Unit 3
REVIEW
Understanding Concepts
1. Calculate the amount of heat in J and kJ that is
required to heat 1.5 kg of water from 20°C to 75°C.
(5.1)
2. The molar enthalpy of vaporization of chlorine is
20.7 kJ/mol. Calculate the enthalpy change during
the vaporization of 2.25 kg of chlorine.
(5.2)
3. In a student lab, 60.0 mL of 0.700 mol/L sodium
hydroxide solution was neutralized with 40.0 mL of
excess sulfuric acid solution. The temperature
increased by 5.6°C.
(a) Calculate the molar enthalpy of neutralization
for sodium hydroxide.
(b) What assumptions have you made?
(5.2)
4. The standard molar enthalpy of formation for vinyl
chloride, C2H3Cl(g), is 37.3 kJ/mol. Express this
information in thermochemical equations as
(a) a heat term;
(b) a ∆H value.
(5.3)
146 kJ/mol, calculate the enthalpy change associated with the combustion of 1 mol of pentane.
(c) How much heat would be released in the combustion of 20 g of pentane?
(5.5)
8. Suggest any three physical properties that may change
during a reaction and that may be used to measure
the rate of a reaction.
(6.1)
9. Table 1 refers to the reaction between carbon
monoxide and nitrogen dioxide:
CO(g) NO2(g) → CO2(g) NO(g)
(a) Predict the missing concentration values.
(b) If the initial concentration of the NO2(g) was
0.250 mol/L, what will be its concentration after
80 s?
(6.1)
Table 1 Concentration of Carbon Monoxide and Carbon Dioxide
Time (s)
[CO(g)] (mol/L)
[CO2(g)] (mol/L)
0
0.100
—
20
0.050
0.050
40
0.033
—
60
0.026
0.074
80
0.020
0.080
100
—
0.083
5. The standard enthalpy of formation of sulfur dioxide
is –296.8 kJ/mol.
(a) Write a thermochemical equation for the formation reaction.
(b) Sketch a potential energy diagram for the reaction, labelling axes, enthalpy of reactants,
enthalpy of products, and ∆H.
(c) If 9.63 g of sulfur dioxide is formed under standard conditions, what quantity of heat is
released?
(5.4)
10. Fire departments warn people about leaving newspapers in large piles in basements. Why would these
newspapers be more of a fire hazard than the same
quantity of wood?
(6.2)
6. Nitromethane is a rapid-burning fuel often used in
dragsters where rate, not energy yield, is important.
11. Observations were made (Table 2) during the decomposition of a compound:
2 X2O5(g) → 4 XO2(g) O2(g)
4 CH3NO2(g) 3 O2(g) → 4 CO2(g) 2 N2(g) 6 H2O(g)
Use Hess’s law and the known thermochemical equations given below to calculate the enthalpy change for
the combustion of one mole of nitromethane.
C(s) O2(g) → CO2(g)
∆H 393.5 kJ
2 H2(g) O2(g) → 2 H2O(g)
∆H 483.6 kJ
2 C(s) 3 H2(g) 2 O2(g) N2(g) → 2 CH3NO2(g)
∆H 226.2 kJ
(5.4)
7. (a) Write an equation for the combustion of one
mole of pentane, C5H12(l), to form carbon
dioxide gas and liquid water.
(b) Given the standard enthalpies of formation in
Appendix C6 and the information that the standard enthalpy of formation of pentane is
414 Unit 3
(a) Using the same concentration and time axes,
draw a graph to show:
(i) [X2O5(g)] vs. time
(ii) [O2(g)] vs. time
(b) Calculate the values to fill the blanks in Table 2.
Table 2 Concentration of Reactant and Products During
Decomposition
Time (h)
[X2O5(g)]
(mol/L)
[XO2(g)]
(mol/L)
0.0
1.20
2.0
0.80
0.20
4.0
0.55
0.325
7.0
0.30
0.45
12.0
0.10
0.55
0
[O2(g)]
(mol/L)
0
NEL
Unit 3
(c) Calculate the overall rates of consumption (or
production) in the first 12 h of:
(i) X2O5(g)
(ii) O2(g)
(iii) XO2(g)
(d) Determine the instantaneous rates of consumption of X2O5 at 2.0 h and 7.0 h.
(e) Describe and explain the observed trend in rate
(6.2)
of consumption of X2O5.
12. Aluminum metal is used in many familiar objects,
from frying pans to screen doorframes and jet aircraft. However, the bottle of aluminum powder in the
chemistry laboratory carries a warning that the contents are potentially dangerously combustible.
Explain these observations.
(6.2)
13. From a kinetics study of the following reaction,
ClO–(aq) I–(aq) → Cl–(aq) IO–(aq)
the rate was found to be first order with respect to each
of the reactants. Predict what would happen to the initial rate, r, as each of the following changes are made.
– ] is doubled.
