Practice Problems for CHGN 124 Final Exam, Spring 2008
1. Explain Le-Chatelier’s Principle, and predict how it will impact the following equilibria:
Fe3+ (aq) + Cl- (aq) ↔ FeCl2+(aq) endothermic reaction
Le Chatelier’s principle states that if a stress is applied to a reaction mixture at equilibrium,
reaction occurs in the direction that relieves the stress.
a. add Fe(NO3)3Adding reactant (Fe3+) causes reaction to go to right
b. add HCl Adding reactant (Cl-) causes reaction to go to right
c. add AgNO3Ag+ will precipitate with Cl-, causing reaction to go to left
d. add a catalyst Catalysts do not change equilibrium concentration
e. increase temperature Endothermic reaction will go to right when temp. is increased
2. The air pollutant NO can react with O2 by the following reaction:
2NO(g) + O2(g) ↔ 2NO2(g)
a. If we add 1 mole of NO and 2 moles of O2 to a 2 liter container and allow the mixture to
come to equilibrium, the final concentration of NO2 is 0.44M. What is the equilibrium
constant?
Initial concentrations: 0.5 M NO and 1.0 M O2
R 2NO + O2 ↔ 2NO2
I 0.5 1.0 0
C -2x -x +2x
E 0.5-2x 1.0-x 2x
2x = 0.44M (final concentration of NO2), x = 0.22
[NO] = 0.5 – 2(.22) = 0.06M; [O2] = 1.0 – 0.22 = 0.78 M
Kc = [NO2]2 / [NO]2[O2] = (0.44)2/(0.06)2(0.78) = 69 = 70 (with one significant figure)
b. If we reduce the size of the container to 1 liter at equilibrium, will the concentrations of
reactants and products change? If so, in what direction will the reaction go (towards the
right or left)?
Reducing the volume of the container will increase the concentrations of all species.
Since there are three reactant gas molecules for every two product molecules, the reaction
will proceed to the right to reduce the overall number of gas molecules present.
3. Describe the major difference between strong and weak acids. Identify the conjugate acid of
each of the following bases, and rank then in order (from strongest to weakest base)
Strong acids completely dissociate when added to water. Weak acids only partially or
slightly dissociate in water (most of the acid molecules remain in their un-dissociated,
protonated form). The value of Ka for each acid indicates the degree to which the acid
dissociates. Similarly, the value of Kb for bases indicates the degree to which the base is
likely to accept protons.
a. CH3NH2CH3NH3+Kb = 3.7 x 10-4 2nd strongest
b. NaOH H2O Strong base Kb >>1 Strongest
c. H2OH3O+Kb ~ 1 x 10-14 Weakest
d. H2PO4-H3PO4Ka = 7.5 x 10-3, so Kb = 1.3 x 10-12 2nd weakest
