982_1st_Exam_990324
MULTIPLE CHOICE. Choose the one alternative that best completes the statement or answers the question.
1) For the overall chemical reaction shown below, which one of the following statements can you rightly assume?
2H2S(g) + O2(g) 2S(s) + 2H2O(l)
A) The reaction is second-order overall.
B) The rate law cannot be determined from the information given.
C) The reaction is third-order overall.
D) The rate law is, rate = k[H2S]2 [O2].
E) The rate law is, rate = k[H2S] [O2].
Answer: B
2) The reaction A + 2B products was found to have the rate law, rate = k[A] [B]2. While holding the
concentration of A constant, the concentration of B was increased from x to 3x. Predict by what factor the rate of reaction will increase.
A) 9 B) 30 C) 27 D) 6 E) 3
Answer: A
3) For the hypothetical reaction A + 3B 2C, the rate should be expressed as
A) rate= 3 Bt B) rate= At C) rate= Ct D) rate= C2 t E) rate= B3 t Answer: D
4) The units for a first-order rate constant are
A) 1/M2 s B) 1/M s C) 1/s D) M/s
Answer: C
5) Nitric oxide reacts with hydrogen to form nitrous oxide, and water. Use the following data to determine the rate law for the reaction.
2NO + H2 N2O + H2O
Expt.# NO 0 H2 0 Initial rate
1 0.021 0.065 1.46 M/min
2 0.021 0.260 1.46 M/min
3 0.042 0.065 5.84 M/min
A) rate = k[NO]2[H2]2 B) rate = k[NO]
C) rate = k[NO]2[H2]
D) rate = k[NO][H2]
E) rate = k[NO]2 Answer: E
6) A certain first-order reaction A B is 25% complete in 42 min at 25 . What is the half-life of the reaction?
A) 84 min B) 42 min C) 21 min D) 120 min E) 101 min
Answer: E
7) The reaction 2NO2(g) 2NO(g) + O2(g) is suspected to be second order in NO2. Which of the following kinetic plots would be the most useful to prove whether or not the reaction is second order?
A) a plot of ln [NO2] -1 vs. t B) a plot of [NO2]-1 vs. t C) a plot of ln [NO2] vs. t D) a plot of [NO2] vs. t
E) a plot of [NO2]2 vs. t Answer: B
8) For the chemical reaction A C, a plot of 1/[A] versus time was found to give a straight line with a positive slope. What is the order of reaction?
A) Such a plot cannot reveal the order of the reaction.
B) zero C) second D) first Answer: C
9) For the hypothetical reaction A + 3B 2C, the rate of appearance of C given by ( [C]/ t) may also be expressed as
A) C
t = A
t B) C
t = -2/3 Bt C) C
t = B
2/3 t D) C
t = -1/2 At Answer: B
10) Which one of the following changes would alter the rate constant (k) for the reaction 2A + B products?
A) measuring k again after the reaction has run for a while B) increasing the concentration of A
C) increasing the temperature D) increasing the concentration of B Answer: C
11) The Arrhenius equation is k = A e-(Ea / RT) The slope of a plot of ln k vs. 1/T is equal to:
A) —Ea/R B) k C) A D) Ea E) —k
Answer: A
12) The rate law for the reaction 2NO2 + O3 N2O5 + O2 is rate = k[NO2][O3]. Which one of the following mechanisms is consistent with this rate law?
A) NO2 + O3 NO3 + O2 (slow)
NO3 + NO2 N2O5 (fast) B) NO2 + NO2 N2O2 + O2 (slow)
N2O2 + O3 N2O5 (fast) C) NO2 + O3 NO5 (fast)
NO5 + NO5 N2O5 + 5/2O2 (slow)
D) NO2 + NO2 N2O4 (fast) N2O4 + O3 N2O5 + O2 (slow) Answer: A
13) Complete this statement: A catalyst
A) increases the concentration of reactants.
B) increases the collision frequency of reactant molecules.
C) increases the activation energy.
D) alters the reaction mechanism.
E) increases the average kinetic energy of the reactants.
Answer: D
14) Peroxodisulfate ion can oxidize iodide ions to iodine according to the balanced equation
S
2O
82-+ 2I- 2SO
42-+ I
2The reaction is catalyzed by certain chemical species. Identify the catalyst in the following mechanism:
step 1: Fe3+ + 2I- Fe2+ + I2
step 2: S2O82- + Fe2+ 2SO42- + Fe3+
A) Fe3+ B) Fe2+ C) I- D) S2O82- E) SO42-
Answer: A
15) Which is the correct equilibrium constant expression for the following reaction?
