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AP Chemistry
Quarter #3 Exam
Spectroscopy, Intermolecular Bonding, Solutions, Kinetics, Equilibrium
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Directions: Each of the questions or incomplete statements below is followed by
five suggested answers or completions. Select the one that is best in each case
and place the letter of that selection in the corresponding space on the first page
of this booklet.
2 NO (g) + O2 (g) —> 2 NO2 (g)
1) A possible mechanism for the overall reaction represented above is the
following:
Step 1: NO (g) + NO (g) —> N2O2 (g) slow
Step 2: N2O2 (g) + O2 (g) —> 2 NO2 (g) fast
Which of the following rate expressions agrees best with the possible
mechanism?
(A)Rate = k[NO]2
(B)Rate = k[NO]1[O2]-1
(C)Rate = k[NO]2[O2]-1
(D)Rate = k[NO]2[O2]
(E) Rate = k[N2O2][O2]
2) Of the following compounds, which is the most ionic?
(A)SiCl4
(B)BrCl
(C)PCl3
(D)Cl2O
(E) CaCl2
HCO3- (aq) + OH- (aq) —> H2O (l) + CO32- (aq) ΔH = -41.4 kJ
3) When the reaction represented by the equation above is at equilibrium at 1 atm
and 298 K, the ratio [CO32-]/[ HCO3-] can be increased by doing which of the
following?
(A)Decreasing the temperature
(B)Adding H+
(C)Diluting the reaction mixture with water
(D)Adding a catalyst
(E) Bubbling neon gas through the reaction mixture
4) The boiling points of elements helium, neon, argon, krypton, and xenon
increase in that order. Which of the following statements accounts for the
increase?
(A)The London dispersion forces increase.
(B)The hydrogen bonding increases.
(C)The dipole-dipole forces increase.
(D)The chemical reactivity increases.
(E) The number of nearest neighbors increases.
Rate = k[M]1[N]2
5) The rate of a certain chemical reaction between substances M and N obeys the
rate law above. The reaction is studied with [M] and [N] each 1 x 10-3M. If a
new experiment is conducted with [M] and [N] each 2 x 10-3M, the reaction rate
will increase by a factor of
(A)2
(B)4
(C)6
(D)8
(E) 16
6) Which of the following describes the changes in forces of attraction that occur
as water changes phase from a liquid to a gas?
(A)H-O bonds break as H-H and O-O bonds form.
(B)Hydrogen bonds between water molecules are broken.
(C)Covalent bonds between water molecules are broken.
(D)Ionic bonds between H+ and OH- ion are broken.
(E) Covalent bonds between H+ ions and water molecules become more
effective.
Use the information below to answer questions 7-10.
PCl5 (g) ⇆ PCl3 (g) + Cl2 (g)
PCl5 (g) decomposes into PCl3 (g) and Cl2 (g) according to the equation above. A
sample of PCl5 (g) is placed into a rigid, evacuated 1.00 L container. The initial
pressure of the PCl5 (g) is 1.00 atm. The temperature is held constant until the PCl5
(g) reaches equilibrium with its decomposition products. At equilibrium, the total
pressure in the 1.00 L container is 1.40 atm.
7) Which of the following is the most likely cause for the increase in pressure
observed in the container as the reaction reaches equilibrium?
(A)A decrease in the strength of the intermolecular attractions among
molecules in the container
(B)An increase in the strength of the intermolecular attractions among
molecules in the container
(C)An increase in the number of molecules, which increases the frequency of
the collisions with the walls of the container
(D)An increase in the speed of the molecules that then collide with the walls of
the container with greater force
8) As the reaction progresses toward equilibrium, the rate of the forward reaction
(A)Increases until it becomes the same rate as the reverse reaction rate at
equilibrium
(B)Stays constant before and after equilibrium is reached
(C)Decreases to become a constant nonzero rate
(D)Decrease to become a zero at equilibrium
9) If the decomposition reaction were to go to completion (not equilibrium), the
total pressure in the container would be
(A)1.4 atm
(B)2.0 atm
(C)2.8 atm
(D)3.0 atm
10)Which of the following statements about Kp, the equilibrium constant for the
reaction is correct?
(A)Kp > 1
(B)Kp < 1
(C)Kp = 1
(D)It cannot be determined whether Kp > 1, Kp < 1, or Kp = 1 without additional
information.
11)Which of the following is a nonpolar molecule that contains polar bonds?
(A)F2
(B)CHF3
(C)CO2
(D)HCl
(E) NH3
2 X (g) + Y (g) —> 3 Z (g)
12)The reaction mixture represented above is at equilibrium at 298 K, and the
molar concentrations are [X] = 2.0 M, [Y] = 0.50 M, and [Z] = 4.0 M. What is the
value of the equilibrium constant at 298 K?
(A)0.50
(B)2.0
(C)4.0
(D)16
(E) 32
13)Which of the following molecules would be able to hydrogen bond to another
identical molecule?
(A) CH4
(B)F2
(C)HCl
(D)HF
(E) HCN
14)Which of the following is the most polar molecule?
(A)H2
(B)HF
(C)HCl
(D)ClF
(E) F2
15)Which best describes the force of attraction that must be overcome to melt
NaCl?
(A)London dispersion forces
(B)Dipole-dipole attraction
(C)Ion-dipole attraction
(D)Ionic bonds
(E) Covalent bonds
Part II
Clearly show the method used and the steps involved in arriving at your answers.
1)
2 HI (g) ⇆ H2 (g) + I2 (g)
Kc = 2.2 x 10-3 at 500 K
HI (g) decomposes according to the equilibrium equation above.
a) Write the expression for Kc
b) If 0.045 moles of HI is placed into a 2.5 liter evacuated flask, determine the
equilibrium concentration of H2 that will result.
c) If 1.0 mole of HI, 1.0 mole of H2 and 1.0 mole of I2 are placed into a 10.0 L
vessel at 500 K, is the reaction at equilibrium, does it proceed toward
reactants, or does it proceed toward products? Justify your answer with a
calculation.
2) Explain each of the following statements using principles of atomic structure
and/or chemical bonding. Be sure to refer to both substances in your
answer.
a) H2O has a higher boiling point than CH4.
b) CH3OH readily mixes with H2O and C5H12 does not.
c) CaO has a higher lattice energy than LiBr.
d) At room conditions, I2 is a solid and F2 is a gas.
3)
!
Dinitrogen pentoxide (N2O5) decomposes to NO2 and O2 at relatively low temperatures in the
following reaction:
2N2O5(soln) → 4NO2(soln) + O2(g)
This reaction is carried out in a CCl4 solution at 45°C. The concentrations of N2O5as a
function of time are listed in the following table, together with the natural logarithms and
reciprocal N2O5 concentrations. The graphs above apply to the data in the table below.
Time
(s)
[N2O5]
(M)
ln[N2O5
]
1/[N2O5] (M
−1)
0
0.0365
−3.310
27.4
600
0.0274
−3.597
36.5
1200
0.0206
−3.882
48.5
1800
0.0157
−4.154
63.7
2400
0.0117
−4.448
85.5
3000
0.00860
−4.756
116
3600
0.00640
−5.051
156
a) Write the rate law for the reaction:
b) Determine the value of the rate constant, k, with appropriate units.
c) How long will it take the concentration of N2O5 to reach 1.0 % of its original
value?
d) How would the following change if the initial concentration of N2O5 was
tripled? Explain.
i) Magnitude and units of k
ii) The initial rate of the reaction