What do we know?What do we know?  Direction of reaction to shift for equilibrium  NEED TO KNOW: quantify amount of reactants and products at equilibrium.

Using K C to determine [products] and [reactants] at equilibrium  Only know K C and initial concentrations of reactants  Changes in concentration of chemical compounds ( Δ C)  Related to STOICHIOMETRIC ratios in chemical equation.  Represents concentration change from initial to equilibrium conditions.

Example 1Example 1  2H 2 + O 2  2H 2 O  So what is [H 2 ] at equilibrium?  [H 2 ] at equilibrium = [H 2 ] initial – Δ H 2 **Use stoichiometric ratios to determine the amount of Δ C**

Example 2: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 K C 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 Δ C 5)Is assumption valid for Δ C? 6)Determine the final concentrations of the chemical compounds.

Ice ChartIce Chart “ICE”ReactantsProducts Initial Concentrations (I): Change in Concentration ( Δ C or x): Equilibrium Concentrations (E):

Assumption with Δ CAssumption with Δ C  If Δ C < 5% of initial concentration, ignore the Δ C 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 Δ C (cont.)Assumption with Δ C (cont.)  If K eq < 1x10 -4, remove the “x or Δ C” value in denominator.  Δ C is very small compared to initial concentration so subtraction would not be a huge difference.  Only works when adding or subtracting Δ C  If concentrations or partial pressures are very small where their magnitude is approximately equal to K eq, CANNOT discount Δ C value.

Example 3:Example 3:  H 2 O (g) is present in a rigid container at 25°C with an initial partial pressure of atm. What are the partial pressures of H 2(g) and O 2(g) at equilibrium? (K P = 2.0 x )

Example 3: continuedExample 3: continued

Example 4: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 SO 3 concentration of 0.100M and K C = 1.6 x

Example 4: continuedExample 4: continued

Example 5:Example 5:  mol of COCl 2 comes to equilibrium in a 5.8L container at 25°C (K C = 7.27 x ). Find the equilibrium concentrations of all chemical compounds in the following equation. ( Hint: first find the initial [COCl 2 ] )  COCl 2 (g)  CO (g) + Cl 2 (g)

Example 5: continuedExample 5: continued

In ClassIn Class  Problems on Equilibrium II Worksheet

Homework  AP Equilibrium and ICE method worksheet (#1-5)