Thermodynamics and Rates of Reactions
Multiple Choice
Identify the letter of the choice that best completes the statement or answers the question.
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1. An endothermic reaction is one where
a. heat is transferred from the surroundings into a system
b. heat is transferred from a system into the surroundings
c. kinetic energy is transformed into potential energy
d. there is no transfer of heat
e. none of the above
2. What is another way to write the following equation?
C2H6(g)  C2H2(g) + 2H2(g) H = 3.2  102 kJ/mol
a. C2H6(g)  C2H6(g) + 2H2(g) + 3.2  102 kJ
b. C2H6(g) + 3.2  102 kJ  C2H2(g) + 2H2(g)
c. 2H2(g) + C2H2(g) + 3.2  102 kJ  C2H6(g)
d. C2H6(g) – 3.2  102 kJ  C2H2(g) + 2H2(g)
e. none of the above
3. 1.00 mol of an element burned in a calorimeter raise the temperature of 100 g of water by 10.00 C. The molar
heat of combustion of the element is
a. 6.36 kJ/mol
d. 4.20 kJ/mol
b. 0.239 kJ/mol
e. 16.9 kJ/mol
c. 41.8 kJ/mol
4. Which statement concerning the accompanying diagram is true?
a. H is positive
b. the system is endothermic
c. the system releases heat to the surroundings
d. the heat content of the reactants is less than the heat content of the products
e. the enthalpy of the products is greater than the enthalpy of reactants
5. Which of the following equations represents the correct heat of formation of water vapour?
a. H2(g) + 1/2 O2(g)  H2O(l)
d. H2(g) + O2(g)  2H2O(g)
b. H2(g) + O(g)  H2O(g)
e. H2(g) + 1/2 O2(g)  H2O(g)
c. 2H(g) + O(g)  H2O(g)
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6. A small 2.0-g candle is consumed in heating 100 g of water from 25ºC to 75ºC. If you ignore loss of heat to
the air, the energy produced per gram of the candle would be
a. 8.56 kJ
d. 2.62 kJ
b. 5.23 kJ
e. 20.65 kJ
c. 10.46 kJ
7. What is the average rate of production of
in the reaction of zinc with hydrochloric acid if after 30 s the
volume of hydrogen is 20 mL, and after 1 minute, the volume is 62 mL?
a. 62 mL/s
d. either a or b
b. 1.4 mL/s
e. either b or c
c. 62 mL/min
8. The presence of a catalyst is thought to increase the rate of a reaction by
a. changing the products that are formed in the reaction
b. decreasing the enthalpy change of the reaction
c. increasing the enthalpy change of the reaction
d. decreasing the activation energy of the reaction
e. increasing the activation energy of the reaction
9. If for the reaction
, the rate law is determined to be
a. doubling the concentration of Y will double the rate of the reaction
b. halving the concentration of Y will double the rate of the reaction
c. doubling the concentration of X will double the rate of the reaction
d. halving the concentration of X will double the rate of the reaction
e. only changes to the concentration of Y will affect the rate of the reaction
, then
____ 10. Consider the above reaction mechanism. The rate-law equation from this reaction would be
a.
d.
b.
c.
e. impossible to tell from this information
Matching
Match these terms with the descriptions below.
a. reaction intermediates
b. activation energy
c. rate-determining step
d. effective collision
e. elementary step
f.
g.
h.
i.
j.
reaction mechanism
activated complex
threshold energy
ineffective collision
temperature
____ 11. a measure of the average kinetic energy of the particles
____ 12. the minimum increase in potential energy of a system for molecules to react
____ 13. a collision that results in breaking bonds and forming new bonds
____ 14. molecules formed as short-lived products in reaction mechanisms
____ 15. the minimum kinetic energy required to convert kinetic energy to activation energy to form the activated
complex
Short Answer (Knowledge) - 8 marks
16. If a reaction produces a gas, give two instruments that could be used to determine the rate of reaction? (2
marks)
17. What is meant by an activated complex? Give an example. (2 marks)
18. Describe three ways how the rate of a reaction such as,
increased. (3 marks)
, could be
19. How are nuclear power stations and conventional power stations similar? (1 mark)
Problem (Inquiry) - 20 marks
20. Wind energy is becoming a more common form of energy that can be used to run household appliances.
a. If a windmill is connected to a hot water tank and is used to heat 60 L (60 kg) of water from 15ºC to 70ºC,
how much energy is required? (3 marks)
b. If energy from the utility company costs 0.35 cents per megajoule, how much money is saved in a year if
the water in the tank is heated 1400 times? (2 marks)
21. a. Calculate Ho for the following reaction. (3 marks)
b. State whether the reaction is exothermic or endothermic. (1 mark)
c. Rewrite the equation as a thermochemical equation to include the heat term. (1 mark)
d. If a calorimeter containing 5.00 kg of water at 20.0oC recorded the heat change, what would the final
temperature of the water? (2 marks)
22. Use the following data to calculate the reaction rate law for the system and to determine the rate constant for
the reaction:
(6 marks)
Experiment
NO (mol/L)
H2 (mol/L)
Initial Rate of Reaction (mol/(L·s))
1
0.001
0.004
0.002
2
0.002
0.004
0.008
3
0.003
0.004
0.018
4
0.004
0.001
0.008
5
0.004
0.002
0.016
6
0.004
0.003
0.024
23. Given the following information, determine the molar heat of vapourization of water. (2 marks)
H2(g) + 1/2 O2(g)  H2O(g) + 242.8 kJ
H2(g) + 1/2 O2(g)  H2O(l) + 286.9 kJ
Essay - Application (6 marks)
1. Explain each of the following instances using the collision theory. Identify the factor responsible for each.
a) a lit match dropped into a pail of gasoline will ignite but not a pail of diesel fuel.
b) hydrogen peroxide decomposes slowly but when some KI(aq) is added, it decomposes rapidly.
c) Fire departments warn people about leaving large piles of newspapers in the basement but are not concerned about an
equal mass of wood.
