Name:____________
Chemistry 114
Fourth Hour Exam
1. (12 points) The reaction P4(s) + 6Cl2(g) 64PCl3(g) is exothermic If the reaction is at
equilibrium, predict what will happen to the PCl3 concentration when the following factors
are changed:
Cl2(g) is added Increase____
Cl2(g) is removed _Decrease__
More P4(s) is added No Change (Solid in in Keq!)
Pressure in increased by adding He gas_No Change__
Pressure is increased by decreasing the container volume __Increase_____
The reaction mixture is heated by 100C __Decrease________
2. (12 points) Fill in the following table:
[H+]
Reagent
added
.0035
[OH-]
pH
pOH
2.86x10-12
2.46
11.54
.0035M HCl
5x10-15
14.30
2.0
-.3
1x10-5
5
2M NaOH
1x10-9
9
-6
_5x10 __M
Ca(OH)2
3. (13 points) A weak acid is only 0.1% dissociated when it is dissolved at a 1M
concentration. What is the Ka of this acid?
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4. (13 points) Use the Lewis acid/base model to explain the following reaction. Make sure
you include Lewis diagrams of the reactants in your explanation to clearly show the
nucleophile and electrophile in the reaction.
CO2 (g) + H2O(l) 6 H2CO3(aq)
Step 1 of a proposed mechanism
I didn’t care what steps were shown, or
which electrons attacked what. All I
wanted to see were some electrons
working in a reaction and labels of
electrophile an nucleophile that were
consistent with the mechanism
5. (12 points) I am going to mix 25 mls of 0.5M HF with 50 mls of 0.1M NaF.
A. Is this solution a buffer?
Mixture of an weak acid and its conjugate base - YES
B. If the Ka of HF is 7.2x10-4, what is the pH of this solution?
pH=pKa+ log [A-]/[HA]
pKa = -log(7.24x10-4 = 3.14
[HF]=25(.5)/75=.1667
[F-]=50(.1)/75 = .0667
pH=3.14 + log (.0667/.1667) = 2.74
6. (12 points) Define the following terms:
Bro/nstead-Lowry Acid
Proton Donor
Lewis Acid
Electron Acceptor
Nucleophile
Likes + charge, hence an electron donor or Lewis base
The Second Law of Thermodynamics
In any spontaneous process the entropy of the universe increases
Positional Entropy
Entropy calculated from the statistics of counting all possible states
Free Energy
G = H -TS; The energy in a system that is available to do work
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7. (13 points) Calculate )S for the reaction 2Cr2O3(s) + 9O2(g) 64CrO6(s).
Given the following So values (in J/K@mol); Cr2O3(s) +81, O2(g) +205, CrO6(s) +72.
)S=Sum of products - sum of reactants
=4(+72)-[2(+81)+9(205)]
=288-162-1845
= -1719J/K@mol
8. (13 points) The process N2O4(l)6N2O4(g) has a )S of +95J/K@mol and a )H of +30
kJ/K@mol.
Will this process be spontaneous at 100K? No
Will this process be spontaneous at 1000K? Yes
From this data calculate the boiling point of N2O4.
Melting point will be where )G = 0
)G = )H -T)S
0 = 30,000 -X(95)
X(95)=30,000
X=30,000/95 = 315.8K
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