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Unformatted text preview: Chemistry 1A
Midterm Exam III
Nov 15, 1995
(Closed Book, 100 minutes, 100 points)
Page 1 of 8
SID: ____________________________ Discussion TA _______________________ The following formulae and data may be useful:
For aA + bB ⇔ cC + dD : ∆ E = q + w Q = [ C]c [ D]d [ A]a[ B]b
At equilibrium, Q = K
pH =pKa - log [HA]/[A-]
R=8.314JK-1mole-1 Photon: E = h υ, λυ = c
°( products) − ∑ ∆H f ° (reactants )
∆ H° = ∑ ∆H f
∆ G° = ∑ ∆G f ° ( products) − ∑ ∆G f °( reactants)
S° = ∑ S°( products) − ∑ S° ( reactants)
h = 6.626 x 10-34 J s
c = 3.0 x 108 m.s-1 ∆ G° = ∆H ° − T ∆S° = − RT ln K = − nF ∆ε ° Thermodynamic Properties
Substance ∆Hf° (kJ mol-1) S° (J K-1mol-1) Substance ∆Hf° (kJ mol-1) S° (J K-1mol-1) O2 0 205.03 H2SO4 -814.0 156.9 FeS2 -178.2 52.93 H2O(l) -285.8 69.91 Fe2O3 -824.2 87.4 NaCl - 411.15 72.13 Standard Reduction Potentials
Half-Reaction ε°(V) Half-Reaction ε°(V) MnO4-+ 8 H++ 5 e- ∅ Mn2+ + 4 H2O +1.49 Sn2+ + 2 e- ∅ Sn (s) -0.14 Cl2 (g) + 2 e- ∅ 2 Cl- +1.36 Fe2+ + 2 e- ∅ Fe (s) -0.41 O2 (g) + 4 H++ 4 e- ∅ 2 H2O +1.23 Zn2+ + 2 e- ∅ Zn (s) -0.76 Cu2+ + 2 e- ∅ Cu (s) +0.34 2 H2O + 2 e- ∅ H2 (g) + 2 OH- -0.83 2H+ + 2 e- ∅ H2(g) 0 Na+ + e- ∅ Na (s) -2.711 Write your name on all 7 pages. This test consists of two parts: Multiple choice and Problems requiring a
longer answer. For the multiple choice section, mark one correct answer for each question AND use a #2
pencil to bubble in one correct answer on your Scantron™ form for each question.
• Budget your time. Anticipate spending about 50 minutes on each part.
• Be sure to leave sufficient time to transfer your multiple choice answers to the Scantron™ form. (for Administrative use only)
MC3 2 Total E3 MC2 3 Total E2 1 4 Name_____________________ Page 2 of 2 Part I: Multiple Choice, 3 points each, 54 points total
MARK THE CORRECT ANSWER ON YOUR EXAM AND SHADE IN THE BUBBLE OF THE
CORRECT ANSWER FOR EACH QUESTION WITH A #2 PENCIL ON YOUR SCANTRON™
1.) The answer to question 1 is A. Bubble in A on your Scantron™ form. 2.)
Consider the endothermic reaction N2O4 (g)
2NO2 (g) in a sealed vessel. If K1 is the
equilibrium constant at 300 K, and K2 is the equilibrium constant at 400 K, which of the following must be
A) K1 = K2 B) K1 = K2-1 C) K1K2 = Kw D) K1 < K2 E) K2 < K1 3.)
What conditions favor the yield of ammonia (NH3) at equilibrium, given the following exothermic
N2 (g) + 3H2 (g)
A) High Temperature B) Low Pressure
4.) D) Open Container E) Thermal Insulation B) =3 C) Between 3 and 4 D) = 4 E) > 4 An HAc (weak acid) solution with pH = 3 is diluted by a factor of 10 with water. The new pH is:
A) <3 6.) C) High Pressure An HCl (strong acid) solution with pH=3 is diluted by a factor of 10 with water. The new pH is:
A) <3 5.) 2NH3 (g) B) =3 C) Between 3 and 4 D) = 4 E) > 4 A sealed can half full of soda expands on a warm day. The reaction below is:
A) isothermal B) exothermic CO2 (g)
C) endothermic D) basic E) acidic 7.)
An electrically conducting solution of Ba(OH)2 (a soluble salt) is titrated with H2SO4 until the
conductance is at a minimum. (Ksp for BaSO4 = 1.1x10-10) At this point:
A) [Ba2+]=[SO42-] B) [Ba2+]<[SO42-] C)[SO42-]<[Ba2+] D) [H3O+]<[OH-] E) [OH-]<[H3O+] Name_____________________
Page 3 of 3
-8 mole speck of AgCl (K = 1.6 x 10-10) is added to 1.0 L of H O. What is [Ag+] in the
A 1.0 x 10
A) 1.0 x 10-16 M
9.) B) 1.6 x 10-10 M C) 1.0 x 10-8 M D) 1.3 x 10-5 M E) 1.0 x 10-4 M Under what conditions will a reaction with ∆H° > 0 and ∆S° < 0 be spontaneous?