Fe2S3(s) + 3H2(g) 2Fe(s) +3H2O(g) A) Kc = [Fe] [H2O] / [Fe2O3] [H2]
B) Kc = [H2] / [H2O]
C) Kc = [Fe2O3] [H2]3 / [Fe]2[H2O]3 D) Kc = [Fe]2[H2O]3 / [Fe2O3] [H2]3
E) Kc = [H2O]3 / [H2]3 Answer: E
16) The following reactions occur at 500°K. Arrange them in order of increasing tendency to proceed to completion (least greatest tendency).
1. 2NOC1 2NO + Cl2 Kp=1.7 × 10-2 2. 2SO3 2SO2 + O2 Kp=1.3 × 10-5 3. 2NO2 2NO + O2 Kp=5.9 × 10-5
A) 1 < 2 < 3 B) 3 < 1 < 2 C) 2 < 3 < 1 D) 3 < 2 < 1 E) 2 < 1 < 3 Answer: C
17) Consider the two gaseous equilibria
SO2(g) + 1
2O2(g) SO3(g) K1 2SO3(g) 2SO2(g) + O2(g) K2
The values of the equilibrium constants K1 and K2 are related by A) K2 = (K1)2
B) K22 = K1
C) none of the above D) K2= 1
K12 E) K2= 1
K2 Answer: D
18) The equilibrium constant for the reaction Ni(s) + 4CO(g) Ni(CO)4(g)
is 5.0 × 104 at 25°C. What is the equilibrium constant for the reaction below?
Ni(CO)4(g) Ni(s) + 4CO(g)
A) 5.0 × 104 B) 2.0 ×10-5 C) 2.0 ×10-3 D) 5.0 × 10-4 E) 2.5 ×109
Answer: B
19) 1.25 moles of NOCl were placed in a 2.50 L reaction chamber at 427°C. After equilibrium was reached, 1.10 moles of NOCl remained. Calculate the equilibrium constant, Kc, for the reaction:
2NOCl(g) 2NO(g) + Cl2(g)
A) 3.0×10-4 B) 4.1×10-3 C) 1.4×10-3 D) 5.6×10-4 E) 1.8×103
Answer: D
20) Kp for the reaction of SO2(g) with O2 to produce SO3(g) is 3 ×1024 . Calculate Kc for this equilibrium at 25°C.
2SO2(g) + O2(g) 2SO3(g)
A) 3×1024 B) 7×1025 C) 5×1021 D) 2×1020 E) 5×1022
Answer: B
21) For the following reactions the equilibrium constants are defined:
A+2B C K1 C D+E K2 Then for the reaction
A+2B D+E Kc The equilibrium constant must be equal to:
A) Kc=K1- K2 B) Kc=K2 / K1 C) Kc=(K1)( K2) D) Kc=K1 / K2
E) Kc=K1+K2 Answer: C 22) For the reaction
PCl3(g) + Cl2 (g) PCl5 (g) Kc = 24.3
a system is prepared with [PCl3] = 0.10 M; [Cl2] = 0.15 M; [PCl5] = 0.60 M Which response is correct? The reaction
A) will form PCl3 and Cl2 until equilibrium is reached.
B) will form more PCl5 until equilibrium is reached.
C) is at equilibrium.
D) None of the above.
Answer: A
23) For the following reaction at equilibrium in a reaction vessel, which one of the changes below would cause the Br2 concentration to increase?
2NOBr(g) 2NO(g)+Br2(g) H°rxn = 30 kJ A) Remove some NOBr.
B) Remove some NO.
C) Compress the gas mixture into a smaller volume.
D) Lower the temperature.
Answer: B
24) For the equilibrium reaction
2SO2(g) + O2(g) 2SO3(g) H°rxn = -198 kJ
which one of the following factors would cause the equilibrium constant to increase?
A) Add a catalyst.
B) Remove O2 gas.
C) Add SO2 gas.
D) Decrease the temperature.
E) None of the above.
Answer: D
25) The reaction
2SO3(g) 2SO2(g) + O2(g)
is endothermic. If the temperature is increased:
A) No change will occure in Kc.
B) Kc will increase.
C) Kc will decrease.
D) The pressure will decrease.
E) More SO3 will be produced.
