Page 1 of 10 CHEMISTRY 151 Midterm Exam #1 – 100 points
#1: Multiple Choice __________/ 24
#2: Half-life __________/ 07
#3: Nuclear equations __________/ 09
#5: Binding energy __________/ 10
#6: Reaction rates __________/ 16
#7: Activation energy __________/ 14
Group-work extra credit: __________/ 04
TOTAL: __________/80
Instructions:
1. Only non-graphing calculators are allowed; cell phones must be put away. 2. Once we begin each section of the exam, you cannot leave and return.
3. Anyone caught cheating in any fashion will receive an automatic F for the entire course, as indicated in the syllabus.
4. Individual exam: You will have 1 hr 45 min to complete this exam individually. Following this “traditional” exam time, you will be permitted a 5-min break, but may not consult your notes, books, etc.
5. Group exam: You will have 1 hr to re-work another copy of the exam. This time you must work together in groups of 4, talking & consulting with one another.
6. If your group effort on the exam is higher than each original exam score in your group, then each person will receive 4 pts extra credit added to the original individual score.
Page 3 of 10 Part #1: Multiple Choice (3 points each)
1. Consider the reaction
2NH3(g) N2(g) + 3H2(g)
If the rate [H2]/t is 0.030 mol L–1 s–1, then [NH3]/t is
A) –0.045 mol L–1 s–1 D) –0.010 mol L–1 s–1
B) –0.030 mol L–1 s–1 E) none of the above
C) –0.020 mol L–1 s–1
2. A bottle of vintage red wine has lost its label. The concentration of tritium (3
1H) in the
wine is 0.34 times that found in freshly bottled wines. If the half-life of tritium is 12.3 years, estimate the time elapsed since the wine was bottled.
A) 37 years D) 5.2 years.
B) 19 years. E) none of these
C) 12 years.
3. Sulfuryl chloride, SO2Cl2(g), decomposes at high temperature to form SO2(g) and
Cl2(g). The rate constant at a certain temperature is 4.68 10–5s–1. What is the order of
the reaction? A) zero B) first
C) second
D) third
E) More information is needed to determine the order.
4. The decomposition of dinitrogen pentaoxide has an activation energy of 102 kJ/mol and
H°rxn= + 55 kJ/mol. What is the activation energy for the reverse reaction?
A) 27 kJ/mol B) 47 kJ/mol C) 55 kJ/mol D) 102 kJ/mol
E) More information is needed, since this is a Hess's law calculation.
5. Which of the following types of radioactive decay does not produce new element?
A) gamma emission D) alpha emission
B) electron capture E) double beta emission
6. A catalyst accelerates a reaction because
A) it increases the number of molecules with energy equal to or greater than the activation energy.
B) it lowers the activation energy for the reaction.
C) it increases the number of collisions between molecules. D) it increases the temperature of the molecules in the reaction. E) it supplies energy to reactant molecules.
7. Exposure to 10 nCi for 10 minutes is more hazardous for a child than for an adult because
A) the child's cells are dividing more rapidly than the adult's and are, therefore, more susceptible to the radiation.
B) the child's smaller body size makes the effective dose larger for the child than for the adult.
C) the child's immune system is not developed well enough to resist damage. D) the child's skin is not as thick as an adult's and cannot block as much radiation. E) None of the above reasons is correct.
8. Sucrose decomposes to fructose and glucose in acid solution. When ln [sucrose] is plotted vs. time, a straight line with slope of –0.208 hr–1 results. What is the rate law for the reaction?
A) Rate = 0.208 hr–1 [sucrose]2 D) Rate = 0.0433 hr [sucrose]
Page 5 of 10 Read the questions carefully. Please show all work in order to receive credit and write legibly. Report numerical answers with appropriate significant figures and units.
2. (7 pts) The decomposition of ammonia gas into nitrogen and hydrogen is a zero-order process. At 1100.0 C, a 0.250 M sample of ammonia has a rate constant of 2.08 10–4 M min–1. Calculate its half-life.
3. (9 pts) Write complete balanced equations for the following situations. Be sure to include full symbols for all reactants & products expected.
b. Alpha decay of uranium-234
c. Beta decay of indium-116
4. (10 pts) Consider the reactions in d) and e) above. The binding energy of silicon-32 is 8.48157 MeV/nucleon.
a. Calculate the binding energy of uranium-234 in MeV/nucleon. The nuclear mass of this isotope is 234.04095 amu.
Page 7 of 10
5. (16 points)The following data are given for the decomposition of hydrogen peroxide:
[H2O2] (M) t (min)
0.300 0
0.221 1
0.107 5
0.065 10
0.047 15
0.037 20
a.) (3 points)Determine the rate law for the decomposition of hydrogen peroxide using the above data. You must justify your answer, just providing the rate law will not receive full credit.
b.) (3 points)Determine the average rate for the reaction.
c.) (5 points)Use the data above to determine the rate constant.
6. (14 pts) Ammonia (NH3, ΔHf = -45.9 kJ/mol) will react with oxygen in the presence of a copper
catalyst to form nitrogen and water (ΔHf = -285.8 kJ/mol).
a. (4 pts.) Write a balanced equation for the reaction and calculate the enthalpy of reaction.
b. (5 pts.) From 164.5°C to 179.0°C, the rate constantincreases by a factor of 4.27. What is the activation energy of this oxidation reaction?
Page 9 of 10
Possibly useful information & scratch paper (periodic table on the next page) YOU MAY REMOVE THIS PAGE FOR CONVENIENCE
mp = 1.0072764669 amu [A]t = – kt + [A]o
mn = 1.0086649156 amu
1 amu = 1.6605389 × 10–24 g = 931.494 MeV 1 Ci = 3.70 × 1010 Bq
c = 2.997 × 108 m s–1
R = 8.314 J K–1 mol–1 1 eV = 1.602 × 10–19 J
A = k N
RT Ea Ae k kt A At [ ]o
