CHAPTER 15 FUELS AND THE FUTURE
Name:
QUESTIONS
15.2 Why are some reactions slow and some fast?
1. Describe and explain the major similarities and differences between the energy
diagrams for exothermic and endothermic reactions shown in figure 15.5.
2. Given what you have learnt about the factors that determine the rate of reactions,
explain why reaction (a) is much faster than reaction (b) at room temperature.
(a)
Ag+(aq) + Cl–(aq)  AgCl(s)
(b)
CH4(g) + 2O2(g)  CO2(g) + 2H2O(l)
3. Hydrogen peroxide decomposes according to the following equation.
H2O2(aq)  2H2O(l) + O2(g)
Draw and label a diagram of potential energy versus reaction coordinate for this
reaction, given that Ea = 75 kJ and H = –98 kJ. Indicate on your diagram:
(a)
where the activated complex exists
(b)
Ea for the reverse reaction and its value in kJ.
15.5 Comparing the energy content of fuels
4. Calculate the heat needed to raise the temperature of 100 mL of water 24 °C.
5. What volume of water could have its temperature raised from 26 °C to 80 °C heated
by 2 kJ of heat energy?
6. Forty-five grams of propane was burned and raised the temperature of 1 L of water by
30 °C. Calculate the molar heat of combustion of the fuel.
7. Copy and complete table 15.2.
8. Taking the specific heat capacity of water as 4.18 J g–l °C–l, calculate the energy
produced from the combustion of 1 gram of each substance. How do the results
compare?
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9. Calculate the molar heat of combustion for each substance. How do the results
compare?
10. Describe a use for each of the fuels tested. Explain why each fuel is better suited to the
purpose it is used for than either of the other two fuels.
11. The molar heat of combustion of ethanol has been found experimentally to be
1120 kJ mol–l.
(a) What was the percentage accuracy of the experiment?
(b) List the sources of error in the experiment and describe how some of these
errors can be minimised.
Review questions
1. Define the following terms.
(a) Activated complex
(b) Activation energy
(c) Catalyst
2. Which of the diagrams below represents the most exothermic reaction?
3. Consider the graphs below. Each shows the potential energy during the course of
a reaction of three liquids, X, Y and Z, with oxygen.
(a) A fuel is a substance that releases heat as it burns. Which liquid could not be
a fuel?
(b) What is the Ea for the reaction of liquid Z?
(c) Which fuel has the greatest heat of combustion?
(d) What is the value of the heat of combustion of liquid Y?
(e) Which fuel would you expect to ignite at the lowest temperature?
4. A very good rocket propellant can be made by allowing hydrogen and fluorine to
react. The product is light in mass and produces a greater amount of energy per unit
mass than any other chemical fuel. The potential energy diagram for the following
reaction is shown below.
H2(g) + F2(g)  2HF(g)
Which of the following would occur if a catalyst was added to the reaction?
A
1 would become smaller and 2 would become larger.
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B
C
D
1 would stay the same and 2 would become smaller.
Both 1 and 2 would become smaller.
Both 1 and 2 would become larger.
5. If dilute hydrochloric acid is added to hydrogen peroxide solution, the rate of
decomposition decreases. The acid, in this case, may be described as an inhibitor, or
negative catalyst.
(a) What would be the advantage of adding a negative catalyst to hydrogen
peroxide solution?
(b) Draw an energy profile diagram for the decomposition of hydrogen peroxide
under the following conditions.
(i) uncatalysed
(ii) with a catalyst
(iii) with added acid.
6. Describe how the addition of oxygenating compounds affects a fuel’s octane rating?
7. List the advantages and disadvantages of the oxygenated additives and alternatives.
8. It is useful to know how much energy can be obtained from different fuels in order to
determine which would be the best fuel for a particular purpose. The apparatus in
figure 15.14 can be constructed in the laboratory to measure the heat given out when a
fuel like ethanol is burned. The heat produced when the fuel burns is absorbed by the
water in the metal can. The temperature can be measured. Therefore, given that the
specific heat of water is 4.18 J g–1 °C–1 and the density of water is 1.0 g mL–1, the heat
of combustion may be determined. The results of one experiment are shown below.
The volume of water in the metal can = 200 mL
Thus, the mass of water in the can = 200 g
The rise in temperature of the water = 11.0 °C
The mass of ethanol burned = 0.5 g.
(a) From these results, calculate the heat produced when 1 gram of ethanol is
burned.
(b) Calculate the molar heat of combustion for ethanol using the above results.
(c) An accurate value for the heat produced when 1 g of ethanol burns is 30 kJ.
Calculate the percentage error in this experiment.
(d) Outline the sources of error in the above experiment and then suggest how the
design of the experiment could be improved so that more accurate heats of
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combustion for different fuels may be determined.
9. When 2 g of sulfur was completely burned in air, the heat evolved raised the
temperature of 222 g of water from 21 °C to 41 °C. Calculate the molar heat of
combustion of sulfur, assuming that the specific heat of water is 4.18 J g–1 °C–1.
