Chapter 15 Equilibrium Test

Tomorrow is the test over Chapter 15, Equilibrium. The test will be all written and consist of six questions. Some of the questions will have multiple parts. You will be given the list of equations and a periodic table supplied for the AP Chemistry exam. Below is a list of what you should know for the test and a key to the LeChatelier's Worksheet.

Know how to perform all calculations from the lab. This includes absorbance to concentration calculations, I.C.E. table set-up, and "k"(equilibrium) calculations using the data from the I.C.E. table.

For all I.C.E. table values, remember that simple stoichiometry must be followed. An example would be the relationship for a balanced equation shown below.
2A + B <--> 3C
A change of "C" would be three times larger than a change in "B". A change in "A" would be twice as large as the change in "B". A change of two "A" would be equivalent of a change of three "C".
Know how to use the equation for converting a k>p to a k>eq and vise versa.

Know how to predict the shift of a system at equilibrium by stressing the system. Remember, LeChatlier's Principle states that a system will try to re-establish equilibrium by partially undoing the stress caused to it. You will have to visually show this like you did on the LeChatelier's worksheet and also be able to discuss what the system must do to re-establish equilibrium.

If you are given a problem where there are no products initially, the shift must occur to the product side to create products.

If you are given a problem where both reactants and products are present initially, you must first determine the reaction quotient, Q, to determine the shift of the reaction. The shift must cause the value of "Q" to eventually equal k>eq.

A right shift would cause you to add "x" to the product side and subtract "x" from the reactant side. A left shift would cause you to subtract "x" form the product side and to add "x" to the reactant side.

You must always keep all "x" values; therefore use of the quadradic equation may be a must OR always first look for easy squares. What is meant by "easy squares" the values of both products and reactants can have their square roots found before doing the hard math of a quadradic equation. Look at the quiz from this past weekend as an example of this.

Use the hand out from today as a guide / reason for disregarding subtraction "x" once the equilibrium values have been placed in an equilibrium expression. If the k>eq value and the initial concentrations differ by a magnitude of 10^5 or more, it is permissible to disregard subtracting "x" in the equilibrium equation to solve for "x". "x" must still be used to determine equilibrium concentration / partial pressure values.

If solving for equilibrium concentrations of reactants and products, be sure to place the values back in the equilibrium expression to confirm that the concentrations equal the equilibrium value given in the problem.