Chemistry 114
Fourth Hour Exam
Name:____________
(4 points)
Please show all work for partial credit
1. Suppose you have 1.00L of an aqueous buffer containing 55.0 mmol of acetic acid
(pKa=4.67) and 45 mmol acetate
Calculate the pH of this buffer.
What volume of 4.5M NaOH would be required to increase the pH to 5.00
2. Define the following terms:
Spontaneous process
A process that will occur without input of energy from a external source.
Second law of thermodynamics
In any spontaneous process the entropy of the universe always increases.
Positional disorder
Randomness that comes from the number of different arrangements of a system.
Molar entropy of fusion
The heat energy that must be added to a solid to make it a liquid when it is at its
melting temperature.
Gibbs Free energy
ÄG =ÄH-TÄS; The energy available to do work.
entropy favored reaction
A reaction where the driving energy term is the increase in entropy. (+ÄSrxn)
3. The molar entropy of fusion for water (ÄH fus) is 6.01 kJ/mole at 0oC. What is the molar
entropy of fusion for water at this temperature?
4. Circle the compound or atom with the highest entropy in each of the following pairs:
He
Ne (more mass so more thermal states)
Br2
Cl2 (more mass so more thermal states)
C2H6
C2H2 (more atoms to wiggle & giggle)
N2O
NO2 (more mass so more thermal states)
The one on the right. The ring system on the left
restricts motion and randomness
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5. Calculate ÄGo rxn for the reaction C3H8(g) + 5 O2(g) 6 3CO2(g) + 4H2O(l) using the
ÄG of values given in the table below:
Compound
ÄGof (kJ/mol)
C3H8(g)
-23.4
O2(g)
0
CO2(g)
-394.4
H2O(g)
-228.6
H2O(l)
-237.1
6A. The reaction O=Cl-O-H(aq) W O=Cl-O-(aq) + H+(aq) has a K of 1.2x10-2 at 25oC.
What is the ÄG for this reaction?
6B. The reaction A WB has a ÄG of -25 kJ/mol at 100K, what is the K for this reaction at
this temperature?
3
7. Explain the difference between KP, KC and the thermodynamic K for a gas phase
reaction.
KP is the K of a gas phase reaction expressed in units of pressure. KC is the K of
a gas phase reaction expressed in units of concentration. K is KP or KC divided by the Q
of the standard state in the same units, so all units are canceled.
8. The value of ÄHorxn for the reaction
2H2O W H3O+ + OH- is 55.8 kJ/mol
if the K for this reaction is 1x10-14 at 250C, what is the K for this reaction at 15oC?
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