10. The specific heat capacity of graphite is 0.71 J °C–1 g–1. Calculate the energy required
to:
(a) raise the temperature of 1.0 mol of graphite by 1.0 °C
(b) raise the temperature of 1.5 mol of graphite by 2.5 °C
(c) raise the temperature of 260 g of graphite by 36.2 °C
(d) raise the temperature of 22 kg of graphite from 15 °C to 30 °C.
11. The specific heat capacity of aluminium is 0.900 J °C–1 g–1.
(a) Calculate the energy needed to raise the temperature of a 7.20 × 102 g block of
aluminium by 10 °C.
(b) Calculate the molar heat capacity of aluminium.
12. It takes 78.2 J of energy to raise the temperature of 45.6 g of lead from 19.2 °C to
32.5 °C. Calculate the specific heat capacity of lead.
13. Kerosene is a hydrocarbon fuel that may be used in lamps, jet engines and camp
stoves. It has a heat of combustion of 44 100 kJ kg–1.
(a) Explain why the heat evolved from the combustion of kerosene is measured in
kJ kg–1 rather than kJ mol–1.
(b) A cup of billy tea contains 250 g of water. How many cups of tea can be made
if 12.5 mL of kerosene are used to heat the water? (Assume that the
temperature of the water increases from 20 °C to 100 °C, the specific heat
capacity of water is 4.18 J g–1 °C–1, and the heat of combustion of kerosene is
37 000 kJ L–1.)
14. The reactions between acids and bases are always exothermic. A student designed an
investigation to determine whether there was a relationship between the volumes
added together and the heat released. She placed 50.0 mL of 1.5M HC1 in a lidded
polystyrene cup. She inserted a thermometer through the lid and used this as a stirrer
as 5.0 mL volumes of NaOH of unknown concentration were added. The temperature
changes during the experiment were recorded in the table below.
(a) Plot a graph of the experimental results.
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(b) Use the graph to determine the heat released in the reaction.
(c) Calculate the molar heat of neutralisation.
15. One of the criteria for the choice of fuels is the amount of heat released on combustion
per gram of fuel. On this basis, which of the following would be the best fuel?
16. An electric car under test is powered by a hydrogen–oxygen fuel cell. The liquid
hydrogen fuel is carried in a 60 L insulated tank and the oxygen is obtained from the
air.
(a) Calculate the energies available from a 60 L tank of liquid hydrogen and a
60 L tank of petrol (assume it is octane), given that the density of liquid
hydrogen at its boiling temperature (–253°C) is 0.071 g mL–1, the density of
octane at 25 °C is 0.70 g mL–1, and the heats of combustion are 286 kJ mol–1
and 5450 kJ mol–1, respectively.
(b) Which tank of fuel will require more oxygen for oxidation, and how many
times more?
(c) The petrol-driven internal combustion engine is a cheap source of energy, with
the technology well developed and understood. Why are efforts being made to
replace these engines with fuel cells?
17. The annual petrol consumption in Australia is around 16 000 ML. The average energy
value of petrol is 34 200 kJ L–1. Ethanol is under consideration as a fuel in motor
vehicles. It reacts as follows.
CH3CH2OH(l) + 3O2(g)  2CO2(g) + 3H2O(l) ∆H = –1367 kJ mol–1
Australia has 26 million hectares of undeveloped land suitable for growing ‘energy
crops’. It has been estimated that such crops could produce grain sugar and cassava
equivalent to 22 Mt of sucrose, and ethanol may be produced by the fermentation of
sucrose:
C12H22O11(aq) + H2O(l)  2C6H12O6(aq)
C6H12O6(aq)  2CH3CH2OH(aq) + 2CO2
Assuming a 90% yield of ethanol from sucrose, what percentage of the country’s
petrol consumption could be saved by the use of this ethanol? List any further
assumptions that you have made.
18. Methanol (CH3OH) is regarded by many chemists as a possible liquid fuel for the
future. The combustion of methanol occurs according to the equation:
2CH3OH(g) + 3O2(g)  2CO2(g) + 4H2O(l)
∆H = –1506 kJ mol–1
(a) Determine the heat of combustion of methanol, in kJ g–1.
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(b) If 0.750 mol of methanol is mixed with 0.750 mol of oxygen and the mixture is
ignited, how much energy will be released?
19. Powdered aluminium or magnesium releases considerable heat when burned. (The
heats of combustion of aluminium and magnesium are 31 kJ g–1 and 25 kJ g–1
respectively.) Would these highly energetic powdered metals be good fuel substitutes
for petroleum? Explain.
20. Compare the advantages, disadvantages and future prospects of methanol, hydrogen
and petrol as fuels for motor vehicles.
21. Australia has large oil-shale deposits at Rundle in Queensland. Assume that the oil
produced has an energy value of 48 × 103 kJ L–1 and the oil shale yields 90 L of oil per
tonne.
(a) If the company needs to produce 7.33 PJ (P = 1020) of energy per day, how
many barrels of oil are needed to be produced daily? (1 barrel = 159 L.)
(b) What mass of shale must be mined and processed daily?
Notes:
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