of SO2 and .00140 M of SO3 at equilibrium, calculate Keq [SO3]2 [SO2]2 [O2] [.00140]2 [.0250]2 [.0172] _______________ _______________ Keq = .18 (reactant favored)

Equilibrium Problems The equilibrium constant can also be used to calculate equilibrium concentrations from initial concentrations Use an ICE table! To start with, we will use a generic equation A(g) + B(g)  2 C(g) [A] = .100 M and [B] = .100 M

A(g) + B(g)  2 C(g) A B 2 C Initial .100 M Change -x +2x Equilibrium Change -x +2x Equilibrium .100 – x 2x A and B are assigned –x values because they are being reacted, while C gets a +x because products are being formed. The 2x comes from the coefficient in front of the C

Sometimes we are given concentrations in a reaction and need to determine if the system is at equilibrium

Use the Reaction Quotient (Q) Number obtained by substituting reactant and product concentrations or partial pressures at any point during a reaction into an equilibrium constant expression Qp or Qc

Comparing K and Q

Closure – Practice Problems Using the 10th edition of the textbook Equilibrium; the Equilibrium Constant 14, 15, 19, 21, 22, 24 Calculating Equilibrium Constants 28-30, 31-33 Applications of Equilibrium Constants 37-38, 41-44, 49

Closure – Practice Problems Using the 15th edition of the textbook Equilibrium; the Equilibrium Constant 15,17,21,23,25 Calculating Equilibrium Constants 31-33, 35-37 Applications of Equilibrium Constants 43-45, 41-44, 47, 49, 51