Answer: D
26) In which of the following gas-phase equlibria is the yield of products increased by increasing the total pressure on the reaction mixture?
A) 2SO3(g) 2SO2(g) + O2(g) B) CO(g) + H2O(g) CO2(g)+ H2(g)
C) 2NO(g) + Cl2(g) 2NOCl(g) D) PCl5 (g) PCl3(g) + Cl2 (g) Answer: C
27) The data below were determined for the reaction below.
S2O82- + 3I (aq) 2SO42- + I3
Expt.# S2O82- I Initial Rate
1 0.038 0.060 1.4 × 10-5 M/s
2 0.076 0.060 2.8 × 10-5 M/s
3 0.076 0.030 1.4 × 10-5 M/s
The rate la for this reaction must be:
A) rate = k [S2O82-] [ I-]
B) rate = k [S2O82-]
C) rate = k [ I-]
D) rate = k [S2O82-]2 [ I-]2 E) rate = k [S2O82-][ I-]3 Answer: A
28) A first-order reaction has a rate constant of 3.0 x 10-3/s. The time requited for the reaction to be 75% complete is:
A) 201 s B) 462 s C) 231 s D) 95.8 s E) 41.7 s
Answer: B
29) The isomerization of cyclopropane to form propene
is a first-order reaction. At 760 K, 15% of a sample of cyclopropane changes to propene in 6.8 min. What is the half-life of cyclopropane at 760 K?
A) 2.5 min B) 230 min C) 29 min D) 3.4 ×10-2 min
E) 23 min Answer: C
30) The isomerization of cyclopropane follows first-order kinetics. At 700 K, the rate constant for this reaction is 6.2
×10-4/min. How many minutes are required for 10.0% of a sample of cyclopropane to isomerize to propene?
A) 1120 min B) 16100 min C) 170 min D) 1.43×10-3 min
E) 3710 min Answer: C
31) At 25 °C, the second-order reaction NOCl (g) NO (g) + 1/2 Cl2(g)
is 50% complete after 5.82 hours when the initial concentration of NOCl is 4.46 mol/L. How long will it take for the reaction to be 75% complete?
A) 17.5 h B) 11.6 h C) 23.0 h D) 8.22 h E) 15.5 h
Answer: A
32) When the following reaction is at equilibrium, which choice is always true?
2NOCl(g) 2NO(g) + Cl2(g)
A) NO 2 Cl2 = NOCl 2 B) NO 2 Cl2 =Kc NOCl 2 C) NOCl = NO
D) 2 NO = Cl2 E) NO Cl2 = NOCl Answer: B
33) The thermal decomposition of acetaldehyde is a second-order reaction.
CH3CHO CH4 + CO
The following data were obtained at 518 °C. The initial pressure of CH3CHO is 364 mmHg.
time, s Pressure CH3CHO, mmHg
42 330
105 290
720 132
Calculate the rate constant for the decomposition of acetaldehyde from the above data.
A) 0.70 mmHg/s B) 2.2 x 10-3/mmHg s C) 2.2 x 10-3/s D) 5.2 x 10-5/mmHg s
E) 6.7 x 10-6/mmHg s Answer: E
34) The thermal decomposition of acetaldehyde is a second-order reaction.
CH3CHO CH4 + CO
The following data were obtained at 518 °C. The initial pressure of CH3CHO is 364 mmHg.
time, s Pressure CH3CHO, mmHg
42 330
105 290
720 132
Based on the data given, what is the half-life of acetaldehyde?
A) 410 s B) 305 s C) 5.4 x 107 s D) 1.5 x 105 s E) 520 s
Answer: A
35) The chemical reaction
2NO(g) N2(g) + O2(g), is exothermic, i.e., H°rxn = -180 kJ which one of the following statements is true?
A) Kp at 1,000K is less than Kp at 2,000K.
B) Kp at 1,000K is larger than Kp at 2,000K.
C) Kp depends on total pressure as well as temperature.
D) The Kp's at 1,000K and 2,000K are the same.
Answer: B
36) For what order reaction does the half-life get longer as the initial concentration increases?
A) concentration B) zero order
C) second ordernone of them because half-life is always independent of the initial D) first order
37) Which one of the following changes would alter the rate constant (k) for the reaction 2A + B products?
A) increasing the temperature B) increasing the concentration of B
C) measuring k again after the reaction has run for a while D) increasing the concentration of A
Answer: A
38) If Ea for a certain biological reaction is 50 kJ/mol, by what factor (how many times) will the rate of this reaction increase when body temperature increases from 37 °C (normal) to 40 °C (fever)?
