Chemistry 1161: Third Midterm A
Monday, April 7, 2025
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RELAX! BREATHE! THINK! DON’T PANIC!
This exam consists of 20 multiple choice questions each worth 3.5 points,
followed by 3 short answer questions worth 10 points each. Your
answers for the multiple choice must be recorded on the on the exam
itself. Fill in your answers as you go – you will NOT receive extra time to
fill in the form, and late forms will not be graded.
You will receive credit for work written on this exam for the short answer
portion. Feel free to use the back of a page for scratch work, but that will
not be graded unless you direct us there from the front.
Good luck!
Multiple Choice
Identify the choice that best completes the statement or answers the question by filling in the bubble that
contains the letter.
1. Which of the following is not true of the heating curve for water?
Both liquid water and ice are present at 0 °C The temperature of liquid water remains at
100 °C as it boils
The temperature of steam cannot exceed 100 ° The temperature of ice remains at 0 °C as it
C melts
The temperature of liquid water increases
linearly as it is heated
2. Heat transfer from the system to the surroundings has a large effect on Ssurr...
when the temperature of the system is low. when the temperature of the surroundings is
high.
when the temperature of the surroundings is when the temperature of the system is high.
low.
at any temperature, as the amount of heat
transferred is independent of temperature.
3. The steps in a reaction mechanism are as follows. Which species could be acting as a catalyst? All species
are aqueous.
Step 1: Ag+ + Ce4+ Ag2+ + Ce3+
Step 2: Ag2+ + Tl+ Ag+ + Tl2+
Step 3: Tl2+ + Ce4+ Tl3+ + Ce3+
Tl+ Ag+
Ag2+ Ce4+
Tl3+
4. To the best of our knowledge, not one molecule of diamond has ever turned into graphite in the history
of the universe. Why is this?
Diamond has a high internal energy. The entropy of the system decreases
significantly when diamond turns into
graphite.
Diamond turning into graphite is The activation energy for the reaction is large.
nonspontaneous.
The entropy of the universe decreases
significantly when diamond turns into
graphite.
5. Hydrogen reacts with nitrogen to form ammonia (NH3) according to the reaction:
3 H2(g) + N2(g) 2 NH3(g).
The value of H° is –92.38 kJ/mol, and that of S° is –198.2 J/mol · K. Determine G° at 25.00°C.
-16.66 kJ/mol +5.897 × 104 kJ/mol
+297.8 kJ/mol -33.32 kJ/mol
+49.50 kJ/mol
6. In the reaction below, if the rate of formation of ammonia is 0.345 M/s, what is the rate of disappearance
of N2?
N2(g) + 3 H2(g) 2 NH3(g)
0.518 M/s 245 M/s
0.173 M/s 0.345 M/s
0.690 M/s
7. The dissolution of ammonium nitrate in water is a spontaneous endothermic process. It is spontaneous
because the system undergoes __________
an increase in enthalpy. a decrease in entropy.
an increase in entropy. a decrease in enthalpy.
an increase in free energy.
8. For the reaction A + B C + D, it is determined that the rate law is given by: rate = k[A]2. Which of the
following statements must be true? Select all that apply.
The reaction must take place in more than one Molecule B does not appear in the rate-
step. limiting step.
Molecule A must appear in more than one The rate-limiting step must be termolecular.
step.
Molecule B must be an intermediate.
9. Determine the standard enthalpy of formation for N2O(g) given the following information about the
formation of NO2 under standard conditions, and Hf(NO2) = 33.2 kJ/mol.
2 N2O(g) + 3 O2(g) 4 NO2(g); Hrxn = -31.4 kJ/mol
+82.1 kJ -164.2 kJ
-82.1 kJ +164.2 kJ
+50.7 kJ
10. Rank the following in terms of their absolute entropy: F2(g), Br2(l), He(g), CO2(g)
F2(g) > CO2(g) > Br2(l) > He(g) CO2(g) > F2(g) > He(g) > Br2(l)
CO2(g) > Br2(l) > He(g) > F2(g) F2(g) > Br2(l) > He(g) > CO2(g)
CO2(g) > F2(g) > Br2(l) > He(g)
11. The reaction enthalpy for a particular reaction is estimated using two different methods: once using
enthalpies of formation, and once using bond enthalpies. The two answers do not agree. Which of the
following statements is most likely true?
The estimate using bond enthalpies is likely to be closer to the experimental result.
The estimate using heats of formation is likely to be closer to the experimental result.
Reaction enthalpies cannot be calculated using bond enthalpies.
Neither estimate is likely to be close to the experimental result.
It is surprising that the two estimates do not agree.
12. The entropy change in a system ( Ssys) during a spontaneous process must be
less than zero. greater than zero.
equal to zero. greater than, less than, or equal to zero.
greater than or equal to zero.
13. Consider the reaction 2 A + B 2 C. The concentrations of A, B,
and C are shown in the figure on the right. Which trace
corresponds to which species?
The dashed trace is A; the gray trace is B; the The dashed trace is A; the gray trace is C; the
black trace is C. black trace is B.
The dashed trace is B; the gray trace is A; the The dashed trace is C; the gray trace is A; the
black trace is C. black trace is B.