(a) The initial [ClO(aq)
–
(b) The initial [I(aq)] is halved.
(c) The same initial numbers of moles of reactants
were placed in a container of half the volume. (6.3)
14. The combustion of propane is represented by the
equation
C3H8(g) 5 O2(g) → 3 CO2(g) 4 H2O(g)
(a) Explain whether you would expect this reaction
to happen in a single step or in a series of steps.
(b) If the rate of consumption of propane gas is
∆[C H8]
– 3
4 102 mol/(Ls),
∆t
write expressions and numerical values to represent
the rate of reaction with respect to oxygen and
carbon dioxide gases.
(6.4)
15. Hydrogen iodide and oxygen react together as shown
by the equation:
4 HI(g) O2(g) → 2 I2(g) 2 H2O(g)
The observations shown in Table 3 are obtained when
initial concentrations of reactants are varied:
Table 3 Rate Evidence for Consumption of Hydrogen Iodide
Initial [HI]
(mol/L)
Initial [O2]
(mol/L)
Initial rate
(mol/(L•s))
0.010
0.010
0.0042
0.010
0.020
0.0084
0.020
0.020
0.0168
NEL
(a) What is the order of this reaction with respect to
the hydrogen iodide?
(b) What is the order of this reaction with respect to
the oxygen?
(c) What is the overall order of this reaction?
(d) Write the rate equation for this reaction.
(e) Determine the specific rate constant for this reaction, including units.
(f) How many molecules are involved in the ratedetermining step?
(g) The overall reaction clearly proceeds in several
steps. Even in the absence of rate data, why would
you predict that the reaction would not proceed
in a single step?
(6.4)
16. Nitrogen monoxide reacts with hydrogen gas to produce nitrogen and water vapour. The mechanism is
believed to be:
Step 1 2 NO(g) → N2O2(g)
Step 2 N2O2(g) H2(g) → N2O(g) H2O(g)
Step 3 N2O(g) H2(g) → N2(g) H2O(g)
(a) Write the overall balanced equation for this
process.
(b) Identify the reaction intermediates.
(c) Write the rate equation for this reaction, given
the information that Step 1 is the slow step. (6.4)
17. Sketch a Maxwell-Boltzmann graph showing the distribution of molecular kinetic energies for a sample
of gas at temperatures T1 and T2, where T2 is the
higher temperature. Label the axes.
(6.5)
18. Which one of the following reactions would you
expect to be faster at room temperature? Explain your
answer briefly.
(a) Pb2
(aq) 2 Cl(aq) → PbCl2(s)
(b) Pb(s) Cl2(g) → PbCl2(s)
(6.5)
19. Diamond and graphite are different forms of the
same element, carbon. Under room conditions, the
enthalpy change from diamond to graphite is negative (1.9 kJ/mol), suggesting that diamond should
spontaneously change into graphite, releasing energy
as it does so. Yet no observable reaction takes place. In
contrast, white phosphorus is so dangerously reactive
that it will ignite and burn if exposed to air, forming
phosphorus oxide and releasing considerable heat. It
has been used in incendiary bombs.
(a) Explain these observations in terms of energy.
(b) Sketch potential energy diagrams for the reactions
diamond → graphite and phosphorus oxygen
→ phosphorus oxide. In each case label the
enthalpy change and the activation energy. (6.5)
Energy Changes and Rates of Reaction 415
Analysis
Applying Inquiry Skills
(a) Analyze the Evidence and answer the Question.
(5.5)
20. A calorimetry investigation involves the use of a
copper flame calorimeter.
23. (a) Outline an appropriate Experimental Design for
the following investigation.
Question
What is the enthalpy of combustion of propanal,
C3H6O?
Question
What is the rate law for the following hypothetical
chemical reaction?
AC2D→2ST
Evidence
Mass of calorimeter
Mass of water in calorimeter
Mass of propanal burned
Temperature increase of
calorimeter and contents
Specific heat capacity of copper
305 g
255 g
1.01 g
Evidence
Table 4 Observations on Rate of Reaction and Formation
28.8°C
0.385 J/(g•°C)
Analysis
(a) Analyze the Evidence and answer the Question. (5.2)
21. The following investigation was conducted.
Question
What is the molar enthalpy of neutralization for
hydrochloric acid?
Solid sodium hydroxide is dissolved in a measured
quantity of hydrochloric acid solution in a Styrofoam
laboratory calorimeter.
3.40 g
100.0 mL
0.850 mol/L
14.5°C
35.6°C
Analysis
(a) Analyze the Evidence and answer the Question.
Evaluation
(5.3)
22. The following investigation was conducted.
Question
What is the molar enthalpy of formation of butane?
Experimental Design
The enthalpies of combustion of butane, carbon, and
hydrogen are used with Hess’s law to determine the
molar enthalpy of formation of butane.