24. cwater = 4.18 J/goC
Hf H2O = - 285.8 kJ/mol
Hf C6H6 = + 49.0 kJ/mol
Thermodynamics and Rates of Reactions
Answer Section
MULTIPLE CHOICE
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
ANS:
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A
B
D
C
E
C
B
D
C
C
REF:
REF:
REF:
REF:
REF:
REF:
REF:
REF:
REF:
REF:
K/U
C
I
C
C
I
I
K/U
K/U
K/U
OBJ:
OBJ:
OBJ:
OBJ:
OBJ:
OBJ:
OBJ:
OBJ:
OBJ:
OBJ:
5.1
5.3
5.2
5.3
5.5
5.2
6.1
6.2
6.3
6.4
LOC:
LOC:
LOC:
LOC:
LOC:
LOC:
LOC:
LOC:
LOC:
LOC:
ECV.01
EC2.02
EC2.03
EC1.05
EC2.05
EC2.03
EC2.06
EC1.04
EC1.03
EC1.06
J
B
D
A
H
REF:
REF:
REF:
REF:
REF:
K/U
K/U
K/U
K/U
K/U
OBJ:
OBJ:
OBJ:
OBJ:
OBJ:
6.3
6.4
6.4
6.4
6.5
LOC:
LOC:
LOC:
LOC:
LOC:
EC2.01
EC2.01
EC2.01
EC2.01
EC2.01
MATCHING
11.
12.
13.
14.
15.
ANS:
ANS:
ANS:
ANS:
ANS:
SHORT ANSWER
16. ANS:
- the investigator can collect the gas and measure its volume and/or pressure increase as the reaction
proceeds versus the time the reaction proceeds
REF: K/U
OBJ: 6.1
LOC: EC1.03
17. ANS:
- if the molecules have enough kinetic energy, they can approach closely enough for their bond structures
to rearrange to form an activated complex
- an activated complex is an unstable chemical species containing partially broken and partially formed
bonds representing the maximum potential energy point in the change
REF: K/U
OBJ: 6.4
LOC: EC1.05
18. ANS:
- since concentration is a factor that affects the rate of reaction, by increasing the concentration of the acid,
the reaction rate would be increased
- temperature is another factor that affects the rate of reaction, by increasing the temperature of the
reaction, the reaction should proceed at a faster rate
- since surface area of a solid would affect the rate of a reaction, by powdering the magnesium, the rate of
the reaction would be increased
- perhaps a catalyst could be added that could speed up the reaction (unknown)
- perhaps using a different acid and/or metal (although this would change the reaction though)
REF: C
OBJ: 6.2
LOC: EC1.04
19. ANS:
- in both systems, heat is used to boil water and the resulting steam drives a turbine
- the spinning turbines, in turn, drive generators that produce electricity
- the difference is that in a conventional power station, combustion of natural gas, oil, or coal supplies the
heat and in a nuclear power station, nuclear fission provides the heat
REF: C
OBJ: 5.6
LOC: EC3.01
PROBLEM
20. ANS:
a.
13.8 MJ of energy is required to heat the water in the hot water tank.
b.
Approximately $67 would be saved over the course of the year on the hot water tank.
REF: I
21. ANS:
a.
OBJ: 5.1
LOC: EC3.01
The heat of reaction is –907 kJ/mol of Benzene
b. Since the sign of the heat of reaction is negative, the reaction is exothermic.
c.
REF: I
OBJ: 5.5
LOC: EC2.05, EC2.02
22. ANS:
From the equation we can write the partial rate law as
Using experiments 1 and 2, the concentration jumps from 0.001 to 0.002 moles/L. IT DOUBLES!! The rates
for these same experiments. The rate jumps from 0.002 to 0.008 moles/L seconds. IT QUADRUPLED!!.
The exponential constant 'm' for the [NO] is the mathematical relationship between these two values. i.e.
2m = 4 therefore m = 2 because 22 = 4
Look at experiments 4 and 5. The H2 concentration DOUBLES and the rate DOUBLES.
2n = 2 therefore n = 1 since 21 = 2
So the rate law expression can be rewritten as
rate = k [NO]2 [H2]1
Now to determine the value of 'k'. Choose any one of the experiments.
Using experiment 1. Using the rate law, above fill in the values from the data table.
0.002 mol/L sec = k (0.001 mol/L)2 * (0.004 mol/L)
0.002 mol/L sec = k * (0.000001 mol2 /L2) * (0.004 mol/L)
0.002 mol/L sec = k * 0.000 000 009 mol3/L3
k = 0.002 mol/L sec
0.000 000 004 mol3/L3
= 500,000 sec/mol2 L2 or sec mol-2 L-2
Therefore the rate law equation for this reaction is
rate = 500,000 sec mol-2 L-2 [NO mole/L]2 [H2 mol/L]
REF: I
OBJ: 6.3
LOC: EC2.06
23. ANS:
Show the addition of equations to get and thus appropriate
H2O(l)  H2O(g)
H2(g) + 1/2 O2(g)  H2O(g)
H2O(l)  H2(g) + 1/2 O2(g)
H2O(l)  H2O(g)
values
The molar heat of vaporization of water is 44.1 kJ/mol based on the given information.
REF: I
OBJ: 5.4
LOC: EC2.04
ESSAY
24. ANS:
Students answers should include some of the following points:
REF: I
OBJ: 5.6
LOC: EC3.01