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Transcript
CH 225H
MT 2
50 minutes
Name: __KEY________________________
Answer all of the following. Each question is worth a total of 10 pts. Show any work clearly for
partial credit.
1.
(a) The nitrite anion (NO2-) disproportionates in aqueous base to form nitrate, NO3- (aq),
and NO (g). Write a balanced equation for this redox reaction.
3 NO2- (aq) + H2O 
2.
NO3- (aq) + 2 NO (g) + 2 OH- (aq)
(a) Rank the following in order of decreasing vapor pressure at 100 °C.
CaO
CO2
H2O
H2S
H2
H2 > CO2 > H2S > H2O > CaO
(b) Using a microscopic (molecular level) approach, briefly explain the origin of surface
tension. .
Surface tension arises from the decreased total intermolecular attractions for molecules at
the surface of a condensed phase, due to the lower average number of neighbors. This
results in a driving force for liquids to decrease their surface areas.
3.
(a) What is the ratio of rms speeds of Ar and He atoms in a gas sample at 290 K and 0.8
atm?
The rms speeds are inversely proportional to the square root of mass, so the ratio of Ar
speed / He speed is ≈ sqrt (4/40) ≈ 0.3
(b) If Ar, He, and Xe are mixed with equal partial pressures in a gas sample, which will
effuse most rapidly through a small hole in the container ? Explain briefly.
He. Effusion is directly proportional to number density (these are all equal) and inversely
proportional to the square root of molecular mass.
4. Describe 2 experimental conditions that will maximize the yield of ammonia that is
prepared by the exothermic reaction of N2 and H2. Explain your answers.
N2 (g) + 3 H2 (g) = 2 NH3 (g)
Page 1
1. Run the reaction at high pressures. This will shift the equilibrium to the right
where there are fewer moles of gas. In practice, this is > 100 atm.
2. Run the reaction at the lowest practical temperature. The reaction is
exothermic, so this will shift the equilibrium to the right. However, if the
temperature is too low, the forward rate constant may be too small. The
optimal temperature is about 500 °C.
3. Extract NH3 while it’s formed , to keep the concentration low.
4. Use an appropriate catalyst (Fe metal based).
5.
Give the relation of osmotic pressure to solution concentration, and explain how the
equation requires this to be a colligative property.
 = i M R T (for a non-electrolyte)
The equation does not depend on any solute properties (other than its
concentration), therefore this is a colligative property.
6.
(a) Sketch one unit cell of the CsI structure.
The figure should show a cubic cell, with either ion at all vertices and the other
ion at the body center.
(b) In relation to the cell edge length, a, what is the closest approach of cation and anion
centers? How many of these nearest neighbor interactions occur for each cation and
anion?
Closest approach = 1.732 a / 2, there are 8 nearest neighbor interactions.
7. (a) What is unusual about the vapor composition above an azeotropic solution?
(b) What happens to the vapor pressure of the solution when the temperature is
increased?
(c) List three compounds besides water that undergo intermolecular hydrogen bonding.
(a) The vapor composition above an azeotrope is the same as the liquid
composition.
(b) The vapor pressures of all solutions increase with increasing temperature.
(c) NH3, HF, CH3OH, other alcohols or amines,
8.
If the pressure on an ideal gas sample with an initial volume of 2 L increases from 7 atm
to 12 atm, while the temperature decreases from 200°C to 100°C, what is the final gas
volume?
Page 2
V = nRT / P
n and R are constants here, so
V2/ V1 = (T2 / T1) (P1 / P2)
V2 = (T2 / T1) (P1 / P2) V1
V2 = (373 K / 473 K) (7 atm / 12 atm) (2 L)
V2 = 0.92 L
9.
At 100 °C, the equilibrium:
2 BI3 (s) = 2 B (s) + 3 I2 (g)
has Kp = 4.3 E-4. When a tube is filled with excess BI3(s) and B(s) and 3 atm of I2(g),
sealed, and then heated to 100 °C, what is the pressure in the tube at equilibrium?
Kp = PI2(g)3 = 4.3E-4
PI2(g) = (4.3E-4)1/3 = 0.075 atm
10.
For the equilbrium:
2 A (aq) + C (aq)
=
D (aq)
Kc = 30. What is Kc for the following equilibrium:
½ D (aq) = A (aq) + ½ C (aq)
Kc = sqrt (1/30) = 0.18
Page 3