A) High T B) Low T C) All T D) No T E) Can’t determine 10.) It snows in the winter (low temperature) but it rains in the summer (high temperature). This implies that
for the formation of snow (H2O (l)
A) H°>0, ∆S°>0 B) ∆H°<0, ∆S°>0 C) ∆H°>0, ∆S°<0 D) ∆H°<0, ∆S°<0 E) Can’t determine 11.) A balloon, containing an ideal gas at 25 °C and 2 atm, adiabatically (q=0) expands to twice its original
volume against an external pressure of 1 atm; the final temperature is:
at 25°C A) < 25 °C B) = 25 °C Pext=1atm C) > 25 ° Ideal Gas
V=2X D) = -RlnKp E) Can’t determine 12.) An ideal gas at 25 °C and 2 atm adiabatically (q=0) expands into a vacuum to twice its original volume,
the final temperature is: Ideal Gas
V = X Vacuum
at 25C A) < 25 °C B) = 25 °C Ideal Gas
V = 2X C) > 25 ° D) = -RlnKp E) Can’t determine 13.) A beam of blue light (400 nm) will eject electrons from an unknown metal surface while a beam of red
light (700 nm) will not eject electrons. Will a beam of green light (475 nm) eject electrons from the same
A) No B) Sometimes C) Yes D) Only at high intensity E) Can’t determine Name_____________________
Page 4 of 4
14.) A photon of blue light with a wavelength of 400 nm splits into two identical infrared photons. Knowing
that energy is conserved, what is the wavelength of each of these two photons?
A) 200 nm
15.) D) 566 nm E) 800 nm B) Cl- C) Cu+2 D) O2 (g) E) Cl2 (g) Which of the following will be oxidized by Fe2+ but not by Na+ under standard conditions?
A) Cl2 17.) C) 400 nm Which one of the following species is the strongest oxidizer under standard conditions?
A) Mn2+ 16.) B) 283 nm C) Na+ B) Zn E) Cl- D) Sn Which of the following is a product of the reaction when NaCl is added to an acidic MnO4- solution?
A) Cl2 B) Cl- C) Na D) H+ E) H2 18.) Aluminum (Al) produces H2 bubbles when immersed in HCl, mercury (Hg) does not. Which is the
19.) B) Al C) Hg D) Hg+ E) H2 Buried copper (Cu) pipes are often fitted with zinc (Zn) strips; why? A) To prevent oxidation of Cu B) To prevent reduction of Cu D) Zn is acid resistant C) Zn is shinier E) Zn is base resistant Name_____________________
Page 5 of 5
Part II: 4 Problems, 46 points total
SHOW ALL OF YOUR WORK AND USE UNITS IN YOUR CALCULATIONS. PARTIAL CREDIT
IS AVAILABLE, SO ATTEMPT EACH PART OF EACH QUESTION EVEN IF YOU WERE
UNABLE TO DO THE PREVIOUS PART(S).
1.) Phosgene (COCl2) is a poisonous gas. At 425 K it will dissociate to establish the following equilibrium:
COCl2 (g) CO (g) + Cl2 (g) K= 1.00 x 10-4 a.) Starting with pure COCl2 gas at PCOCl2 = 2.00 atm, when the system reaches equilibrium the total pressure
(in atm) will be: (circle the correct answer)
A) = 1.00 B) Between 1.00 and 2.00 C) = 2.00 D) Between 2.00 and 4.00 E) = 4.00 b.) Calculate the partial pressure of CO for the equilibrium system of part a.). Justify any assumptions made. PCO = _____________
c.) The container for the equilibrium system of part a.) is doubled in volume. When the system reaches the new
equilibrium state the total pressure (in atm) will be: (circle the correct answer)
A) = 1.00 B) Between 1.00 and 2.00 C) = 2.00 D) Between 2.00 and 4.00 E) = 4.00 d.) Calculate the partial pressure of CO for the new equilibrium system of part c.). Justify any assumptions
made. PCO = _____________ Name_____________________ Page 6 of 6 (9pts)
2a.) Starting with a 1.0 liter solution of 1.0 M sodium acetate (Ac-), how many moles of HCl do you need to
add to make a buffer with pH= 4.75? (For HAc pKa = 4.75 ). Show your work. Moles HCl = _____________
b.) To 1.0 L of buffer solution (pH=pKa) produced in part a.) 9.0 L of H2O are added, what is the new pH?
Show your work. pH = _____________
c.) How many moles of acetic acid (HAc) must be added to H2O to make 1.0 L of a solution with pH = 4.75?
Show your work. Moles HAc = _____________ Name_____________________ Page 7 of 7 (12pts)
3.) You make the following observation:
• Addition of tin metal (Sn) to a solution of silver ions (Ag+) results in the formation of Ag and Sn2+.
a.) Write a balanced equation for the net spontaneous reaction that describes this observation. b.) Will Ag reduce Sn2+? Explain. c.) For the standard cell Sn (s) | Sn2+ (1.0 M) || Ag+ (1.0 M) | Ag (s) when current flows, at which electrode,
Sn or Ag, does oxidation occur? Explain in terms of electron flow for the operating cell. d.) In the operating cell in part c.), for each mole of anode (- electrode) that dissolves, how many moles of
metal plate out onto the cathode (+ electrode)? Explain. Moles = _____________ Name_____________________ Page 8 of 8 (15 pts)
4.) A study of the ancient geology of the earth shows that rocks greater than 2 billion years old contain iron in
the form of FeS2. In rocks less than 2 billion years old, iron appears mostly in the oxidized form Fe2O3
4FeS2 (s) + 8H2O (l) + 15O2 (g) 2Fe2O3 (s) + 8H2SO4 (l) a.) Calculate ∆H° for the above reaction. ∆H° = _____________
b.) Calculate ∆S° for the above reaction. ∆S° = _____________
c.) Over what temperature range is the reaction spontaneous? Show your calculations. From T = ___________ to ___________ ...
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