CHEM104
Exam 1 KEY
Spring 2002
MULTIPLE CHOICE QUESTIONS: correct answer in red. 3 points each
1. For the following reaction:
2SO3 (g) <=> 2SO2 (g) + O2(g) DH = 197 kJ/mol
Le Chatelier's Principle predicts that when the temperature is increased the reaction will:
a) speed up
b) form more product
c) go in reverse
d) be prevented from reaching an equilibrium
2. If the reaction quotient, Q, calculated for a reaction is less than the value of the equilibrium constant, Keq:
a) the reaction has stopped.
b) the reaction will proceed in the forward direction to form more product.
c) the reaction will proceed in the reverse direction to form more reactants.
d) the reaction can never reach equilibrium.
3. A "p"-function :
a) is the negative natural logarithm of another function
b) equals [- log(the same function)]
c) raises a function to some power of ten
d) is none of the above
4. HNO2 as compared to HNO3 is a:
a) weaker acid
b) stronger acid
c) comparable acid
d) cannot tell from information given
5. The pH of a solution of sodium acetate will be:
a) the same as pure water.
b) > 7
c) < 7
d) .... cannot determine without additional information.
6. Chloride ion has no effect on solution pH because:
a) it is small
b) it is such a weak acid
c) it is the conjugate base of a strong acid
d) it has only a 1- charge
7. A reaction must be spontaneous if:
a) DSsys is positive
b) DSsurr is positive
c) a gas is a reagent
d) none of the above
8. If Q = Keq for a system then
a) DG < 0
b) DStotal < 0
c) DG = 0
d) DStotal > 0
SHORT ANSWER QUESTIONS:
9. Will the equilibrium of the acid/base reaction below go primarily to the right or to the left?
(pKa (HF) = 3.17; pKa (HClO) = 7.53) (3 points)
F- + HClO ‹ ClO- + HF
...because HF is a stronger acid than HClO
10. Give the chemical formulas for the chemicals named below and circle whether an aqueous solution of the substance is acidic, neutral, or basic. (8 points)
carbon dioxide CO2 acidic neutral basic
sodium nitrate NaNO3 acidic neutral basic
potassium acetate KCO2CH3 acidic neutral basic
ammonium chloride NH4Cl acidic neutral basic
11. The value of the "constant" Kw in fact is not constant but varies with temperature. For example, at very high temperatures (above 600o F), the value of Kw increases to 10-11.
What is the pH at neutrality for water at 600o F? (5 points)
Neutral pH = 5.5
Because: Kw = [H3O+][OH-] = [H3O+]2 = 1.0 ^ -11
12. Cement is made from solid calcium oxide powder mixed with water.
Calcium oxide, CaO, is made from decomposing the mineral calcite, CaCO3 with heat at very high temperatures to form calcium oxide and gaseous CO2.
a) write the balance equation for the formation of calcium oxide. (3 points)
CaCO3 --> CaO + CO2
b) write the equilibrium expression for this reaction. (3 points)
Kc = [CO2]
c) explain why the industrial process of decomposing calcite to prepare CaO is done in an open vat, rather than in a closed vat. (5 points)
If the pressure due to the carbon dioxide gas formed is not released, as would happen in a closed vat, the reaction would eventually come to equilibrium. When the reaction is done in an open vat, the gas escapes, leaves the equilibrium and so the reaction is constantly thermodynamically favored to proceed in the forward direction to make more carbon dioxide gas.
13. The free energy of formation for N2O4 is +97 kJ/mol.
Is this compound thermodynamically stable? Why? (6 points)
It is not stable because the reverse reaction, decomposition to the elements ( nitrogen and oxygen gas) has a negative value for the free energy and may spontaneously occur.
14. Many cultures worship the sun as a god. Give a thermodynamic justification for these spiritual practices. (8 points)
These cultures are humans who live by respiration, a process that produces energy (free energy of respiration is negative and spontaneous) to keep them alive through the oxidation of carbohydrates, like glucose. The carbohydrates originate from plants, who make the carbohydrates through the process of photosynthesis. Photosynthesis is not a spontaneous process (free energy is negative) but is made possible through the energy in the form of light from the sun.
The sun is responsible for all life on earth.
PROBLEMS:
15. Many minerals in Nature slowly react to convert to another mineral type. For example, oxidation of magnetite, Fe3O4, forms the mineral hematite, Fe2O3 .
(17 points)
balance equation! 2 Fe3O4 (s) + 1/2 O2 (g) Æ 3 Fe2O3 (s)
(a) Does entropy contribute to making this reaction spontaneous? (explain why or why not)
No. The overall entropy is expected to decrease, so delta S is negative, since a gas is a reagent but not a product.
(b) Calculate the free energy change for the conversion of the mineral magnetite into the mineral hematite.
DG (Fe3O4 ) = -1015.4 kJ/mol; DG (Fe2O3 ) = -742.2 kJ/mol
delta G(rxn) = 3 delta G(Fe2O3) - 2 delta G(Fe3O4 ) = 3(-742.2) - 2(-1015.4) = -1958 kJ/mol
(c) Based on your answer in (b) above, predict whether the conversion of magnetite to hematite is exothermic or endothermic.
Since the overall free energy change for the reaction is a large negative number, and since the entropy change does not contribute ( because it is also negative) then the reaction MUST BE exothermic.
delta G = delta H - T delta S
(-) (-) - (-)
16. Sodium benzoate, Na+[C7H5O2-], is a salt of the conjugate base of benzoic acid. You can frequently find it listed as an ingredient of many food products where it is a common preservative. What is the pH of a solution made from 0.20 mol sodium benzoate dissolved in 500 mL of water? The pKb of benzoate is 9.80. (10 points)
17. The ferric ion, Fe3+, is nearly colorless in aqueous solution. An old method of detecting iron in water is to add thiocyanate SCN- and look for a red color. (15 points)
Fe3+(aq) + SCN-(aq) <=> [Fe3+ (SCN)]2+(aq) Kc = 1.1 x 103 , 25 deg.
Pale yellow colorless dark red
What are the equilibrium concentrations of ferric ion, [Fe3+], thiocyanate, [SCN-], and ferric-thiocyanate ion, [Fe3+ (SCN)]2+ if 0.0010 mol Fe3+ is added to 1.0 L of 0.02 M [SCN-]?
18. You have just purchased (for $20,000) a marble stature of an angel to grace your garden. Unfortunately, you live in an area of very acid rain. Rain falling on your garden ( and your angel statue) typically has a pH of 4.5. (15 points)
a) Remembering that marble is calcium carbonate, write the reaction(s) involved as acid rain dissolves marble.
b) If your angel weighs half a ton, how many liters of rain at pH 4.5 will dissolve all of it? (one ton = 907 kg)
19. During one winter break, I read a book about winemaking at the Iron Horse Vineyards in California. I learned that makers of expensive wines often pay more than $1.00 per each cork that is used to stopper the wine bottles. The ethanol in wine forms acetic acid (vinegar! sour! yuck!) according to the following reaction. (15 points)
C2H6O (ethanol) + O2 (g) <=> C2H4O2 (acetic acid) + H2O Kc = 1.2 x 1082
a) Explain why the quality of the cork is so important to winemakers.
b) oops! I forgot to replace the cork in the wine bottle three nights ago! Since I am a chemist, I decide to check on how much acetic acid formed by measuring the pH of the wine. If the pH is 4.23, what is the concentration of acetic acid in the wine? (You may assume that acetic acid is the only contributor to acidity)