A) 2.0 times B) 1.20 times C) 1.05 times D) 2.0×105 times
E) 1.0002 times Answer: B
39) The isomerization of methyl isocyanide (CH3NC) CH3NC CH3CN
Follows first-order kinetics. The half-lives were found to be 161 min at 199°C, and 12.5 min at 230°C. Calculate the activation energy for this reaction.
A) 31.4 kJ/mol B) 6.17 x 10-3 kJ/mol C) 78.2 kJ/mol D) 163 kJ/mol
E) 124 kJ/mol Answer: D
40) The activation energy for the reaction C4H10 C2H6 + C2H4
has an activation energy (Ea) of 350 kJ/mol, and the Ea of the reverse reaction is 260 kJ/mol. Estimate H, in kJ/mol, for the reaction as written above.
A) +90 kJ/mol B) -90 kJ/mol C) 350 kJ/mol D) +610 kJ/mol E) -610 kJ/mol Answer: A
41) The activation energy for the reaction O + O3 2O2 is 25 kJ/mol, and the enthalpy change is H= -388 kJ/mol. What is the activation energy for the decomposition of O2 by the reverse reaction?
A) 363 kJ B) 413 kJ C) 25 kJ D) 388 kJ E) 50 kJ
Answer: B
42) An increase in the temperature of the reactants causes an increase in the rate of reaction. The best explanation is: As the temperature increases,
A) the activation energy decreases.
B) the collision frequency increases.
C) the activation energy increases.
D) the concentration of reactants increases.
E) the fraction of collisions with total kinetic energy > Ea increases.
Answer: E
43) The rate law for the reaction H2O2 + 2H + 2I I2 + 2H2O
is rate = k[H2O2][I ]. The following mechanism has been suggested.
H2O2 + I HOI + OH slow OH + H H2O fast HOI + H + I I2 + H2O fast
Identify all intermediates included in this mechanism.
A) H and HOI B) HOI and OH C) H only D) H and I
E) H2O and OH Answer: B
44) The equilibrium constants (expressed in atm) for the chemical reaction N2(g) + O2(g) 2NO(g)
are Kp = 1.1 × 10-3 and 3.6 × 10-3 at 2,200K and 2,500K, respectively. Which one of the following statements is true?
A) Kp is less than Kc by (RT).
B) Higher total pressure shifts the equilibrium to the left.
C) The total pressure at 2,200K is the same as at 2,500K.
D) The partialpressure of NO(g) is less at 2,200K than at 2,500K.
E) The reaction is exothermic, H° < 0.
Answer: D
45) For the reaction X2 + Y + Z XY + XZ
it is found that the rate equation is rate = k [X2][Y]. Why does the concentration of Z have no effect on the rate?
A) Z must react in a step after the rate determining step.
B) Z is an intermediate.
C) The activation energy for Z to react is very high.
D) The fraction of molecules of Z that have very high energies is zero.
E) The concentration of Z is very small and the others are very large.
Answer: A
46) When the concentrations of reactant molecules are increased, the rate of reaction increases. The best explanation is: As the reactant concentration increases;
A) the order of reaction increases.
B) the average kinetic energy of molecules increases.
C) the rate constant increases.
D) the activation energy increases.
E) the frequency of molecular collisions increases.
Answer: E
47) A reaction mechanism usually is
A) obvious if the activation energy is known.
B) obvious if the reaction order is known.
C) difficult to prove.
D) restricted to only one possible explanation.
E) the same as the balanced chemical equation.
Answer: C 48) For the reaction
BrO3 + 5Br + 6H 3Br2 + 3H2O
[BrO3 ] / t = 1.5 ×10-2 M/s at a particular time.
What is [Br ]/ t at the same instant?
A) 13 M/s B) 3.0 ×10-3 M/s C) 7.5 ×10 -2M/s D) 330 M/s
E) 1.5 ×10-2 M/s Answer: C
49) The reaction A + 2B products was found to have the rate law , rate = k[A][B]2. Predict by what factor the rate of reaction will increase when the concentration of A is doubled and the concentration of B is also doubled.
A) 9 B) 2 C) 4 D) 6 E) 8
Answer: E
50) In general, the unit of reaction rate is:
A) s-2 B) s-1 C) L2mol-2s-1 D) mol L-1s-1 E) L mol-1s-1
Answer: D
