CHAPTER 15 BLUFFER’S GUIDES
Bluffer’s Guide for 15.1
Section 1 dealt most significantly with the concept of equilibrium in general.

Chemical Equilibrium: A reaction is at equilibrium if the rate of the forward reaction is equal to the
rate of the reverse reaction.
o The rate constants of the forward and reverse reaction are also related, and can be used to find
the Equilibrium Constant (kf / kr = Keq)
The Equilibrium Expression
For the reaction
aA + bB
cC + dD
Keq = [C]c[D]d
[A]a[B]b
1. [A], [B], [C], and [D] are molar concentrations or partial pressures at equilibrium
2. Products are in the numerator, and reactants are in the denominator
3. Coefficients in the balanced equation become exponents in the equilibrium expression
4. Solids and pure liquids are not included in the equilibrium expression – only aqueous reactants and
products are included
5. Units are not given for Keq
Bluffer’s Guide 15.2
1. Haber Process
a. Mole ratio affects equilibrium
b. No matter the starting composition of reactant and products the ratio of concentrations is
achieved at equilibrium
2. Law of Mass Action
a. Expression of concentration of the reactants and products present at equilibrium
i. aA + bB ↔ cC +dD
ii. the equilibrium constant expression
1. Keq =
=
2. Small letter is the stoichiometry
3. No units
3. Magnitude of equilibrium constants
a. Keq »1 : lies to right; products predominate
b. Keq «1 : lies to left; reactants predominate
4. Direction
a. Keq »1; forward
b. Keq «1; reverse
c. 1/Keq gives the reverse constant
5. Manipulations
a. Double the moles equals squared equilibrium constant
15.3 Heterogeneous Equilibria
homogeneous equilibria- equilibria substances found in the same phase
Heterogeneous equilibria- Equilibria substances found in different phases
While writing the equilibrium-constant expression it is not necessary to include solids or pure substances in
the equilibrium expression. Partial pressures of gasses and molar concentrations are the only values
included.
When solid calcium carbonate decomposes to solid CaO and gaseous carbon dioxide the equilibrium
constant depends solely on the partial pressure of carbon dioxide.
Although they do not appear in the constant expression they are still present at equilibrium.
Chapter 15 Section 4: Calculating Equilibrium Constants
In order to solve for the solve for the equilibrium constant or for one of the concentrations or partial
pressures of a reactant or product, use an ICE Chart:
I-
The initial concentration of the products and the reactants
 The gas law may need to be used if the question is asking for a partial pressure. Use PV
= nRT
(where R = 0.08206 L*atm/mol*K)
 If a value of the initial concentration is not given for a product, assume its initial
concentration or pressure is zero
C- The change in concentration or pressure


Is negative for the reactants and positive for the products
Is the difference in initial concentration and pressure and equilibrium concentration or
pressure
If there is a coefficient take that into account by multiplying the coefficient by the
change (only for that particular reactant or product)

E- The concentration of the reactants or products at equilibrium

This is the initial concentration or pressure plus or minus the change
Bluffer's Guide to Chapter 15 Section 5:
Use of the equilibrium constant, Keq:
1). Predict the direction of reaction
- Substituting reactant and product concentrations/partial pressures into the
constant expression yields the reaction quotient, Q
- At equilibrium and only at equilibrium Q = Keq
- If Q > Keq, than the reaction will proceed to the left, producing more
reactants.
- If Q < Keq, than the reaction proceeds to the right, producing more
products.
equilibrium
2). Calculating equilibrium concentrations
- If both Keq and initial concentrations are known
- First, determine Q and direction reaction must proceed
- Next, set up an ICE (initial, change, equilibrium) chart with change done in terms of a
variable, x - Be careful of stoichiometry!
- Using the initial and change values, write expressions at equilibrium for each concentration
- Finally, plug the concentration expressions in the equilibrium constant
solve for the variable using algebra.
expression and
- Often a quadratic that yields multiple answers - check to make sure that answers make
sense, as all concentrations/pressures must be non-negative.
Section 15.6

Le Chatelier’s Principle

If a system at equilibrium is disturbed by a change of pressure temperature or the
concentration of one of the components, the system will shift its equilibrium to counteract the
effect of the disturbance


Change in Concentration

The reaction will move in the opposite direction of an addition of reactants or products

The reaction will shift toward a decrease in products or reactants
Volume and Pressure

Reducing volume causes the reaction to shift in the direction that reduces the number of
moles of gas



System will shift to reduce pressure
Temperature

If Enthalpy is positive an increase in temperature favors products

If enthalpy is negative an increase in temperature favors reactants
Catalysts

Speed up reaction but do not change equilibrium concentrations