CHAPTER 14 THE POWER OF PETROL
Name:
QUESTIONS
14.1 Getting petrol
1.
Which fraction from the distillation of crude oil would most likely contain the following
hydrocarbons?
(a)
C35H72
(b)
C4H10
(c)
C20H42
(d)
C8H18
2.
What type of hydrocarbon is each of the above? Explain your answer.
3.
The following hydrocarbons are all found in petrol. In each case, draw one possible
structural formula and state the type of hydrocarbon.
(a)
C7H16
(b)
C4H8
(c)
C5H12
14.3 Refining and blending
4.
Below is a list of alkanes. For each one, state whether it:
(a)
(i)
could be used unchanged in petrol
(ii)
should go through isomerisation before being blended with other compounds to
form petrol
(iii)
should be cracked before being blended with other compounds to form petrol.
CH3(CH2)15CH3
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(b)
(CH3) 3CCH(CH3)CH3
(c)
CH3CH2CH2CH3
(d)
CH3CH2CH2CH2CH2CH3
14.4 Where does the energy come from?
5.
Describe and account for the similarities and differences in the equations above.
6.
Suggest why the energy (heat) content of fuels is most often stated in kJ kg–1 rather than
in kJ mol–1.
14.5 Exothermic and endothermic reactions
7.
For each of the processes (a) to (c) below:
(i)
state whether it is exothermic or endothermic
(ii)
identify whether the reactant or products have the higher enthalpy
(iii)
state whether heat would be released to or absorbed from the surroundings.
(a)
Burning kerosene
(b)
Melting ice
(c)
Condensing steam at 100 °C
14.6 Thermochemical equations
8.
Write the thermochemical equation for the formation of 4 mol of carbon dioxide from
methane and oxygen.
9.
Calculate the amount of heat released when the following quantities of methane are
consumed according to the equation:
1
2
CH4(g) + O2 (g)  12 CO2(g) + H2O(g) + 105 kJ
(a)
10 mol
(b)
1 kg
10. The reusable booster rockets of the space shuttle use a mixture of aluminium and
ammonium perchlorate as fuel. An equation for the reaction is:
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3Al(s) + 3NH4ClO4 (s)  Al2O3 (s) + AlCl3(s) + 3NO(g) + 6H2O(g) + 2677 kJ
Calculate the amount of heat released when the following quantities of aluminium
are consumed:
(a)
2 mol
(b)
500 g
11. When 0.5 mol of metallic iron is oxidised in air to iron(III) oxide, 4.12 kJ of heat energy
are released. Write a balanced thermochemical equation for the reaction. Express the heat
of reaction as a ∆H value.
12. For each of the reactions (a) to (c) below:
(i) state whether it is exothermic or endothermic
(ii) identify whether the reactants or products have the higher enthalpy by drawing
reaction diagrams
(iii) state whether the temperature of the reaction mixture would increase or
decrease
(a)
(iv) rewrite the equation to include the heat of reaction in the equation.
2SO2(g) + O2 (g)  2SO3(g) ∆H = –20 kJ
(b)
KNO3 (s)  K+ (aq) + NO3 (aq)
(c)
Ag+(aq) + Cl–(aq)  AgCl(s)
∆H = +35 kJ
∆H = –112 kJ
13. Write the balanced equation and calculate ∆H for each of the following reactions.
(a)
1.2 g of magnesium are burned in air to magnesium oxide, releasing 12.4 kJ of heat.
(b)
1.1 g of zinc are dissolved in excess dilute hydrochloric acid, releasing 2.4 kJ
of heat.
14. Using the ∆H values in question 12, calculate the heat of reaction for each of the
following reactions.
(a)
At SLC, 10 L of sulfur dioxide are burned in 2 L of oxygen.
(b)
10 g of potassium nitrate are dissolved in water.
(c)
100 mL of 0.2M silver nitrate solution are added to 40 mL of 0.04M potassium
chloride solution.
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14.7 Exo or endo?
15. Use the data in table 14.3 to calculate the heat of formation of hydrogen fluoride
according to the equation:
H2(g) + F2(g)  2HF(g)
16. Given that the bond energy for the OH bond in water is 460 kJ mol–1, calculate the
approximate heat of reaction for the combustion of hydrogen gas.
H2 (g) +
1
2
O2(g)  H2O(g)
Review questions
1.
Explain the meaning of a fuel’s octane rating.
2.
Premium petrol has an octane rating of 97. To what 2,2,4-trimethylpentane to heptane
mixture does this rating compare?
3.
List two ways to increase a fuel’s octane rating.
4.
Compare the molecular structures of octane and iso-octane.
5.
Why is tetraethyl lead no longer used as a petrol additive?
6.
List some molecules that might be formed when the following substances are cracked.
(a)
C16H34
(b)
C18H38
7.
What is meant by the enthalpy change of a chemical reaction?
8.
When writing a thermochemical equation, why must the physical states of each reactant
and product be included?
9.
Energy is ‘lost’ during an exothermic reaction. Explain why this observation is
still consistent with the Law of Conservation of Energy.
10. Propane (C3H8) and octane (C8H18) are important constituents of the fuels LPG and petrol
respectively. Each compound burns with excess oxygen to produce carbon dioxide and
water. Experiments show that 2.48 moles of C3H8 are required to yield the same quantity
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of heat of combustion as 1.00 mole of C8H18. Which fuel would consume the greater
quantity of oxygen in generating the same amount of heat? Justify your answer.
