Revised Q 5 Practice, Chemistry 111, Lycoming College, Feb. 23, 2005, Dr. Mahler
Fair Game for Friday’s Quiz: All previous material (Ch. 11, 12, 13). New Material –Chapter 14 Sections 3, 4 and 5. Relevant Homework: Ch. 14 problems - all assigned (keys on web: http://www.lycoming.edu/chem/spring2005/111/keys.htm)
Practice problems:
1) Possible essay / short answer type questions:
· Chapter 14 Review Questions 1-9 (page 628)
2) For the reaction N2 (g) + 3 H2 (g) ó 2 NH3 (g), Kp is 41 at 400 K. a) Calculate the value of Kc at 400 K. b) Say that the initial pressure of each gas is 0.234 atm at 400 K. What is the value of Qp ? c) Which way will the reaction in b) shift to reach equilibrium?
d) If you start with only 0.234 atm NH3, what will be the equilibrium pressures of each gas? Show all work and reasoning.
3) For the reaction 4 NH3 (g) + 5 O2 (g) ó 4 NO (g) + 6 H2O (g) at 298 K, ∆H = -905 kJ/mol. Starting at equilibrium in each case, what would a separate change in the system have as an effect? Specifically, the table below gives a change in the first column, then asks you to consider the effect of this change on something else, given in the second column. Your answer can be increase, no change, or decrease (check one of these).
|
Change |
Quantity |
Increase |
No change |
Decrease |
|
Add NO
|
Amount of H2O |
|
|
|
|
Add NO
|
Amount of O2 |
|
|
|
|
Remove H2O
|
Amount of NO |
|
|
|
|
Remove O2
|
Amount of NH3 |
|
|
|
|
Add NH3
|
Kc |
|
|
|
|
Remove NO
|
Amount of NH3 |
|
|
|
|
Add NH3
|
Amount of O2 |
|
|
|
|
Raise temp.
|
Amount of NO |
|
|
|
|
Raise pressure
|
Amount of H2O |
|
|
|
|
Add catalyst
|
Amount of NH3 |
|
|
|
|
Lower temp.
|
Amount of H2O |
|
|
|
Key for 3) The basic reasoning here is to use le Chatelier's principle. Look at what is added or removed or changed in the first column. Then see what the shift caused by the change is, and see how that shift changes the quantity asked for in the second column.
As an example, for the first one, adding NO is adding a product. Adding a product causes the reaction to shift to the left as written, towards reactants (it makes more reactants). This causes the amount of all products (including H2O) to decrease.
Adding reactant shifts the reaction to the right (make more products). Removing reactant shifts the reaction to the left (make more reactants). Finally, removing product shifts the reaction to the right (make more products).
Exothermic reactions have a negative ∆H and heat is essentially a product. Endothermic reactions have a positive ∆H and heat is essentially a reactant. So increasing the temperature is like adding product if exothermic and adding reactant if endothermic. The reaction given is exothermic, so raising the temperature is like adding product and shifts the reaction to the left (make reactants), decreasing the amount of NO. Lowering the temperature is like removing product, shift the reaction right (make products) and increasing the amount of products, including H2O.
Pressure increases cause the reaction to shift to the side of the equilibrium with fewer moles of gas, here the reactants (with 9 moles of gas vs. 10 moles for the products). So raising the pressure causes the amount of all products (including H2O) to decrease.
Finally, Kp (or Kc) is a constant and is not changed by adding or removing chemicals (their amounts shift to get back to the value of Kp or Kc). Catalysts will speed up the reaction but not change Kp or Kp or the concentrations of reactants or products at equilibrium.