Chem 112 Class Guide: Equilibrium Constants (Chapter 15, Sections 1-5)
Chapter 15, Sections 1-5 Learning Goals:
Upon completion of Chapter 15, Sections 1-5, you should be able to determine the following:
Write an expression for K, Kc, or Kp given appropriate reactions
Calculate any variable in an equilibrium expression given equilibrium values
Chapter Reading Guide: Chapter 15, Sections 1-5 (Note: For Chapter 15.3, page 625 through Sample
Exercise 15.8 ONLY)
Section 1: THE CONCEPT OF EQUILIBRIUM
Read Chapter 15.1
Chemical Equilibrium is the state where the concentrations of the reactants and the
products do not change. They do not necessarily need to be equal, but they no longer
change.
In order to reach equilibrium, we cannot add any more reactants to the reaction nor can we
remove products.
Section 2: THE EQUILIBRIUM CONSTANT
Read Chapter 15.2
Given a generic reaction aA + bB ↔ dD + eE, then the equilibrium constant K 
[ D]d [ E ]e
[ A]a [ B]b
TIP! The value of K does not depend on the initial amounts of reactants or products and
does not usually have units.
Your book uses many different subscripts for K. Kc is concentration-based K while Kp is
pressure-based. They may be mixed depending on the states of the compounds in
equilibrium.
n
We can relate Kc to Kp using the equation: K p  Kc ( RT ) , where Δn is the change of moles
of gas from products to reactants.
Try Practice Exercise 15.1
Try Practice Exercise 15.2
Section 3: UNDERSTANDING AND WORKING WITH EQUILIBRIUM CONSTANTS
Read Chapter 15.3, page 625 through Sample Exercise 15.8 ONLY
If K is larger than 1, then we say that the equilibrium favors the product side. The
concentrations or pressures of the compounds on the product side are larger than those on
the reactant side (the concentrations or pressures may be larger, but recall that the values
do not change)
If K is smaller than 1, then we say that the equilibrium favors the reactant side. The
concentrations or pressures of the compounds on the product side are smaller than those
on the reactant side (the concentrations or pressures may be smaller, but recall that the
value do not change)
Try Practice exercise 15.3
Section 4: HETEROGENEOUS EQUILIBRIA
Read Chapter 15.4
Solids and liquids have no definable concentration nor pressure, so they never appear in
expressions for K.
For example, in the reaction:
2
PbCl2 (s) ↔ Pb2+ (aq) + 2 Cl- (aq), K would equal: K   Pb 2  Cl   .
Because PbCl2 is a solid, it is not written in the expression for K.
Try Practice exercise 15.6
Try Practice exercise 15.7
Section 5: CALCULATING EQUILIBRIUM CONSTANTS (only page 625)
Read Chapter 15.5
Expressions for K are largely plug-and-chug algebra-based problems. You could be given the
equilibrium concentrations of all species and asked to solve for K or given K and some
equilibrium concentrations and asked to solve for one particular equilibrium concentration.
Try Practice exercise 15.8
Learning Resources
Chapter Learning Goals
Chapter 15, Sections 1-5 Learning Goals
Pre Class Assignment: This assignment must be completed prior to the next class. Check your syllabus
for the exact due date and time.
Complete the pre class assignment
(http://berks.psu.edu/clt/chem112/EquilibriumConstants_HW.docx)
Submit a copy to the dropbox located in ANGEL called “Pre Class Assignment Submission:
Equilibrium Constants”
End of Chapter Problems:
Practice with these problems if you are having difficulty with any of the concepts covered in this
class guide AFTER we have met in class. If you cannot easily complete these problems, seek
help from your instructor, your mentor, or the learning center.
Chapter 15: 15, 17, 19, 21, 23, 29, 33, 35