What do we know?
 Direction of reaction to shift for equilibrium
 NEED TO KNOW: quantify amount of reactants and
products at equilibrium.
Using KC to determine [products] and
[reactants] at equilibrium
 Only know KC and initial concentrations of reactants
 Changes in concentration of chemical compounds
(ΔC)
 Related to STOICHIOMETRIC ratios in chemical
equation.
 Represents concentration change from initial to
equilibrium conditions.
Example 1
2H2 +
O2 
2H2O
 So what is [H2] at equilibrium?
 [H2] at equilibrium = [H2]initial – ΔH2
**Use stoichiometric ratios to determine the amount of
ΔC**
Example 2:
 1 mole of butane is in a 1L flask at 25°C with no
isobutane present. What will be the equilibrium
concentrations of butane and isobutane with a KC of
2.5 for this reaction?
Equilibrium Key Elements
in Problem
 Balanced Equation
 Equilibrium constant
 Initial Conditions
 Problem’s Objective
Steps for Complex
Problems
1) Decide if the reaction is at equilibrium
2) Set up equilibrium expression
3) Make an ICE Chart
4) Solve for ΔC
5) Is assumption valid for ΔC?
6) Determine the final concentrations of the chemical
compounds.
Ice Chart
“ICE”
Initial Concentrations
(I):
Change in
Concentration (ΔC or x):
Equilibrium
Concentrations (E):
Reactants
Products
Assumption with ΔC
 If ΔC < 5% of initial concentration, ignore the ΔC for
adding and subtracting in equilibrium expression.
 If Q< K, both small values so initial concentrations
close to equilibrium
 Always validate this assumption in an equilibrium
problem
Assumption with ΔC (cont.)
 If Keq < 1x10-4, remove the “x or ΔC” value in
denominator.
 ΔC is very small compared to initial concentration so
subtraction would not be a huge difference.
 Only works when adding or subtracting ΔC
 If concentrations or partial pressures are very small
where their magnitude is approximately equal to Keq,
CANNOT discount ΔC value.
Example 3:
 H2O(g) is present in a rigid container at 25°C with an
initial partial pressure of 0.784 atm. What are the
partial pressures of H2(g) and O2(g) at equilibrium? (KP
= 2.0 x 10 -42)
Example 3: continued
Example 4:
 Sulfur trioxide decomposes to form sulfur dioxide and
oxygen at 300°C°. Calculate the concentrations of all
chemical compounds at equilibrium with an initial SO3
concentration of 0.100M and KC = 1.6 x 10-10.
Example 4: continued
Example 5:
 0.194 mol of COCl2 comes to equilibrium in a 5.8L
container at 25°C (KC = 7.27 x 10-38). Find the
equilibrium concentrations of all chemical compounds
in the following equation. (Hint: first find the initial
[COCl2] )
 COCl2 (g)  CO(g) + Cl2 (g)
Example 5: continued
In Class
 Problems 16-20 on Equilibrium II Worksheet
Homework
 AP Equilibrium and ICE method worksheet (#1-5)