11. The heats of combustion of ethyne (acetylene) (C2H2), hexane (C6H14) and ethane (C2H6)
are 1630 kJ mol–1, 1300 kJ mol–1 and 1560 kJ mol–1 respectively. Assuming all three are
readily available, which fuel would be most suitable in a situation where the mass used
must be kept as low as possible?
12. Each person in Australia, on average, utilises 500 MJ of energy daily.
(a)
Assume that all this energy is obtained solely from oil at 100% efficiency. What
mass of oil would be consumed daily for each person? (Oil has an energy value of
4.1 × 104 MJ tonne–1.)
(b)
If Australia’s oil reserves are 462 GL, for how many years will our oil last,
assuming the rate of consumption found above, a population of 18 million and no
oil imports? (Oil has a density of 0.824 kg L–1.)
(c)
If, in reality, oil continues to contribute 36% of Australia’s total energy
consumption, for how long will our oil last?
13. For each of the following reactions, state whether they are exothermic or endothermic:
(a) H2(g) + Cl2(g)  2HCl(g)
∆H = –185 kJ mol–1
(b)
H2(g) + CO2(g)  CO(g) + H2O(g)
∆H = +41 kJ mol–1
(c)
SnO2 (s) + 2C(s)  Sn(s) + 2CO(g)
∆H = +360 kJ mol–1
(d)
N2(g) + 3H2 (g)  2NH3(g)
∆H = –92.2 kJ mol–1
14. Compare endothermic and exothermic reactions with respect to:
(a)
the sign of ∆H
(b)
heat exchange with the surroundings
(c)
energy contents of reactants and products.
15. (a)
Prepare a list of common everyday chemical reactions that are exothermic, and one
or two reactions that are endothermic.
(b)
From your list of exothermic reactions, discuss which of the reactions give off heat
that can subsequently be used for a specific purpose, and which of the reactions
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give off heat that is subsequently not used.
(c)
From your list of endothermic reactions, discuss which of the reactions absorb heat
that can subsequently be used for a specific purpose, and which of the reactions
absorb heat that is subsequently not used.
16. Iron can react with carbon dioxide according to the following equation:
2Fe(s) + 3CO2(g)  Fe2O3(s) + 3CO(g) ∆H = +26.3 kJ mol–1
(a)
Explain why this equation may be described as a thermochemical equation.
(b)
Calculate the enthalpy change, in kilojoules, when 2 g of iron react completely with
excess carbon dioxide.
(c)
Calculate the heat change, in kilojoules, when 200 mL of carbon dioxide is used
at SLC.
(d)
What mass of iron would be needed to absorb 1000 kJ of heat energy?
(e)
What volume of carbon dioxide would be needed at SLC to absorb 1000 kJ
of energy?
17. During the last 30 years of rapid development in space exploration, many different rocket
fuels have been tried. Two of these fuels are hydrogen and dimethylhydrazine. The
equations for the reactions of these fuels are:
(i) 2H2(g) + O2(g)  2H2O(l) ∆H = –576 kJ mol–1
(ii) (CH3)2N2H2(g) + 4O2(g)  N2(g) + 4H2O(g) + 2CO2(g)
∆H = –1694 kJ mol–1
(a)
Why do both of these reactions have negative ∆H values?
(b)
Which fuel releases the greater amount of energy per gram?
(c)
What amount of energy would be released if the combustion of a sample of
dimethylhydrazine produces 1.100 kg of carbon dioxide?
18. The average family car in Australia travels 16 000 km annually. If the petrol consumption
is 7.5 km L–1, 17 000 MJ of energy is actually used in propelling the car for the
16 000 km. Assume that the density of petrol (taken as octane) is 0.70 kg L–1, and that
the combustion is represented by this equation.
2C8H18(l) + 25O2(g)  16CO2(g) + 18H2O
∆H = –10 901 kJ mol–1
(a)
Calculate:
(i)
the mass of CO2 emissions per litre of petrol consumed
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(b)
(ii)
the mass of CO2 produced by the car annually
(iii)
the percentage efficiency of the car [(energy produced/energy used) × 100]
(iv)
the total mass of CO2 produced annually by ten million cars in Australia.
Why is there concern over the large quantities of CO2 produced by cars?
19. Butane (C4H10) is a gaseous fuel. For the combustion of butane at SLC,
∆H = –2880 kJ mol–1.
(a)
Write a balanced thermochemical equation.
(b)
Calculate the quantity of heat that would be produced by combustion of 12 L of
butane, measured at SLC.
20. Burning petrol can be represented by the following equation:
C8H18(l) + 12.5O2(g)  8CO2 (g) + 9H2O(g) + 5555 kJ
(a)
Calculate the heat energy released from the burning of 1.00 L of C8H18(l).
(The density of C8H18 = 0.70 g mL–1.)
(b)
What volume of O2 (g) at SLC would be required to burn this quantity of C8H18(l)?
(c)
In motor vehicles, only 25.0% of the heat energy produced by petrol combustion is
used to propel the car. If a car has a fuel consumption of 8.00 km L–1, how many kJ
of heat energy is wasted when the car travels 100 km?
21. Calculate the average bond energy of the O—O covalent bonds in O3, given that the
thermochemical equation for its formation is:
O(g) + O2(g)  O3(g)
∆H = –107 kJ
22. The bond energy for a C—C single bond is 346 kJ mol–1, whereas the bond energy for
a C==C bond is 609 kJ mol–1.
(a)
Explain this difference in bond energies.
(b)
What would you expect the value for a C≡≡C bond to be?
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Notes:
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