Evidence
13
C4H10(g) O2(g) → 4 CO2(g) 5 H2O(g)
2
∆Hc –2657.3 kJ
C(s) O2(g) → CO2(g)
∆Hc –393.5 kJ
H2(g) 1/2 O2(g) → H2O(g)
∆Hc –241.8 kJ
416 Unit 3
0.15
0.20
0.20
12.0
0.30
0.20
0.20
24.0
0.30
0.40
0.20
24.0
0.30
0.40
0.40
(b) Analyze the Evidence and answer the Question.
(6.3)
Making Connections
24. “Nuclear reactors should be used instead of burning
fossil fuels to supply electrical energy.” Do you agree
or disagree with this statement? Justify your answer,
from economic and environmental perspectives. (6.6)
Evidence
(b) Evaluate the Experimental Design.
6.0
Analysis
Experimental Design
mass of NaOH(s)
volume of HCl(aq)
concentration of HCl(aq)
initial temperature of HCl(aq)
final temperature of HCl(aq)
Initial rate of formation Initial [A] Initial [B] Initial [C]
of S (mol/L·s)
(mol/L)
(mol/L)
(mol/L)
25. “The concept of half-life can apply to more than
radioactive decay.” Research this assertion, and
explain it by making reference to at least two examples each of radioactive half-life and chemical halflife.
(6.3)
26. Use collision theory to provide an explanation of the
following observations:
(a) Magnesium metal reacts much more rapidly in
concentrated HNO3(aq) than in dilute HNO3(aq).
(b) Paints and stains often have instructions that they
should not be applied below 10°C (Figure 2).
(c) Food spoils more rapidly on a counter than in a
refrigerator.
(d) A natural gas furnace requires a pilot light or
electronic igniter in order to operate.
(e) Dust in grain elevators has been blamed for several violent explosions, which have completely
demolished the elevators.
NEL
Unit 3
(e) The car is completely warmed up after travelling 2
km. If it uses fuel at a rate of 0.150 L/km when
fully warmed up, what is the rate of consumption
during the first 2 km? The density of octane is
0.800 kg/L. What assumptions must you make in
order to solve the problem with the data provided?
(5.5)
Figure 2
(f) People who have contact lenses sometimes use a
sterilizing kit containing hydrogen peroxide and
a platinum-coated disk. Hydrogen peroxide
decomposes to release bubbles of oxygen gas only
when the disk is placed in the solution.
(g) Baking powder, containing a mixture of a solid
acid and a solid base, does not react when dry but
reacts rapidly when dissolved in water.
(6.4)
Extension
27. When octane is burned completely in a well-tuned
engine, the products are gaseous carbon dioxide and
liquid water. A poorly tuned car engine may produce
equimolar amounts of carbon monoxide and carbon
dioxide, and water is produced in the gaseous state.
(a) Write balanced equations for the reactions that
occur in both the efficient and non-efficient
engines.
(b) Calculate the energy produced in each of the
reactions, using standard heats of formation.
(c) Determine what percentage of available energy is
being wasted in the non-efficient engines.
(d) The engine block and radiator of the car have a
mass of 200 kg and a specific heat capacity of
0.50 J/(g•°C). The cooling system contains 15.0
kg of water. The operating temperature of the
engine is 95°C, but when the engine is started up
on a cold winter morning, the air temperature is
–15°C. If the engine has been tuned to make it
run efficiently, calculate the mass of octane that
must be burned to raise the temperature of the
engine to operating temperature.
NEL
28. The concept of half-lives can be applied to biological,
as well as chemical, systems. There is some concern
that antibiotics, fed to livestock to speed their growth
by killing disease organisms, are excreted in the animals’ urine, thereby contaminating ground water.
This rate of excretion appears to be a first-order reaction. Research this issue, and find out how long
animals keep 50% of the administered antibiotics
in their bodies.
(6.3)
GO
www.science.nelson.com
29. Papain is an enzyme derived from tropical papaya
fruit. It acts as a catalyst to break down the peptide
bonds in proteins. Research to find how this enzyme
is being marketed for consumer use, and comment on
the potential effectiveness of these uses.
(6.5)
GO
www.science.nelson.com
30. The hydrolysis of dimethyl ether, an exothermic reaction, is represented by the equation
CH3OCH3(aq) H2O(l) → 2 CH3OH(aq)
For the reaction to proceed, acid (H(aq)) is required.
The experimentally determined rate law equation is
r k [CH3OCH3]1[H(aq)]1
(a) Draw structural diagrams for the reactants and
products.
(b) What is the function of the acid?
(c) What reactant species must be involved in the
rate-determining step?
(d) Design a simple two- or three-step mechanism,
consistent with the above information. You may
find it useful to consider CH3OH as an intermediate. Clearly identify the slowest step in your
mechanism.
(e) Draw a reasonable potential energy diagram, consistent with your mechanism, with clearly labelled
axes, reactants, intermediates, and products. (6.5)
Energy Changes and Rates of Reaction 417