The dashed trace is C; the gray trace is B; the
black trace is A.
14. Dinitrogen pentoxide rapidly decomposes in the atmosphere according to the reaction 2 N2O5(g) 4
NO2(g) + O2(g). At a particular temperature, the slope of a plot of ln[N2O5] versus time in seconds
is -0.00693. What is the half-life of this reaction if [N2O5]o = 1.20 M?
4.79 s 86.6 s
120 s 100 s
72.2 s
15. This diagram shows Arrhenius plots for four different reactions. Rank
the reactions from highest frequency factor to lowest frequency factor.
d>c>b>a a>b>c>d
a=c>d>b b>a=c>d
d>a=c>b
16. Which statement below regarding rate laws and rate constants is true?
The rate constant is independent of temperature.
The units of the rate constant depend on the stoichiometry of the reaction.
The rate constant typically depends on concentration of reactant(s).
The rate constant does not change when a catalyst is introduced.
The rate typically depends on concentration of reactant(s).
17. Use the bond energies below to estimate the enthalpy change associated with the chlorination of
methane:
CH4(g) + Cl2(g) CH3Cl(g) + HCl(g)
C-H: 412 kJ/mol
Cl-Cl: 242 kJ/mol
C-Cl: 338 kJ/mol
H-Cl: 431 kJ/mol
115 kJ/mol -188 kJ/mol
-115 kJ/mol 77 kJ/mol
-77 kJ/mol
18. The reaction of one mole of sulfur dioxide gas with oxygen to form sulfur trioxide gas at 25°C and 1 atm
releases approximately 198.5 kJ of heat while absorbing approximately 2.5 kJ of work. Which of the
following is true?
The reaction enthalpy cannot be determined without more information.
The reaction is exothermic and E is more negative than H.
The reaction is endothermic and E is more positive than H.
The reaction is endothermic and E is less positive than H.
The reaction is exothermic and E is less negative than H.
19. Water has a molar heat capacity of 75.38 J/(mol · °C) and its vaporization enthalpy at 100°C is 40.7 kJ/mol.
How much energy is needed to convert 124 g liquid H2O at 20.0°C to steam at 100°C?
322 kJ 239 kJ
56.5 kJ 1030 kJ
5790 kJ
20. When molecules with small dipole moments, like CO, form crystalline structures at low temperatures,
they sometimes form imperfect crystals. Some molecules will form local structures with different
orientations compared to the rest of the crystal. What is the entropy of such a crystal as the temperature
approaches 0 K?
S>0 S=0
S<0 S 0, depending on how carefully it is cooled
S 0, depending on how carefully it is cooled
21. EXTRA CREDIT: In the space below, tell me which was your favorite under-represented scientist of the
semester (1 point) and why (2 points). It's ok if you don't remember the name or the details so long as
there's enough to identify them.
Short answer question A1: Show your work!
In a galaxy far, far away, scientists have studied the following reaction:
2 TuR (g) + 2 Sa2 (g) ⇌ Tu2 (g)+ 2 Sa2R (g)
When various reaction mixtures were studied, the following initial rates were observed.
Initial [TuR] (M) Initial [Sa2] (M) Initial rate (M s-1)
1.25 x 10-2 1.30 x 10-2 1.23 x 104
2.30 x 10-2 1.30 x 10-2 4.16 x 104
2.30 x 10-2 2.10 x 10-2 6.72 x 104
A) Write the rate law for this reaction, including a numerical value for the rate constant. Include units.
B) The following mechanism has been proposed. Determine which of these steps is rate-determining, and
demonstrate that this leads to the experimental rate law you determined above.
2 TuR (g) ⇌ Tu2R2 (g)
Tu2R2 (g) + Sa2 (g) ⇌ Tu2R (g) + Sa2R (g)
Tu2R (g) + Sa2 (g) ⇌ Tu2 (g)+ Sa2R (g)
Short answer question A2: Show your work!
A) Scientists from a galaxy far away have determined that the enthalpy of combustion of nutorium is given by
the following equation:
Nt(s) + O2(g) → NtO2(g) ΔHº = −213.5 kJ/mol
And the enthalpy of combustion of regvis is given by:
Rg(s) + O2(g) → RgO2(g) ΔHº = −106.8 kJ/mol
In addition, nutorium and regvis are known to react as follows:
Nt(s) + 2 Rg(s) → NtRg2(l) ΔHº = -487.9 kJ/mol
Using only this information, determine the standard reaction enthalpy for the following reaction:
NtRg2(l) + 3 O2(g) → NtO2(g) + 2 RgO2(g)
B) ΔSº for the listed reaction is 126 J/molK. Is this reaction spontaneous at all temperatures, at low
temperatures, at high temperatures, or at no temperature? If the reaction changes spontaneity at some
temperature, what temperature is that transition?
Short answer question A3: Show your work!
A) The reaction rate for A → B doubles when the temperature is increased from 25 °C to 65 °C. Calculate
the activation energy for this reaction. Include units.
B) The rate constant for this reaction at 55 °C is 4.75 x 10-3 M s-1. Calculate the pre-exponential factor, A.
Include units.
